(^nmll Uttivmitg J \%XM% BOUGHT WITH THE INCOME | PROM THE SAGE ENDOWMENT FUND ' THE GIFT OP Menirs m. Sage Z891 ........k.U-U-9i , ..Ay^mM Cornell University Library arV18700 Elementary manual of magnetism and elect 3 1924 031 273 836 olin,anx The original of tliis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924031273836 ELEMENTARY MANUAL MAGNETISM AND ELECTEICITY, WORKS BY ANDREW JAMIESON, M. Inst. C.E., F.E.S.E., &c., PROFB550R OF ENGINEERING, THE GLASGOW AND WEST OF SCOTI-AND TECHNICAL COLLEGE. 'Seventh Edition. Pocket-Size, 680 pp., Leather, Bs. 6d. A POCKET-BOOK OF ELECTRICAL RULES AND TABLES for the use of Electricians aiid Engineers. By John Munro, C.E., and Andrew JAMIESON, F.R.S.E., M. Inst. C.S., M.I.E.E., &c. " Wonderfully PERFECT. .... Worthy of the highest commendation we can give it." — EUcirician. "The STERLING VALUE of Mcssrs. Munro and Jamieson's Pocket-Book."— £/f^/wa^ Review^ Second Edition. Crown 8vo, with very numerous Illustrations, Cloth, 3J. 6if . MAGNETISM AND ELECTRICITY (an Elementary Manual on). Specially arranged for Elementary or First- Year Students. With Examination Questions. "A capital text-book. , , . The diagrams are an important feature."— 5'<:A(7tf/»Mr/?r. " Arrangement of subjects as good as well can be . . . diagrams are also excellent, _. . . The subject throughout treated as an essentially practical one, and very clear instructions given."— iVff^w/w, Sixth Edition, Revised. Crown 8vo, Cloth, 75. 6rf. A TEXT-BOOK ON STEAM AND STEAM ENGINES, specially arranged for the Use of Science and Art, City and Guilds of London Institute, and other Engineering Students. With numerous Illustrations, four folding Plates, and many Examination Questions. " The BEST BOOK yet published for the use of Students." — Engineer. " The most valuable and complete handbook of reference on the subject tfiat now exists,' --Marine Engineer, Second Edition. With numerous Illustrations, Crown 8vo, Cloth, 3*. &d, STEAM AND THE STEAM ENGINE (an Elementary Manual of). Forming an Introduction to the Larger Work by the same Author. With numerous Illustrations and Examination Questions at the end of each Lecture. •* The whole subject is admirably treated." — School Board CkronicU, "Quite the kight sort op book for first-year students."— £«;?»»«?■. Specially arranged for the use of Teachers and Students preparing for the Science and Art Departments and other Uzaminations in these Subjects. ADVANCED AND HONOURS QUESTIONS ON MAGNETISM and ELECTRICITY. Crown 8vo, wrapper, price 6d, net ; interleaved for use of Teachers, price i^. London : CHARLES GRIFFIN & CO., Exeter Street, Strand. ELEMENTAEY MANUAL MAGNETISM AND ELECTRICITY SPECIALLY ARRANGED FOR THE USE OF FIRST-YEAR SCIENCE AND ART DEPARTMENT AND OTHER ELECTRICAL STUDENTS. ANDREW _JAMIESON, M.Inst.C.E. FSOF£saOR OF ENOINEERING, THB GLASGOW AND WEST OP SCOTLAND TECHNICAL COLLEGB ; FORMERLY ELECTRICIAN' ABROAD TO THE EASTERN TELEGRAPH COMPANY ; MEMBER OTf THE TNaTITUTtON OF ELECTRICAL ENGINEERS ; FELLOW OF THE ROYAL SOCIETY, EDINBURGH ; ATTTHOR OP '* ELECTRICAL RULES," AND TEXT-BOOKS ON " STEAM AND STEAM-ENGINES," ETC. SECOND EDITION. aaitj iSiinui-ous Jllustratelr ©.vperimeiits antt (ffi.wminatioit-C|uestioits. LONDON: CHARLES GRIFFIN AND COMPANY, EXETER STREET, STRAND. 1891. [AU rights reserved,] A. 5r^7^ CORNELL JUNIVERSiTYj ^LIBRARY^ e PREFACE. This Manual has been ^v^itten expressly for Elementary ov First- year Students of Magnetism and Electricity. It covers the Elementary Stage of the Science and Art Department's Exa- mination; but, at the same time, the treatment is sufficiently general for students of Public and Private Schools who have not these examinations specially in view. The book is divided into three distinct parts. Part I. treats of Magnetism; Part II. of Electro-Magnetism and Current Elec- tricity under the general heading of Voltaic Electricity; and Part III. of Electro-Statics, or what is commonly termed Frictional Electricity. Each of these parts is sub-divided into seven or eight short Lectures, full of illustrations of apparatus and experiments. A Test-question with Answer has been given at the end of mo^t of the Lectures; and, in addition, a series of carefully selected questions has been arranged in the precise order of, and re- lating solely to, each Lecture, so that teachers and students may have a minimum of trouble in finding suitable examples. Full advantage has been taken of the excellent and searching questions set annually by the Science and Art Department's Examiners in IV PKEFACE. this subject; in fact, all those given in their Elementary exami- nations for the last eight years have been incorporated, together with others. At the end of each part will be found a short Appendix on the making of Experimental Apparatus by students, and on their conducting experiments with the same. Hitherto students have, for the most part, been treated simply to lectures and exercises, with few or no opportunities of handling tools or electrical and magnetic appliances. Now, however, a new and most praiseworthy feature in the teaching of this subject is fast coming into vogue in Schools and Colleges, whereby students have not only to make their own apparatus, but to take laboratory notes of the results obtained. There can be no doubt that a combination of lectures, exercises, practical workshop and laboratory testing creates the most lasting impressions and the most thorough understanding in the minds of students. It often happens that a lad, compara- tively dull in the lecture-room, will pull ahead of his more theoretically advanced feUow-students in the workshop and laboratory, and ultimately become the better Electrician or Elec- trical Engineer. The different parts of the book may be taken up in any order most agreeable to the teacher, but a continuous experience of ten years' lecturing on this subject has proved to the Author that it is preferable to begin with Magnetism, on account of the greater ease with which fresh or elementary students understand and appreciate the magnetic phenomena. Moreover, since the session generally commences in October, the early winter months PKEFACK. V in thjs country, owing to the natural humidity of our atmosphere, with its attendant fogs, are most unsuitable for successful experi- ments on Frictional Electricity. Magnetism naturally leads on to Electro-Magnetism and the laws of Current Electricity, and the student here gets a grasp of the accepted notions of potential, resistance, positive, and negative Electricity, &c., and is thus better prepared to follow and understand the peculiar phenomena observable when dealing with high tension or high pressure elec- tricity in a statical condition. The book, as a whole, will form an easy introduction to the Author's more Advanced Text Book upon the same subject, now in preparation. In the latter, a knowledge of Elementary Mathematics will be assumed, whereas in the present Manual any youth who has merely had a course of Elementary Arithmetic wUl be able to follow the expositions from beginning to end. Of course, I have been indebted to other writers on the subject, but, at the same time, I have endeavoured, as far as possible, to carry out the method (adopted in my Lectures to my own students) of drawing attention to the pi-aetical applications of experimental facts in Telegi'aphy, Telephony, Electric Lighting, Transmission of Power, Electro-Metallurgy, &c. Most of the figures have been specially drawn for this book to represent the forms in which they are used at my Lectures. I have to thank my Electrical Assistants — Mr. James Livingston, A. Inst. E.E., Mr. Thos. Crichton Fulton, Mr. Ernest Payne, B.A. (Cantab.), and Mr. Philip S. Stewart, Wh. Exh., for kind assistance La the preparation of this little work. VI PKEFACB. If any errors should be observed by readers, or if they will kindly send me any suggestions or communications tending to increase the usefulness of the book, I shall feel greatly obliged for an early note of them, and will gratefully acknowledge the receipt of the same. AliTDKEW JAMIESON. The Glasgow and West op Scotland Technical College, September 1889. PREFACE TO THE SECOND EDITION. All errors observed in the first edition have been corrected, and all the Elementary Questions set at the 1890 and 1891 Science and Art Department's May Examinations have been printed at the end of the respective Lectures to which they moht naturally belong. A. J. ikplember 1891. CONTENTS. LECTURE I. „G„ Natural Magnets — Artificial Magnets — How to Distinguish a Mag- net — Definition of a Magnet — The Poles of a Magnet — How to Make Artificial Magnets — Specimen Question and Answer — Questions 1-8 LECTURE II. Permanent Magnets — Common Forms of Permanent Magnets and their Uses (Simple Bar, Compound Bar, Simple Horse-shoe, Compound Horse-shoe, Horizontal Needle, Vertical Needle) — Attraction and Repulsion — First Law — Polarity — Speci- men Question and Answer — Questions .... 9-17 LECTURE III. Magnetic Curves or Lines of Force— External and Internal Mag- netic Fields — Second Law — Graphic Representation of Magnetic Fields — Different Cases of Magnetic Curves — Magnetic Axis and Magnetic Equator of a Bar Magnet — Specimen Question and Answer — Questions , . . 18-27 LECTURE IV. Molecular Theory of Magnetisation — Magnetic Saturation — Reten- tivity and Resistance — Effect of Vibration on Magnetisa- tion — ^Effect of Temperature on Magnetisation — Questions , 28-34 LECTURE V. Distribution of Free Magnetism along a Bar Magnet — Another Proof of the Molecular Theory of Magnetisation, Breaking a Magnet — Magnetic Screens, Magnetic and Non-Magnetic Substances — Pole Pieces, Armatures and Keepers — Speci- men Question and Answer — Questions .... 3S-44 VIU CONTENTS. LECTURE VI. PAOM Magnetic Induction — Definition of Induction — Secondary Induc- tion — In the Case of Induction tlie Attraction always takes place between Two Magnets — Action and Reaction are Equal and Opposite — Inductive Effects of Like and Unlike Poles — Polarity Reversed, or Consequent Poles produced by Induction — Questions 45-5° LECTURE VII. The Earth Regarded as a Magnet — Geographical and Magnetic Poles and Meridians — -True Polarity of the Earth — Declina- tion or Variation — Inclination or Dip — Earth's Magnetic Axis and Equator — Questions 51-59 LECTURE VIII. The Mariner's Compass — Magnetisation by the Inductive Effect of the Earth's Magnetism — Magnetisation of Iron and Steel Ships — The Earth's Influence on a Magnet is Directive, but not Translative — A Compass-needle always obeys thp Stronger Force — Astatic Pair — Questions .... 60-70 APPENDIX. Practical Notes on Making Experimental Apparatus for Study- ing Magnetism 71-77 INSTRUCTIONS TO BE FOLLOWED IN THE WEITING OF HOME EXEECISES. 1. Put the date of handing in each exercise at the right-hand top comer. 2. Leave a margin an inch wide on the left-hand side of each page ; and in the margin place the number of the question, and nothing more. 3. Leave a space of at least three lines between your answers for remarks or corrections. 4. Be sure you understand exactly what the question requires you to answer, then give aU it requires, but no more. ■ If unable to answer any question, write down its number and the reason why. 5. Make your answers concise, clear, and exact ; and accompany them, whenever practicable, by an Uhistrative sketch. 6. Make all sketches large, open, and in the centre of the page, and do not crowd any writing about them. Note. — The character of the sketches will be considered in awarding the marks to the several questions. Neat sketches and an " Index to Parts," with the first letter of name of Part, will always receive more marks than a bare written description. 7. Every sketch must be accompanied by an " Index to Parts " written immediately beneath it, and must accompany the- answer it is designed to illustrate. Note. — The initial letter or letters of the name of the Part must be used, and not A, B, C, or i, 2, 3, &c. 8. Unless otherwise specially requested by the question, every sketch must be accompanied by a concise written description, 9. Every answer which receives less than half of the full marks awarded to it, must be re-written correctly for next evening, before the usual class work and headed "He-written." Remabks. — Students are strongly recommended to write out the answer in scroll first, and then to compare it with the question. After committing it to their book, they should then read it over a second time, so as to correct any errors they may thereby discover. Reasonable and easily intelligible contrac- tions are permitted. Students are invited to ask questions and explanatiohs regarding anything they do not understand. Except in special cases, arrears of Home work will not receive marks. N.B.— Students who from any cause have been absent from a lecture, must send a post-card or note of explanation to the teacher. If tliey miss any exercise or exercises, they must state the reason (in red ink, or underlined) in their exercise hooks when handing them in next night. Tf these rules are not complied with, then marks wUl he deducted. ELEMENTARY MANUAL MAGl^ETISM AI^D ELECTRICITY. LECTURE I. Contents. — Natural Magnets — Aitificial Magnets — How to Distinguish a Magnet — Definition of a Magnet — The Poles of a Magnet — How to Mate Artificial Magnets — Specimen Question and Answer — Questions. Natural Magnets. — It does not appear to be known who first discovered magnets. The word " magnet " is supposed, however, to have been derived from the name of an ancient district in Lydia, Asia Minor, called Magnesia, where a mineral and certain brown-coloured stones (now known as the magnetic oxide of iron, FejOj) were observed to possess the magic property of attracting imall pieces of iron or steel. The Chinese claim to have dis- covered that when a piece of this magnetic stone Ls freely suspended by a thread, it naturally takes up a definite position, pointing nearly north and south, and they have been credited with being the first to make use of this device, in a.d. 1122, for the purpose of assisting them in navigat- ing their ships. Hence the term Lodestone, or " leading stone," has been given to these natural magnets, which are found in various parts of the world, such as Sweden, Spain, the United States, &c., and from which class of ore excellent iron is obtained. Artifloial Magnets. — If we take a piece of lodestone, and rub a bai' of hardened steel with it, we shall find that the steel has been imbued with the magnetic properties of the natural magnet Natueal Magnet with IKOH Filings. LECTUKE I. without any apparent loss of magnetism in the stone. The strength of the magnetism found in lodestone is, however, not great, and since it cannot impart to the steel or artificial magnet a greater intensity of magnetisation than is possessed hy the lode- stone itself, we shall have recourse later on to more convenient and effective methods of making the magnets to be used in illus- trating these Lectui'es. Experiments I. — How to Distinguish a Magnet. — Suppose you are given two bars of the same kind of steel, which are to all outward appearances alike in every respect, but the one is a magnet and the other is not ; how would you find out which bar is magnetised ? First, lay each piece of steel amongst iron filings, and then lift each of them clear of the filings. You immediately observe that some of the iron filings adhere to one of the bars, more espe- cially to its ends, whilst the other has no filings attached to it. Magnbtisbd Bae with Iron Filings. Unmagnetisbd Bak. You conclude from this simple experiment that the bar which has attracted the iron filings is a magnet, and that the other is not. The magnetic force may, however, be so weak that few filings (especially if these are coarse) attach themselves to the magnet ; consequently, now try another and more delicate test. Second, suspend each bar horizontally by means of a fine thread, without any twist in it, and a copper-wire stirrup, as shown by the figure, keeping the bars a few feet apart, so that the magnetism of the one may not affect the other ; you observe that one of the bars comes to rest with its length pointing in a northerly and southerly direction, and that, although you deflect it from this position, it always comes back to it again, whereas the other bar rests indifferently in any position. The former is evidently a magnet, whilst the latter is unmagnetised. We shall discuss in a future Lecture how it happens that the natural magnetism of the Earth acts upon the magnetised bar, and turns it until it takes up the definite position pointing nearly north and south. Copper Thread Steel " " Bar Suspended Hard Steel Bae. HOW TO MAKE ARTIFICIAL MAGNETS. 3 Definition of a Magnet. — A magnet is a piece of steel, or other iiuKjnetised substance, which possesses the i^'operties of aitractiny other pieces of steel or iron or other niagnetisahle bodies, and of 2)oiHtiny, when freely suspended in a horizontal position, towards the poles of the earth. The Poles of a Magnet. — The ends of a magnet are termed poles. The end or pole which turns towards and points to tlie North, we shall call the " North Pole," or N-end, or marked, or red end of a magnet ; and the other, the " South Pole," or S-end, or unmarhed, or blue end.* ExpEEiMENTS II. — How to Make Artificial Magnets.f — Take a piece of hard steel (say 6" x |" x |") and make a mark near one end of it with a a file. This end we wish to make a north pole. I. Magnetisation by Single Touch. — Lay the piece of steel flat on a table, and take a strongly-magnetised steel bar magnet in the right hand, placing the unmarked or S-end of the same on the unmarked end of the bar to be magnetised. Then stroke the magnet along the steel bar to the opposite end, lifting it clear from the marked end, and letting it come down on the unmarked end in the direction indicated by the arrow and dotted curved line in the gs^ figure. Repeat this a dozen times. Now || turn the steel bar on its side, and repeat I __,, *->i>^ the stroking process until all four sides llli'' ^'> of the bar have been acted upon. The ||| '', bar of steel will then be found to be /l J magnetised, having the marked or, N', ^^^Jps" ^'"^ '-f ^ end as a north pole, since that end was ■ "' ""^'<" «««««««< i i mi muuJ'^ always last touched with the S-end of Single Touch. the magnet. A better result may be obtained if one pole of the magnet be stroked from the centre of the steel bar to one end, and the other pole from the centre to the other end, following the directions in- dicated by the dotted lines and arrows in the two following figures. Then turn the bar over and repeat the process, until all four sides have been thus acted upon. A horse-shoe shaped magnet may be used to magnetise a bar of steel of simUar shape by simply placing a piece of soft iron across * Makers of magnets usually stamp the north-seeking or north-pointing end of a magnet with the letter, If, or make a file-mark across that end, or paint it red, to distinguish it from the unmarked, south, or blue-painted end. ' t See the Appendix to this section, " Magnetism," for a more detailed explanation. LECTURE I. the ends of the latter and stroking it with the magnet, following the direction of the curved dotted line and arrows as shown by the C ^.. ■"■N, s' B s' n' Magnetising Each Half Separately by Single Touch. figure. When the p bar being acted upon is turned over, care must be taken to also reverse the f| magnet so as to keep the opposite poles N and S next each other. Magnetising Hobse-shoe Magnet. 2. Magnetisation hy Divided Touch. — Place the steel bar N'S', to be magnetised, upon the ends of two bar magnets as shown '\ /■ Magnetisation by Divided Touch. by the figure. Take two other bar magnets, one in each hand (with their poles as marked), and holding them in an inclined HOW TO MAKE ARTIFICIAL MAGNETS. position simultaneously stroke the bar, N'S', from the centre to the ends, following the direction of the dotted curved lines and arrows. Turn the bar, N'S', over, and repeat the process a dozen or more times on each side, until it is as strongly magnetised as you can make it under the circumstances. This may be tested by trying the number or weight of naUs which the bar under treat- ment will lift. 3. Magnetisation hy Double Touch. — The only difference between this and the preceding method is, that the two magnetising bars are moved together with a piece of wood between them, their opposite poles being next each other. Now, with one set of ends of this double magnet, rub the steel bar to be acted upon from middle to one end backwards and forwards, finishing at the middle. Then rub from the middle to the other end, giving it the same number of rubs. Finally, do the same to each of the other sides until you have rendered it as strongly magnetised as can be effected. 4. Magnetisation by an Electric Current. — None of the previous processes will render a thick, large bar of steel a strong magnet. "When a strong bar magnet is required, the hard steel bar should be placed inside a coil of covered copper wire (as shown by the following figure). / Magnetisation by an Electric Current. Through this coil a strong current of electricity is passed from any convenient source, such as a battery or a dynamo, and the ends of the bar are tapped with a hammer during the passage of the current. Should it happen to be a horse-shoe shaped magnet that is required, insert first one limb into the coil, put on the current, tap the bar. Then take it out of the coU, and insert the other limb 6 LECTURE I. into the opposite end of the coil ; put on the current, tap the bar, take off the current and extract the bar, when you wiU have a magnet proportionately strong to the strength of the current employed and the number of turns of wire in the coil if the steel is not saturated. cc i.- c. Magnetisation hy an Electro-Magnet.— Aiioth&o very effective method is represented by the next figure, which illustrates the magnetisation of a bar of steel by an electro-magnet. Magnetisation by an Electro-Magnet. Index to Parts, EM represents Electro-Magnet bobbins or solenoids. NS „ North and South poles of iron cores. Y -....-. WB Th-T- B N' Yoke of soft iron fixed to cores. Wooden Base to which Y is fixed. Terminals positive and negative. Battery or generator of electric current. Arrows showing direction of current. North pole of bar being magnetised. The construction and action of Electro-Magnets will be fully described in a future Lecture ; but as this is a most useful piece of apparatus, its outward appearance and the method of using it for the purpose of making artificial magnets should be explained to the student at the very commencement of his studies of Magnetism; SPECIMEN QUESTION AND ANSWER. 7 for with it, he will get far better results than by any of the previous processes. An ordinary electro-magnet for lecture and workshop use consists of a, U, or horse-shoe shaped piece of very soft wrought iron, called the core, upon each limb of which is placed a bobbin or coil of covered copper wire. This combination is fixed to a wooden base, WB, and two of the ends of the coUs of wire are joined together, whilst the other two are fixed to ter- minals marked, T + , and, T — , which in turn are connected to the ends of a battery, B, or other suitable source of electricity. When the wires are properly joined together and connected to the battery, the current of electricity which passes through the wire strongly magnetises the soft iron cores, with a north pole at one end and a south pole at the other as marked, N, and, S. All you have to do, is to clasp with the right hand the steel bar which you wish to magnetise, and stroke the bar from its middle to one end, as indi- cated by the curved dotted line and arrows, turning the bar quarter round after each stroke, so as to present each side of it to the core of the electro-magnet. Thereafter reverse the steel bar, and operate upon the other end with the other pole of the electro- magnet, until the bar will attract and lift two or three times its own weight of wrought iron. Specimen Question and Answer. Question. — You are required to magnetise a darning-needle so that the eye-end shall be a south pole. For this purpose, you are given a bar magnet, and are allowed to use only its south pole. How would you do so, and prove that you had done so correctly ? Answer. — Stroke the darning-needle several times from the centre to the point on the S-pole of the bar magnet, taking care to bring the needle .back to its centre each time with a curve as indicated by the curved line and arrows in the figure. [The student should make two neat freehand sketches in his note-book, the one to illustrate the experiment, and the other the proof. Always give a sketch in answering a question if the answer admits of it.] Proo/!— ^Suspend the needle near its centre by a fine thread without twist ; if the needle (after being disturbed from its posi- tion) invariably comes to rest with its eye-end pointing southward, then you know that that end must have a south pole.. LECTUEE I. QUESTIONS. Lectubb I. — Questions. 1. State what you know about natural magnets. Where are they found 1 What name has been given to them, and why ? 2. What is the difference between a natural and an artificial magnet ? Can artificial magnets be produced by aid of a natural magnet ? If so, how? 3. What do you understand by a magnet ? What are its ends called, and why ? 4. How would you ascertain whether a given piece of steel was magnet- ised or not ? J. You are provided with a bar magnet, and a steel knitting-needle, one end of which has been marked by a iile or dipped into ink. State con- cisely (illustrating your remarks by freehand sketches) what you would do in order to magnetise the needle so as to make the marked end a Worth pole, and the other end a South, pole. How would you find out whether you had succeeded ? 6. Distinguish by sketches and concise explanation between the follow- ing methods of magnetisation: — (i) Single touch, (2) Double touch (i) Divided touch. ' ^'' 7. You are given a coil of covered copper wire, a battery for producing electric currents, and a bar of hard steel in the form of a horse-shoe How would you magnetise the bar ? Illustrate your remarks by freehand sketches. 8. You are provided with an electro-magnet, a battery, and a bar of hard steel. Explain and illustrate by a sketch how you would magnetise the bar. How would you find out which was the north pole of the bar when it was magnetised ? and how would you test whether it was stronelv mas- netised or not J -s j "s ^ 9 ) LECTURE II. Contents. — Permanent Magnets— Common Forms of Permanent Magnets and their Uses (Simple Bar, Compound Bar, Simple Horse-shoe, Com- pound Horse-shoe, Horizontal Needle, Vertical Needle) — Attraction and Repulsion — First Law — Polarity — Specimen Question and Answer — Questions. Permanent Magnets.* — In the last Lecture several well-known methods of magnetising steel were explained. All bar magnets, if made from the most suitable kind of steel properly tempered and magnetised, will permanently retain their magnetism, if care be taken not to subject them to rough usage or other demagnetising influences ; and hence such magnets have been termed Permcment Magnets. Every kind of steel, however, will not make a good permanent magnet. In fact, steel containing a certain percentage of manganese refuses to become magnetised, whilst other kinds, such as certain brands of spring and cast steel, as well as mild plate steel, although easily magnetised, readily part with their mag- netism. It, therefore, requires considerable knowledge of the different kinds of steel in the market, as well as practice in the different methods of tempering and magnetising, before an experi- menter or a student can with certainty make really good, strong, permanent magnets. Even the very best permanent magnets that can be obtained from the best makers will become demagnet- ised if subjected to a high temperature, and they will part with a large percentage of their magnetism if carelessly permitted to come into contact with similar poles of other strong magnets, or roughly handled, or subjected to severe vibration, or suddenly severed from their soft iron keepers. Common Forms of Permanent Magnets and their Uses. — Simple Bar Magnets. — As may have been gathered from the last Bab Magnet of, □, Section. * See Appendix to this section on Magnetism for a detailed description of how to select the best kind of steel, and how to treat it in order to make good Permanent Magnets, and Magnetic Needles. lO LKCTURE n. Lecture, a permanent bar magnet is a plain solid ste&l bar, either rectangular or cii-cular in section, which has been properly tem- pered and magnetised. Bab Magnet of, O. Section. For the purposes of class-illustration, as well as for simple laboratory instruction and experiments, it is found indispensable to have a pair of strong simple bar magnets, about lo or 12 inches long, always at hand. When not in use, these magnets should be carefully inserted into a wooden case with theii- opposite poles, N, 8, next each other, and with large wrought-iron keepers, K, placed across their ends as shown by the following figure. iJl&iefa /aid ^M€. " '■•■•■■I ^ c: Two Bar Masnets, Kbepebs, and Case. The precise action of the keepers will be explained afterwards, but at present the student is simply informed that they help to prevent the magnets from losing their magnetism. Experiments III. — Compound Bar Magnets. — If a strong bar magnet be made of one piece of steel, \ inch or more in thicltness, and then put into a bath, say, of nitric acid, whereby its surface is " eaten off," it will be found that the bar has lost the whole of its magnetism ; thus proving that it is merely the surfaee-slcin of the steel bar that has been strongly magnetised. The discovery that the strength of a magnet does not increase in simple propor- tion to its size, or, rather, that thin magnets are able to sustain a greater weight in proportion to their cross-sectional area than thick ones, naturally led to the introduction of wJiat are termed " com- pound " magnets. Compound magnets are compased of very thin flat nbbons of hard steel, magnetised separately and fixed parallel COMMON FORMS OF PET?MANFNT MAGNETS. II together (with tneir similar poles adjacent, and turned the same ^1 BC Ul^I li^ ^S' w Ps Compound Bar Magnet. way) by means of brass clamps, BC, as indicated by the above figure. A more expensive form of compound bar magnet (illus- trated by the following figure) is composed of nine thin bars arranged three on one side by three on the other. The ends of these bars are firmly embedded in and clamped to soft wrought- iron poles, IP, (by brass bolts and clamps, BC) so as to concen- CoMPOUND Bak Magnet with Wbought-ieon Poles. trate and direct the magnetism from the separate bars into each of these common pole-pieces, N, and, S. Should magnets of either of the above compound forms lose their magnetism from any cause, then they can easily be taken to pieces, and the elementary bars be re-magnetised separately, and then put together again. Simple Horse-shoe Magnets. — A simple permanent horse-shoe House-shoe Magnet or i Section with Keeper. HoBSE-SHOB Magnet op 1 Section without Keeper. 12 LECTUKE II. magnet consists of a plain solid steel bar, either rectangular or circular in section, that has been bent into the form of a horses shoe, tempered, and magnetised. Horse-shoe shaped magnets are useful for many practical pur- poses besides the mere illustration of the fact that a greater weight of iron oi steel can be lifted by a magnet of this form, than can be lifted by one of the bar type, although both may be of the same sectional area, weight, and magnetic strength. This leads us to state here that the absolute " strength " of a magnet is not measured scientifically by the weight of iron or steel that it will lift, but by the quantity of free magnetism at either pole. How this quantity of free magnetism is measured and reckoned, must be left unexplained until the student has mastered the elementary stage. It will, however, be quite easily understood that, given a certain strength or attractive force at each pole of a magnet, if both poles can freely act svniultaneously upon the attracted substance, a greater weight of the substance will he lifted than if only one pole were brought into action at one time. We may apply the illustration thus : — If a person can lift a certain weight with one of his hands, he will be able to lift about double this weight with both hands, if both hands can be simul- taneously and effectively brought to bear on the weight. The load which a magnet can lift, therefore, depends not only upon the strength of its poles, but also upon the shape of the magnet, i.e., upon the freedom with which one or both poles can bring their magnetic strength to bear upon the load. A very good steel horse-shoe magnet wUl lift about twenty times its own weight of soft wrought iron, being about four times the weight that can be lifted by a straight bar magnet of the same material and linear dimensions. Compound Horse-shoe Magnets. — For pre- cisely the same reasons as were given under the heading, " Compound Bar Magnets," we find that, when a very strong permanent horse-shoe magnet is required, recourse is had to the device of clamping several thin plates of magnetised steel together. It must be borne in mind, however (and the causes will be explained when ^' !■ "Tl- ^^'^ ^® come to discuss magnetic induction), that y) the load of iron which a compound magnet will ^ attract and lift is not so great as the sum of the loads lifted by each separate plate of which the HOESB^SHOB magnet is composed. In testing the truth of Magnet, N, S, *'^i® statement by experiment, it is well to AND Keeper, K. remember that the keeper, K, should fairly butt COMMON FOkMS of PEEMANENT MAGNETS. 13 close up against all the N and S ends of the plates forming the compound magnet, and not merely in contact with the central plate, as shown by the last figiu-e. ExpEBiMENT IV. — Another curious and interesting experiment may now be tried, viz., attach a pail or scale pan to the hook of the keeper, and very gradually add small additional weights by dropping an ordinary No. 6 gun shot every minute or so into the pail or pan. The ultimate weight which the magnet wUl support is much greater than the weight which it could bear at the commencement of the experiment; but whenever the keeper has been severed from the poles, their attractive force is again reduced to the original value, if not slightly below that amount. This peculiar behaviour has never received a satisfactory scientific explanation. Horse-shoe permanent steel magnets are very useful wherever strongly concentrated magnetism is desired without having re- course to electro-magnets. In Sir William Thomson's Siphon Recorder for receiving messages on submarine cables, in De Meriten's 'alternate current dynamos at The Foreland, May Island, and other lighthouses for generating electricity for arc lighting, and in many kinds of telegraphic relays and receivers and telephonic transmitters, we find this form of magnet adopted. Horizontal Magnetic Needle. — ^This useful piece of apparatus con- SLsts of a thin piece of magnetised steel usually in the shape of an elongated lozenge with a small hole bored through its centre. Into this hole is fixed an agate, glass, or brass centre (yy) ac- cording as the needle is desired to be delicate or rough. The combined needle and centre are poised upon a fine hard steel point fixed into a wooden or brass base, so that the former may move with a mini- mum of resistance in a hori- zontal plane. This form of magnet is much used in instruments for detecting and measuring electric currents. When attached to the mariner's card, it forms the well-known nautical compass, by means of which the steering of ships is regulated ; consequently, without this little simple magnetic needle, the sailor would have a very much more difficult and dangerous task. Vertical, Inclination or Dip Needle. — This form of magnetised HoBizoNTAL Magnetic Needle. 14 LECTURE II. needle has its centre, or axis, so adjusted and supported that the needle can only move freely in a vertical plane. The needle is generally of the same shape and constructed in the same manner End View Sec- tion THBOUGH Bearings. Dip Needle and Stand. as the horizontal needle just described. The centre, however, instead of being a single bearing, is either a fine axis fixed to and protruding on each side of the magnet, as illustrated by the above left-hand figure ; or, it is pointed and rests in, V, agate, glass, or brass centres as shown by the right-hand figure. The single needle telegraph instrument as used at many railway stations and signal boxes, is an instance of the prac- tical application of this form of needle, and as we shall see when we come to discuss the subject of the " Earth as a Magnet," there is an interesting scientific piece of apparatus called the " Dip Needle and Circle " which also embodies this kind of needle. Experiment V. — Attraction and Repulsion. — (i) Suspend a simple bar magnet in the way described in the last Lecture, and thus find out its IT and S poles ; mark them. (2) Take a magnetised needle of either the horizontal or vertical type, as described above. Find out in the same way their N and S poles, and mark them. (3) Present the N-pole of the bar magnet to the N-pole of the needle, and observe its deflection away frmn, that end of the bar magnet. ATTKAOTION AND REPULSION — FIRST LAW. IS (4) Present the S-pole of tlie bar magnet to the S-pole of the needle, and again observe the repulsion of the needle. (5) Present, however, the S-pole of the bar magnet to the A N-PoiiE Repels N-Polb. N-pole of the needle, and observe that the needle now deflects towards the bar or exhibits attractimi. S-PoLB Atteacts N-Poi'is. First Law. — Hence the law for attraction and repulsion between magnets is — Like Poles Repel, hut Unlike Poles Attract each other. Polarity. — ^The term Polarity has been adopted to express this duality or two-endedness, or distinctive manifestations of the two poles of a magnet. l6 LECTURE II. Specimen Question and Answer. Question. — Two sewing-needles are magnetised so that the eye of each is a IT-pole. The needles are stuck by their points into separate bits of cork, so that when each is thrown into water it floats upright with the eye downwards. How will the needles behave towards each other when the corks are brought dose togetlier in the water ? Answer. — As indicated by the following sketch : — Where- C„ Cj, stand for the corks. N,, Nj, „ „ needles. •< > „ „ repulsion. LECTURE II. — QUESTIONS. I ^ Lecture II. — Questions. I. What are permanent magnets 3 Of what are they made, and how ? 2". Sketch and describe the construction of a simple and of _a compound bar magnet. How would you prove by an experiment that it is only the surface of a bar of steel that is strongly magnetised ? Deduce from your experiment the advantage of making strong magnets of thin ribbons or fine wires of steel. 3. Sketch and describe the construction of a simple and of a compound horse-shoe magnet. Name three practical applications of horse-shoe magnets. . ji j 4. Sketch and describe a horizontal and a vertical magnetic needle, and name two different practical applications of each. 5. If required to demonstrate the law that " like " magnetic poles repel, and that " unlike " poles attract each other, how would you do it ? Give 6. A steel sewing-needle is drawn over the north pole of a magnet from eye to point. What is the subsequent condition of the needle ! The point is now presented to the north pole of a magnetic needle; state what occurs. Illustrate your answer by a sketch. 7. Two long magnets, made of small steel wire, are suspended from their upper ends by threads, so that their lower ends are on a level with each other Suppose the lower ends to be both north poles, how will the mag- nets act upon each other » Again reverse one of them, and sketch how they will now act upon each other. 8. What do you understand by the polarity of a magnet ? ( i8 ) LECTURE III. Contents. — Magnetic Curves or Lines of Force — External and Internal Magnetic Fields — Second Law — Graphic Representation of Magnetic Fields — Bifferent Cases of Magnetic Curves — Magnetic Axis and Magnetic Equator of a Bar Magnet — Specimen Question and Answer — Questions. In Lecture I. we described how magnets are made, in Lecture II. we explained )land illustrated common forms of magnets. We shall now give details of how the directions of the force which emanates from and surrounds magnets may be found by simple experiments. Experiments VI. — Magnetic Curves or Lines of Magnetic Force. — First, take a lorig bar magnet and a short vertical or dipping needle of the form first illustrated and explained in the last Lecture. Lay the bar magnet flat upon a level table, and place the base of the needle-stand on the middle of the bar magnet. Observe that the needle lies horizontal, with its N-pole pointing in the direction of the S-pole of the bar magnet.* Now move the needle along the bar to the different positions indicated by the following figure, and observe how the needle dips more and N > Testing foe the Dieections op the Magnetic Foece Above a Bab Magnet. more as it approaches the ends, becoming truly vertical when close to either end. Carry it beyond the ends, and it still inclines towards a point near to but slightly within the end of the bar to which it is nearest. This clearly manifests that there are invisible forces acting between the bar and the needle, and further that the * The student should note that when it Is required to illustrate a small magnetic needle, an arrow-head is invariably used to denote the N-pole of the needle. MAGNETIC CURVES OE LINES OF MAGNETIC FORCE. 19 lie of the needle indicates the mean or resultant direction of these forces. Second. — ^You may, however, say, Is this the case for the under, as well as the upper, side of the bar magnet ? Yes, it is also true for the under side. In order to prove this, suspend or elevate the bar a foot or two above the table, still maintaining its horizontal position. Now take the needle (or for the sake of variety take the second form of vertical needle illustrated in last Lecture, or tie a fine thread to the centre of a short magnetised sewing-needle) and bring it over as well as under the bar to the several positions indicated by the following figure. You find that the needle dips to the poles from below as well as from above the bar when brought towards the ends, and lies horizontal at the centre. Finding Dihections of Magnetic Foece Above and Below A Bae Magnet. Third. — You may now say that, this proves only a directive force acting on the needle when it is placed above and below the flat sides of the bar magnet. Does this force exist on all sides as well as at the ends 1 Yes, it exists in every plane. For, turn the bar magnet on its edge, and you will get similar results to those last observed. Or, perhaps, a still more conclusive proof will be to lay the bar magnet down flat on a sheet of white paper upon a table, taking the end of the thread attached to the short needle in your left hand and holding it up until the needle almost touches the paper. Now, move the thread until the needle is brought into the various positions indicated by the following plan, and mark by means of a pencil (held in the right hand) small crosses ( x x x ) at the places where the N and S poles of the needle come to rest. Remove the needle and join by short lines the various crosses +r -1-. tions of the magnetic forces ^ ^*+ along these two paths. You / \ may go still further, for you / jtj may hold the thread in several ? JS( difierent sets of positions, and — -^ \ thus plot out the directions of I the magnetic lines or " curves y of force " for any number of v" J* positions all round about the >g -< magnet in this plane, and then *»,,^ y' by turning the magnet first 20 LECTURE III. ( X X X ), when you will have a couple of curves, one on each side of tho bar magnet, like that shown below, which graphically represents the resultant direo- partially, and finally fully „ „ over, on its side, do the same Plan. — Findino Dihbctions op » i, ,■, j-J. , , MAGNETIC FOECB ON THE SIDES ^°'' ^^ ^^^<^ different pkuBS, AND Ends of a Bae Magnet. thus proving conclusively that the force of magnetism emanates in all directions from a magnet. External and Internal Magnetic Fields. — The magnetic force which we have just proved to exist on all sides of a bar magnet is assumed by physicists and electricians to start from the N-pole, and to pass through the surrounding medium, entering the magnet by the S-pole, and completing its path through the magnet itself to the N-pole, thus forming a complete circuit. Second Law. — Jfo magnetic line or curve of magnetic force can exist without completing its own circuit, and magnetic lines or curvet never cut, cross, or m,srge into one another. Consequently, a magnet- is never found, and cannot be made, with a single pole. One portion of the magnetic circuit lies outside the magnet, and the remainder is inside the magnet. The space outside the magnet throughout which its magnetism acts (as graphically represented by " the lines or curves of force " in the foregoing and following experiments) is termed the " External Field," or "Apparent Field," or shortly the "Field." The space occupied by the magnet may be termed the " Internal Field," or " Non- apparent Field." The cross-section of the magnet or internal field being, as a rule, less, and its natural aptitude for accommodating or conveying magnetic lines being much greater than the external field or air, there is, of necessity, great crowding or concentration of the lines of force where they leave the N-pole and where they enter the S-pole. The magnetism is therefore, found to be more intense at the poles than in other parts of the external field, This is very well illustrated by the two following figures, which GEAPHIO EEPRESENTATION OF MAGNETIC FIELDS 21 reprCvSent a magnet as if it were an arrow-sheath, with the arrows entering by the S-pole and leaving by the W-pole. N '"-M^. Magnetic Lines op Foece Leave and Entee a Magnet ON ALL Sides. Experiment VII. — Graphic Representation of Magnetic Fields. — In order to repre- sent graphically or map out magnetic fields for any par- ticular form of magnet, or combination of magnets, take a foolscap sheet of strong impressed (ungiazed), white paper, and draw it through a bath of melted white paraJ^ne wax. Gently shake the paper over the bath so as to let the super- fluous wax drop off, and hang the paper up by one corner to dry, whilst en- gaged treating several other pieces of foolscap in the manner just described. Now lay the magnet or magnets down upon a table in the precise way required, with pieces of wood of the same thickness as the mag- net on each side of it, and upon these place the waxed paper. From a pepper-pot or muslin bag containing iron filings, held in the right hand about a foot or so above the paper, sprinkle the filings upon the paper, at the same time gently tapping the table. You instantly observe that the filings so arrange them- >XB. Covering Papbe with Paeappine Wax. P represents Paper. TB „ Tin bath. MPW „ Melted paxaffine wax. BB „ Bimsen burner. 22 LEOTUBE III. selves as to form curves between the poles of the magnet or magnets, similar to the curves which were discovered by aid of Spbinkling Ikon Filings upon Peepaebd Paper. Experiments VI. The fact is, that each iron filing when it comes within the influence of the external magnetic field of the magnet or magnets, becomes a small magnet by induction,* and of Photogeaphbd pkom a Spbcimen Cakd of Magnetic Field made IN ACCOKDANCE WITH THE PBEVIOUS DlKECTIONB BY ONE OF Pbopessoe Jamieson's Students. * TMs inductive action of magnets upon iron wiU be explained in a lutuie ^^ecture. ^ DIFFERENT CASES OF MAGNETIC CUEVES. 23 necessity takes up a definite position under this influence or force in the same way that the magnetic needle did. When a sufficient quantity of filings has been showered upon the paper, so that the delineation of tiie whole magnetic field in that plane has been rendered apparent, pass a hot copper soldering-bolt, or the flame from a Bunsen burner, over (close to, but not touching) them. This melts the wax, and fixes the filings in their position, and they now present a permanent and beautiful picture of the resultant direction at every point and the relative intensity of the magnetic lines of force in the plane of the paper. There are several other methods of obtaining the same result. A sheet of glass thinly gummed and dried may be used instead of the paper, and when the filings have been sifted over its sur- face (with due attention to tapping), a jet of steam is caused to play gently over the surface of the glass, which softens the gum and thus fixes the filings in their places. Or, the weU-known blue ferro-prossiate paper used by draughtsmen for obtaining photo prints of drawings, may be Tised instead of the waxed paper. When the filings have been peppered over its surface, the whole is exposed to sun-light or to an electric arc-light for a short time. The filings are then shaken off and the paper washed with a fixing solution, when the curves will appear white on a deep blue ground. Different Cases of Magnetic Curves. — The following figures are intended to illustrate a few of the simpler and more common cases of magnetic fields. AH of these cases can be easily demon- strated and made apparent to a large class by means of the first process described above, or recourse may be had to a magic lantern, whereby the gradual delineation of the field may be most interestingly and instructively exhibited to an audience. Each of the following figures should be carefully copied by the student, or he should be encouraged to make them experimentally, and to see clearly for himself how the First and Second Laws of Magnetism, hold good in every instance. When he has mastered the Laws of Magnetic Induction and of magnetic fields due to electric currents, his teacher may with advantage request him to predict by dia- grams the direction and relative intensities of magnetic fields which would result from different arrangements of permanent magnets, and of electro-magnets with pieces of steel or of iron placed in various positions relatively to the magnetic field. Magnetic Axis and Magnetic Equator of a Bar Magnet. — The straight line joining the N and S poles of a bar magnet is termed the Magnetic Axis of the magnet, and the line at right angles to this axis at the centre of the magnet where no free or external magnetism is apparent, and consequently where no iron filings are attracted, is termed the Magnetic Equator of the 24 LECTURE HL _3__ . 1^ Jlggnetie^AA- LSfe^cr: ' / Simple Bab MAeivET. N -<- ---..< s ■''■"'■"-■ V I Uiin- ->- N -■ rv^ — ^ // \Vx J I \ ^ V Two Bae Magkets with Uslike Poles Adjacbkt. '.''■■ - - -' — v-i s aa» ~'''''^T^ — I — X : ^ ^^ — = ->N "■---":->, '',''-'--:--"- --~~"---c~"-^ '~"^v "v^ - — ~: Two BAE Magnets with Like Poles Adjacent DIFJFEKENT CASES OF MAGNETIC CURVES. 35 -?- N J > M I , , I I, S Wf / Bab Magnets with Unlike PoiiES Opposite bach othee. -> N -' / 1 \ \ \ \ \ \ / / / / / / / \ I '. '. \ \ I • I I Bae Magnets with Like Poles Opposite each other. Note.—\Ti the above figures the remainder of the fields (between the N and S poles of each magnet j has been left out, due to want of space. • '• i ■'' ' y'' • ' .' ,"'- i i //,'''--- -- ^ V ^ * J . '■ 1 ! .'''■' ^ \ ?f ', I ^ / -^ I -^^ ^. ^ \: \ ' i I ' ^ ——— — — — — -4HECDI -* V \ ^ \ ^ I ' ' ' (( — ,^\\m ^\ ' ""'--^^--'-" < \^^ ' '1 \ '■ \ 1 i il'y'y 1 ! --'' --'/ /tv\~x~~~-~, \ -^ ^^l-y / , ,,-' / / I \ \/N, .,.,,, t ■ r. ,-' ,■ ' ,' / I \ \ "- //.'.'\\\W "..--' / .'' ! \ \ H0BSE.SHOE Magnet. End of Cibcular Bae Magnet. magnet. The positions of these two imaginaxy lines are shown by the first figure on p. 24. 26 LECTURE m Spbcimkn Question and Answer. Question. — A horizontal or compass-needle stands upon a table. You bring the North pole of a bar magnet towards it from the north. What happens, and why ? Answer. — The needle turns round until it takes up a position with its S-pole facing the If -pole of the bar magnet, for the fol- lowing reasons : — I. The compass-needle naturally lies with its W-pole pointing northwards; and, consequently, its lines of force issue in the opposite direction, and oppose the magnetic lines of force from the approaching bar magnet, thus — Magnetic Lines teom Needle and prom Bar Magnet Complete their Circuits Indepekdbntlt. 2. The bar magnet, being held rigidly by the hand, camiot turn, but the needle, being freely suspended, is turned round in obedience to the forces or couple acting upon it, until a position is reached which naturally permits of the maximum lines of force from the bar magnet completing their circuits through it, as shown by the following figure ; — Some op the Magnetic Lines prom Bar Magnet Complete their Circuit through the Needle. LECTUKE III. QUESTIONS. 2^ Lectueb III. — Questions. 1. Explain what you understand by a magnetic field. Distinguish between the external and the internal field in the case of a bar magnet. 2. Given a bar magnet and a freely suspended magnetic needle, how would you determine the paths of the lines of force whaoh lie outside the former ? 3. How would you prove experimentally that a magnetic field sur- rounds a bar magnet on all sides ? Illustrate your answer by sketches. 4. Account briefly for the different behaviour of the lines Of force in the two first figures on page 25. 5. Describe and illustrate a complete process whereby you can map out by aid of iron filings, and obtain a permanent record of the field of force surrounding a bar magnet in one plane. 6. A long strip of hard steel is magnetised ; when your small magnetic needle is passed along the strip, its north pole is attracted by one end of the strip, its south pole by the other, the centre of the strip appearing to attract neither point of the needle. Explain this by aid of sketches. 7. What is meant by the magnetic axis and the magnetic equator of a magnet ? 8. Sketch neatly how the Unes of magnetic force arrange themselves in the following cases : — (i) A bar magnet ; (2) two parallel bar magnets with urJJSce. poles adjacent ; (3) two parallel bar magnets with lihe poles adjacent. 9. Sketch neatly how lines of magnetic force arrange themselves in the foUowing cases : — (i) A horse-shoe magnet ; (2) two bar miagnets in line with unlike poles adjacent; (3) two bar magnets with like poles adjacent ; (4) one end of a bar magnet of circular section. 10. Suppose the south pole of a bar magnet. A, is brought near the north pole of a bar magnet, B, what chaise occurs in the directions of the lines of force about. A, and about, B, and why ? Illustrate your answer by sketches showing (i) the natural condition of the fields when A and B are far removed from each other ; (2) when they are brought near to each other in a straight line ; (3) when A is brought near to B, but at right angles to it. 11. Three precisely similar magnets are placed vertically with their lower ends on a horizontal table. Iron filings are scattered over a plate of glass which rests on their upper ends, two of which are north poles, and the third a south pole. Give a diagram showing the forms of the lines of force mapped out by the filings. (S. and A. Exam., i88g.) ( 28 ) LECTUKE IV. Contents. — Molecular Theory of Magnetisation — Magnetic Saturation— Retentivity and Resistance — Effect of Vibration on Magnetisation- Effect of Temperature on Magnetisation — Questions. ExPKRiMENTS VIII. — MolGcular Theory of Magnetisation.— In order to assist in comprehending what takes place when a piece of steel or iron is magnetised, take a glass tube (say lo or 12 inches long and ^ inch internal diameter), and fill it lightly with steel or hard iron filings. Plug up the ends with corks and shake the filings. Glass Tube with Unmagnbtisbd Ibon Filings. The filings present the appearance indicated by the above figure — in other words, they are not arranged in any definite order, but are mixed in a haphazard fashion. First, present this tube of fihngs to a compass-needle ; it will be found that each end attracts, and is attracted equally by, each end of the freely suspended needle. Or if you suspend the tube by a thread and stirrup (in the same way that we suspended the steel bar in Lecture I.), you will find that it comes to rest indifferently in any position. This proves that no free magnetism proceeds from the filings, or in popular language you would say that the tube of filings was not magnetised. Second, place the tube inside a coU of insulated copper wire through which a strong electric current is passing, or strongly magnetise the filings as a whole by any of the methods described in Lecture I., and again present it to the compass-needle. You now find that one end strongly repels the N-pole of the needle, whilst it as strongly attracts the S-pole, and that the other end of MOLECULAE THEORY OF MAGNETISM. 29 the tube repels the S-pole and attracts the N-pole, thus exhibiting all the eifects of a permanent bar magnet.* Glass Tube with Magnetised Ieon Filings. Or, if you now hang up the tube by the thread and stirrup, you will find that it always comes to rest with one end pointing to the north and the other to the south, just in the same way that a freely suspended permanent bar magnet does. Third, mark one of the ends of the tube, say the north- pointing end, with a piece of chalk or with ink. Now shake the tube vigorously, so as to thoroughly intermix the iron fi.lings, and then bring the marked end towards each end of the needle in turn, when it will be found to attract them equally (as will the un- marked end) just as in the first test. Or, suspend the tube as before, and it will come to rest indifferently in any position, thus proving that all free magnetism, or recognisable polarity, has dis- appeared from the filings. The whole is, therefore, no longer a magnet in the ordinary acceptation of the term, for the filings as a whole have become demagnetised. What has happened may be thus explained. In the first and third cases, the iron filings were so arranged or rather disarranged that the lines of force proceeding from each found a short and easy path through its nearest neighbours. Their magnetic circuit was thereby completed wiihin themselves, just as if they had been arranged like any of the three or four sets of small magnets in the following figures : — Shoet Ciecuiting op M/gnbtism in Small Magnets, Exhibiting Nbdteal Condition op the Molecules in a Magnetic Body. * You must ahoays observe a repulsion of one or other of the poles of the needle before you can be certain by this test whether a body is magnet- ised or not, i.e., whether it possesses free magnetism or magnetic polarity. Never depend solely upon attraction. 30 LECTURE IV. The natural conditions of a magnetic circuit were therefore satisfied by a short circuiting of the lines of force between each of the neighbouring sets of small magnets or filings, and thus left no tendency to project their lines beyond themselves. In other words, the whole field was entirely internal, and without free magnetism. In the second case, the filings having become turned round or symmetrically arranged (each one lying fair in line or parallel with its neighbours), the path of the magnetic lines springing from the set of north poles at the centre of the tube, found their easiest path to be through their neighbours in front of them, and so on towards the N-end of the tube. A rapid arithmetical accumuk- tion of the lines of force thus took place, because each little filing or magnet not only projected its own set of lines, but con- ducted the lines of force from those immediately behind it. The naturally self-repellant action which like poles or lines in the same direction have for each other, forced some of the lines to take an external path before they reached the end of the tube. Those lines which did reach the end had no other route left for them but the air path, whereby to complete their circuit to the opposite end or free south poles. This condition of the second case is illustrated by the following enlarged sketch showing two sets of polarised filings or small magnets with external dotted lines to represent the external field. By referring again to the illustrations of mag- netic curves at the end of the last Lecture, it will be seen how faithfully the above reasoning is borne out. 1 , s n ~~~> "■"■* ^^^*, 1 M ,'t "C. ''--,. s n __J" Enlarged Figueb op the Condition of Magnetised Ikon Filings IN THE Glass Tube, ob of the Molecules of a Magnet. Another very interesting experiment to prove this same action, or partial rotation of the molecules of iron on their axes, BETENTIVITY AND RESISTANCE 3 1 when a magnetising influence is brought to bear upon the bar, is that of mixing fine particles of the magnetic oxide of iron with water, and pouring the mixture into a glass tube fitted with clear glass ends. When the tube is shaken, you cannot see a light placed at its further end, but set the tube within a coil of in- sulated wire, and pass a current of electricity through the wire so as to magnetise the iron particles, and the light at once becomes apparent, from the fact that the particles arrange themselves longitudinally, as shown by the last figure, and thus offer less obstruction to the rays of light. Magnetic Saturation. — In the case of a solid bar of steel, the molecules of which the bar is composed represent the mixed or higgledy-piggledy condition of the particles of hard iron or steel in the glass tube, only they are in much closer and firmer contact with one another. During the first stroke which the bar receives with a permanent magnet, some of the molecules are turned round a little and remain there ; during the second stroke, a few more are turned round, and those that were already turned a little round are still further turned on their axes through a greater angle ; and so on, until, when all the molecules have been fairly turned round, as portrayed by the last illustration, the bar is said to be saturated or completely magnetised, and, therefore, it will receive no more magnetism. Betentivity and Besistance. — The rigidity with which the molecules remain in this stressed condition represents the reten- timiy or permanency of the material for magnetism. Generally speaking, the greater the retentivity of a magnetisable body, the greater the resistance to become magnetised. Thus, the harder steel is, the more difficult is it to magnetise, but its capability of retaining magnetism is correspondingly increased. This may be explained by the fact that the molecules of hard steel are very closely packed together, and consequently they are more difficult to turn from their natural position ; but when once turned, their great intermolecular friction prevents their returning to the normal or neutral condition. ExPKRiMENTS IX. — Magnetise in turn first a bar of hardened steel, and then a bar of soft wrought iron of the same size and shape as the steel bar, by means of a coil of wire and the same current strength of electricity as explained in Lecture I. With- out removing either bar when under the magnetising influence of the electric current, observe what weight of iron each wUl attract and lift. It will be found that the wrought-iron bar will lift a much greater weight than the steel bar, thus proving that iron is more easily and strongly magnetised than steel. Steel, therefore, ofiers a greater resistance to magnetisation than iron, and eonse- 32 LBOTUKE IV. quently its natural magnetic resistcmce * is said to be greater than that of iron. a • i- Effect of Vibration on Magnetisation.— Agam, magnetise each of the bars in turn, and remove them from the coil, letting them fall several times roughly upon the floor; or hit them with a hammer so as to cause them to vibrate. _ Upon now testing them for magnetism with iron filings, or by aid of the compass- needle, it will be found that the soft wrought- iron bar has parted with its free magnetism, whereas the hard steel is still strongly magnetised. The molecules of the iron bar being more loosely put together are much more easily turned than those of the steel bar, and also regain their normal condition more readily. They represent in a marked degree the condition of matters previously illustrated by the tube of iron filings. In fact, the slightest^ shock or vibration is sufficient to demagnetise soft wrought iron. This property of being easily magnetised and demagnetised is largely taken advantage of in the practical applications of elec- tricity and magnetism. For example, in the well-known Morse telegraph instruments, as well as in telegraph sounders, the cores of the electro-magnets are made of short pieces of soft iron, so that weak electric currents may quickly magnetise them, and that, when the current has ceased to flow, they may quickly lose their magnetism. The shorter the bar is, the quicker it will magnetise and demagnetise. Again, the armature-cores of continuous-current dynamo-machines are made up of thin plates of soft wrought iron, in order that they may readily become magnetised, demag- netised, and remagnetised, many hundred times in a minute. Eflfeet of Temperature on Magnetisation. — ^We have just seen that if the molecules of magnetised iron or steel are free to move and are vibrated, they become demagnetised. Now, when a body is heated, vibration is set up amongst its molecules, and the higher the temperature to which it is raised, the more rapid is the vibration ; at the same time, the body expands, and thus permits of greater freedom of movement amongst its molecules. Experiments X. — Heat a steel magnet to ioo° 0., and you * The single term " coercive force " was employed mitll lately by most writers to signify both the property of resisting magnetisation and of retaining magnetism ; but the expressions magnetic resistance and reten- tivity, which have now become more general, are certainly more appropn- ate. The term magnetic resistance is very wide in its signification, and may be applied to all substances through which magnetic lines of force are caused to flow. Tables of the relative magnetic resistances' of several different bodies (or their reciprocals, which are termed their permeabili- ties) have been derived from elaborate and accurate experiments, and are to be found in modern advanced treatises on magnetism. See Index to Munro and Jamieson's Electrical Poolcet Booh, published by Ohas. Griffin & Co.,, for " Permeability." EFFECT OF TEMPEBATUEE ON MAGNETISM. 33 will find that it loses some of its magnetism. Raise its tempera- ture up to 700° C. or a bright-red heat, and it loses the whole of its magnetism. If, on the contrary, you decrease its temperature to freezing point or 0° C, the magnetism is increased in strength ; Magnet and Red-hot Ieon Ball. but Prof. S. P. Thompson states that if the temperature is decreased to 100° C. below zero, the magnetism disappears. Heat a soft iron ball red-hot, suspend it by a chain or wire, and bring it near a strong magnet, you will find that the ball is not attracted by the magnet ; when it cools down however, it becomes strongly attracted thereby. 34 LECTURE IV. — QUESTIONS. Lbctubk IV.— Questions. A glass tube, with its ends marked A and B, and nearly full of steel filings is stroked several times from A to B with the north-seeking pole of a strong magnet. The tube is then brought with its end B near to the south-seeking pole of a compass-needle. What is the effect upon the needle ? The tube is now shaken so as to mix the filings, and put near the needle as before. What is the effect upon the needle ? Why is the effect on the needle different in the two cases 7 2. A glass tube, fitted with thin, clear, flat glass ends, is fiUed with watei and a quantity of particles of magnetic iron oxide. The tube is shaken, and a lighted candle placed at one end cannot be seen from the other end of the tube ; but when the tube is placed in a, strong magnetic field pai-allel with the lines of force, the light now becomes clearly visible. Explain this. 3. How would you illustrate and explain the natural magnetic condition and relative position of the molecules of a bar of nnmagnetised steel or iron? 4. How do you figure to yourself the change that occurs in a bar of un- magnetised steel when the pole of a magnet is rubbed along it ? What do you understand by the pole of a magnet ? Give sketches. 5. Explain and illustrate what is meant by saying that a bar of iron or steel has been saturated with magnetism. 6. A hard steel bar is said to have greater retentivity for magnetism than a similar bar of soft iron. How do you explain this 1 7. A bar of hard steel is said to offer greater magnetic resistance than a bar of iron of the same size and shape. How do you explain this ? 8. You have given to you two rods, one of soft iron, the other of hard steel; also a compass-needle and a bar magnet. Describe experiments with the things provided whereby you could find out which was the iron and which the steel rod. (8. and A. Exam,, 1886.) 9 A bar of hard steel and a bar of soft wrought iron are both magnet- ised to saturation. Each bar is then hit hard several times with a hammer. What is the result in each case, and why ? 10. If you make an iron ball red-hot, will it be attracted by a strong magnet ? If you make a strong magnet red-hot, what is the effect 1 Give concise and clear reasons for your answers. ( 35 ) LECTURE Y. Contests,— Distribution of Free Magnetism along a Bar Magnet — Another Proof of the Molecular Theory of Magnetisation. Breakin" a Magnet — Magnetic Screen?. Magnetic and Non-Magnetic Substances — ^Pole-Pieces — ^Armatures and Keepers — Specimen Question and Answer — Questions. Distribution of Free Magnetism along a Bar Magnet. — By an inspection of the magnetic curves illustrated in Lecture III., and from what was explained in Lecture lY. regarding the paths of the lines of force through and from the molecules con- stituting a magnetised bar of steel, the student no doubt gathered that the relative strength or distribution of free magnetism along a bar magnet increases from nothing or zero at the centre, or neutral line, or equator of the magnet, to a maximum at the poles ; and then diminishes slightly to the end of the magnet. We shall, however, endeavour to still further impress these facts upon him by two simple experiments. Bough Test for the Relative Disteibutios op Feef ilAGSETISM AI/OXG A BAE MaGSET. 36 LECTURE V. Experiment XI.— Take a long strong bar magnet and a number of small soft iron nails* Firmly suspend or support the bar magnet, and commence by applying the nails to the under- side of the magnet in chains, as shown by the figure on page 35, You will find that no nails are attracted at the neutral line or equator, that a gradually increasing number will attach themselves and hang on to each other as you get towards the poles, and that a less number will adhere to the very end of the bar than at a short distance inward from the end, thus proving that the quantity -Folo 20\- sV- FoU- h N.B.— All the arro\*-henfl8 on this side of She fi^uref Bhould be turned the opposite way. E-»-.-*fO -^•S5 .->B4 ■^0 Test foe Disteibution op Feeb Magnetism B¥ Oscillations ov Needle. of free magnetism gradually increases from zero at the equator to the poles, and then diminishes slightly. On. the upper side of the bar magnet have been drawn dotted vertical lines and outline dotted curves through their extremities, to graphically illustrate the relative quantities of free magnetism which also exist on that side; for, as we proved in Lectui-e III., lines of force proceed from a bar magnet in all directions. Experiment XII. — Take a Itmg bar magnet and fix it up ver- tically, with its S-pole pointing downwards, as shown by the * French nails are best for this experiment. DISTRIBUTION OP FKEE MAGNETISM. 37 figure on page 36. Place a sniaM freely poised (or suspended and weighted) magnet capable of moving horizontally at each of the various positions close to the magnet, but always at the same fixed distance from it, as indicated on right hand of figure. Deflect the needle whilst held at each position, and count the number of complete oscillations (to and fro swings) which it makes in a given time; or, in other words, find the rate of oscillation for each position along the bar magnet. Now move the needle away from the magnet until it is quite beyond the influence of its magnet's field, and count the number of oscilla- tions which it makes in the same time as before under the in- fluence of the Earth's magnetism alone. This number will be the same as the needle made when opposite to the equator of the bar magnet, thus giving you another proof that no free magnetism is found close to and at the magnet's neutral line. Finally, squwre the nitmber of oscillations per minute as found at eacA posi- tion, subtracting from each square the square of the number under tlie EartKs ma,gnetism alone, amd you get a series of values re- presenting the quantities of free magnetism along the bair. If you take these final results and plot them to scale along a figure of the magnet, for each position, by lines drawn at right angles to the length of the bar magnet, then, by joining the extremities of these lines, you obtain two curves, the ordinates of which graphically re- present the relative distribution of free magnetism along one side of the bar magnet, or the relative number of hnes of force which leave the magnet at diflferent points. For example, suppose that the magnetic needle gives 4 complete oscillations (or to and fro swings) in one minute when under the influence of the Earth's magnetism only; but that when opposite the pole of the bar magnet the needle gives 10 oscillations. Then, (10' - 4") = 100 - 16 = 84. Therefore 84 parts represent the length of the lines drawn at right angles to the magnet opposite the poles. And so on for all the other positions. Thus — Oscillations. OBciUations squared. Oscillations squared minus Earth's OscillatioDB squared. Relative Quantities of Free Magnetism. At centre 4 :: \ 7 „ 8 9 At pole 10 9-8 16 % 49 64 81 100 96 16- 16 25-16 36-16 49-16 64-16 81-16 100-16 96-16 9 20 1 80 38 LECTURE V. Another Proof of the Molecular Theory of Magnetisa- tion. — Some of you may have been thoroughly satisfied from what was said in the last Lecture, explanatory of the similar magnetic conditions of the hard iron filings in the glass tube and a steel bar magnet, that the molecules of the latter are each complete little perfect magnets; but we shall hei-e introduce one other simple experiment as an additional proof. Bxi'ERiMENT XIII. — Take a thin bar of magnetised steel, or a magnetised knitting-needle. Test its polarity by bringing it towards a compass-needle and mark its N-pole. Now break the magnet at the neutral line, or equator, into two parts, and test each half by the needle. You find that each part is a perfect magnet with N and S poles nearly as strong as the original whole magnet. The pole which was originally a N-pole is still Nm-th, and the pole which was originally a S-pole is still South, but we have a new S-pole and a new N-pole at the place of fracture. If you break each of the halves, and test them as before, you pro- duce four complete magnets, as shown by the following figure ;-— Breaking a Magnet as a Test op the Molbculae Polae Thboky op Magnetisation. You may proceed in this way, breaking each piece into two, until you make them so very small that you cannot break them any further. Still, however small you make them you will find that each little bit is a complete magnet. You therefore conclude, not only that a steel or iron mctgmt is composed of a congregation of timj {molecular) magnets, each o/tchich is a complete magnet; but also that it %s vmpossihle to prodiice a magnet wUh only one pole. n a n s V an j> n H " ... » n s Ji a n a n s t^ — i n an a n a n a n a .1 Magnipied Moleoulae Polae Condition of a Magnet. The above figure helps to convey the meaning of the foregoing remarks, and serves to indicate that wherever the magnet happens to be broken the fracture shows a W set of poles on one side and a S set on the other. Experiment XIV. — Magnetic Screens ; Magnetic and Non-Magnetic Substances.— Lay an ordinary compass-needle MAGNETIC ANt) .NON-MAGNETIC SUBSTANCES. 39 down upon a table. It points in a north and south direction, under the influence of the Earth's magnetism, if no pieces of soft iron or other magnet be near to disturb it. Bring a long bar magnet up to- wards, and on a level with, the centre of the needle from the west or the east, until the needle deflects 45° j'-,-'''' to ijo" from its former natu- W #''' ^ ,, To MaSnjticJS.J'ai^ Compass-Needle deflected by Bar Magnet, although a Non-Magnetic Body is placed between them. ral or zero position. Now interpose between the needle and the magnet, a board of wood. The needle still remains deflected to the same extent as before. Substitute in turn for the board of wood, a piece of cardboard, a sheet of brass or copper or ebonite, or a pane of glass, or a vacuum jar, and still the needle maintains its deflected posi- tion. But replace these by a thin sheet of soft iron and the deflection is sensibly reduced. Insert a very thick _-^ large plate of soft wrought iron and the needle now swings back to almost its zero po- sition, but still shghtly inclined towards the iron plate. The thick soft iron plate acts as a magnetic screen, shielding the needle from the force of the magnetic lines ema- nating from the bar magnet. In the first case Compass-Needle Shielded fkom Magnetism you observe that of Bae Magnet by a Thick Plate op Soft the interposition of ^^' \k_ ( r V>%^~- : "^ Weought Ikon placed between them. 40 LEOTUEE V. boards, plates, or slabs of wood, cardboard, brass, copper, ebonite, or glass produces very little more or less retarding or shielding eflFect to the magnetic lines of force which pass between the magnet and the needle, and vice versd, than if the intervening medium had been air or a vacuum. These and many other substances are therefore termed non-magnetic bodies, in contra-distinction to iron, steel, nickel, cobalt, chromium, cerium, and manganese, which are magnetic or magnetisable bodies. What occurs is precisely similar to the action and reaction which always take place between a magnet and a compass-needle, and which was so far ex- plained in answering the specimen question at the end of Lecture III. Before the magnet is brought near the needle, the lines of magnetism passing from the needle (to the Earth's poles and from the Earth's poles to the needle's) pull it round untU they he in a line with and flow in the same direction as those of the Earth's magnetic lines. Again, suppose for a moment that there was no Earth's magnetism present when the bar magnet is brought near the needle, the latter would swing round through 90° until its axial lines of force and those from the magnet were fairly in a Une with each other and flowing in the same direction. Consequently, when the "magnetic force of the Earth and that of the magnet simultaneously act upon the magnetism of the needle, the latter must take up an intermediate position so as to permit of its lines being shared between the Earth's poles and the magnet's poles in proportion to the respective strengths of their magnetic fields at the position of the needle. In other words, the needle takes up a resultant position under the action of two forces — ^viz., that due to the Earth and that due to the bar magnet. The stronger the bar magnet and the nearer it is to the needle the greater will be the deflection of the latter from zero, and proportionately more of the needle's and of the magnet's lines of force will pass through each other. In the second case the whole of the lines of force (which in the first case passed from the magnet to and through the needle) find a path of less resistance through the nearer side of the thick soft iron plate, and therefore they do not reach or produce any effect upon the needle. Their force is occupied in magnetising a portion of the plate. In effecting this they naturally turn with the polarised iron molecules, and thus obtain a shorter circuit back to the further pole of the bar magnet as indicated by the last figure. _ The needle also being a magnet and being close to the other side of the thick iron plate produces a similar effect; but, being free to move, it deflects slightly from zero until its lines of force are proportionately accommodated between the Baiih's mag- netism and the induced magnetism in the plate. This deflection is, POLE-PIECES, AEMATDKES, AND KEEPERS. 4 1 of course, small compared with the deflection in the first case, because the induced lines of force in the plate are few compared with the number of lines which reached the needle from the bar magnet. You can now understand that if a magnet is entirely surrounded by a thick hollow sphere of soft iron it will be entirely screened from the action of external magnets. Sir William Thomson takes advantage of this fact in his Marino Ironclad Galvanometer in order to protect the needle from the influence of the steel ship's and the Eai'th's magnetism by placing it and the coil of wire inside a cylinder of thick soft wrought iron. Two important facts brought out by these experiments should be carefully noted and applied to reasoning out similar cases. 1. Lilies of farce always choose ilie imili of least magnetic resist- aiice. 2. If a magnetic body, free to move in any direction, he placed in a magnetic field, it always moves so as to accommodate through itself the greatest possible number of the lines of force of the field, in the same direction as its own internal lines. Pole-Pieces, Armatures, and Keepers. — From the previous experiments and remarks the student will have gathered that some of the magnetic lines of force which leave and enter a magnet extend to considerable distances in all directions from the poles ; and that they are constantly exerting themselves to polarise all magnetisable bodies within there circuit. In order to concen- trate or direct as many of the lines as possible from the poles of a magnet in a definite desired direction, the poles are often fitted with pole-pieces or extensions of wrought iron, cast iron, or steel, of a form most suitable for the special purpose in view. For example, the pole-pieces fitted to the compound bar magnet illus- trated in Lecture II. are so arranged as to guide the lines of force from each of the separate plates of which the magnet is composed into one common channel at each end. In the following figure, the pole-pieces are shown bored out between the facing ends so as to concentrate the lines y % of force from the horse-shoe magnet upon a / /^~ '\ central iron-cored cylinder termed an Arma- ture. The armature in the case of a dynamo- machine (or machine for generating electri- city by means of mechanical power) consists 1 p ((a)]" of a soft wrought-iron core covered longitu- L-— as.^4. cUnally with insulated copper wire. In the ^"i'l^.l^TUBS case or a Morse telegraph instrument the qj, j^ Magneto- armature consists of a cyhndrical tube of Elbctkio Dynamo. wrought iron, which is free to be attracted by the poles of the electro-magnet against the tension of a spiral 42 LECTUKE V. AEMATUEB (A) AND Electro - Magnet (EM) OF A Morse Telegbaph Ikstbu- MENT. N. N spring when the poles are excited, and to return to its normal position under the action , of the spring when the poles become demagnetised. In telegraphic I'elays and other electrical instruments the ai-mature consists of a piece of magnetised iron or steel so pivoted that one end may move between and be attracted or repelled by tlie poles of the magnet according as they are rendered TS or S. In this case the arma- ture is tei-med a polarised or permanently magnetised armature (see next figure). Generally speaking, then, an Armature is a magnetic body, placed between or near, hut not touckini/, the poles of a magnet; and which is free to be rotated, or moved to and from them m- between them, by or against the concentrated magnetic force of the poles. A Keeper differs from an Arnmture, for it is a soft piece of wrought iron placed across and actually connecting the unlike poles of a horse- shoe magnet (or of a pair of straight or curved magnets) for the purpose of concentrating the lines of force as entirely as possible upon itself. You may tlmrefore consider a keeper as a simple device fm- furnishing a complete short circuit or dosed magnetic path for the lines of force between unlike poles of The lines of force which pass from the mag- net through the keeper polarise the latter, and thereby render the keeper a magnet with opposite poles to those of the magnet, as illustrated by the accompanying figure. The magnet- ism thus induced in the keeper reacts on, and induces further magnetism in, the magnet ; so that the keeper not only helps to maintain, but also to strengthen, the magnet. Deprived of its keeper a magnet gradually loses its magnetism, for the stray lines of force (or those which make a POLAEISBD AEMATUBE (N'S') OF A Tele- GBAPHic Relay Elec- tbo-Magnbt (EM). a magnet or magneU. AND HoBSE-sHOE Magnet „„„ Kbbpek, SHOWING PoLABiTY ^°^^ circuit between the poles) are OP Each, and Closed Mag- very easily cancelled or lost by vibra- tion, &c., from the fact that it is much NETIC ClECUIT. SPECIMEN QUESTION AND ANSWER. 43 easier to demagnetise than to magnetise steel. It may take more than a dozen strokes from a strong permanent magnet to strongly magnetise a bar of steel, but one or two strokes with the reverse pole will very seriously deprive the bar of its magnetism if they do not actually demagnetise it. Specimen Question and Answer. Question. — An iron ball is held over a pole of a horse-shoe magnet. Will the attraction exerted on the ball be altered if the poles of the magnet are connected by a soft iron keeper, and, if so, in what way, and why? (S. and A. Exam., 1889.) Answee. — Yes. When the poles of the magnet are connected by a soft iron keeper all the lines of force between the poles are concentrated within, and find their circuit through, the keeper ; consequently, no free magnetism is left to act inductively upon the iron ball. Note. — The student should make two sketches to illustrate the above answer — (i) Showing the direction of the lines of force when the ball is held over a pole of the horse-shoe magnet without the keeper, and marking the polar- isation of the ball and direction in which it is attracted. (2) Showing the path of the lines of force through the keeper, and the unpolarised uuattracted ball. 44 liBOTUKE V. QUESTIONS. Lectube v.— Questions. 1. How does the middle of a magnet act upon a piece of iron ? How do the ends of the magnet act upon the same iron ? Does any change occur in the iron when the magnet acts upon it ? 2. How OQuld you ascertain the relative quantities of free magnetism which exist at different points along a bar magnet? Having found them by experiment, how would you plot them out so as to represent a con- tinuous curve showing the variation from the equator to each pole of the magnet ? 3. A long strip of hard steel is magnetised, and when your small mag- netic needle is passed along the strip, its north point is attracted by one end of the strip, its south point by the other, the centre of the strip appearing to attract neither point of the needle. When the strip is broken across at the centre, what is the action of its two halves upon the mag- netic needle ? If each half is again broken, what happens, and why ? 4. The pole of a magnet is brought within an inch of one side of a sphere of very hard steel. It manifestly attracts the steel, but is not quite able to draw it into contact. A sphere of iron of the same weight is now substituted for the sphere of steel, and the magnet is found able to draw this new sphere quite up against itself. Explain, according to the mole- cular theory, this difference of action. 5. If a compass-needle is deflected when a steel bar is brought near it, how can you find out whether the deflection is due to magnetism already possessed by the bar, or to the bar becoming magnetised by the compass- needle at the time of the experiment ? (S. and A. Exam., 1886.) 6. Illustrate and fully explain the difference between, and the uses of, Pole-Pieces, Armatures, and Keepers. 7. What are magnetic keepers ? Give the theory of their use. 8. A magnet is placed near a compass-needle so as to pull the needle a little way round. If a large and thick sheet of soft iron is put between the magnet and the needle, what happens, and why ? 9. A horse-shoe magnet is placed near a compass-needle so as to pull the needle a little way round. On laying a piece of soft iron across the poles of the horse-shoe magnet, the compass-needle moves back towards its natural position. Explain this. (S. and A. Exam., 1885.) , 10. A piece of soft iron, placed in contact with both poles of a horse- shoe magnet at the .same time, is held on with more than twice the force with which it would be held if it were in contact with only one pole of the same magnet. Why is this? (S. and A. Exam., 1886.) II. You have three equal bar magnets without keepers. How would you arrange them so that, when not in use, they might preserve their mag- netism ? Give a sketch. ( 45 > LEOTtJEE ri. Contests. — Magnetic Induction — Definition of Induction — Secondary Induction — In the Case of Induction the Attraction alwaj's takes place between Two Magnets — Action and Reaction are Equal and Opposite — Inductive Effects of Like and Unlike Poles — Polarity Reversed', or Consequent Poles produced by Induction — Questions. Experiments XV. — Magnetic Indilction. — Take a strfitght strip of soft iron (abtitrt lo inches loftg and abotft i incli broad), and hold it close above some iron filings. No filings will be found adhering to the strip. Now lay along the strip a thin piece of wtood, and on the wood a strong bar magnet of about the same length as the soft iron strip. Tie the three together with string, as shown by the fi-gWTe. Again hold the soft iron close above the fi'Hftgs, tak- ing care that the i&ag- uet does not approach tew Hear them. This tiBie it #111 be foTind that the iron strip has beeblne a magnet, for it SSttraets some of the fiKhgS' to itSfelf, al- tlitough it is iKJt eiren tdfl^hfed by the magrnet. This peculiarity pos- sessed by magnetic force, of being able to act upon other magnetisaftle" boffies at a dJsta'rt'ee, is kflown' as rreeteiveetic ijiduclion. DEFINITION-. — Mdgimlm Indmtion ig thi name ^eh to the action etntd reaeUort ivMdh taKe pltiide token tfm magnetic Jot'66 sprmging frcmi (me hody malies evident the ieHlent m^ffMtimt in another hody, ii^r, vnth or wUhditt aetimi conti^ heMbM the bodies. The body from which the fof ee emanates is calTed tHe" indueing body; wM'te that upon which the force acts is called the Eod^i^Met jrno Iriiyi Tilmgi Soft Iron Sar Magnetised Inductively BY BAE itfAONET. Strictly speafciflg, there ought to be an interval or gftp between' the Hiducing body and the one whicfr ite ufider induction ; but i^ 46 LECTURE VI. has become customary, in the case of magnetism, to extend the meaning of induction so as to include actual contact. The space between the bodies may be a vacuum, or it may be occupied by such non-magnetic substances as air, wood, glass, or copper ; but one condition must be fulfilled — whatever lies between rmtst be non- magnetic. Secondary Induction. — Referring again to the experiment just performed, it will be noted that the filings were attracted even although the strip of iron did not touch them. This is an instance of the magnetisation of filings by the secondary inductive action of the force fir.st induced in the soft iron by the magnet. And so, in every case of magnetic attraction, there must first of all be induction. Experiments XVI. — In the Case of Induction the Attrac- tion always takes place between Two Magnets. — Procure a piece of very hard unmagnetised steel, and also a piece of good soft wrought iron of the .same size and shape as the steel. Place a .„ a compass-needle on the table, and lay the steel bar on a level with it. You observe that the needle is slightly de- Vnmagnetwi j--^\, ^ fleeted from its natural position. Now, .j M-^— ---■—.— ,|- -- ^ I jjj accordance with a Law of Magnet- i Ward steel \\\ . ^. , . . ^^ \ 1 V ism mentioned lu a previous Lecture, that end of the bar which is nearer either pole of the needle must become *t^ V 1 a pole of opposite kind, for some of the '*"^" ' lines of force from the magnetic needle have passed through the bar of steel, and have polarised it by giving its Attraction op Needle molecules a definite, set. It thus be- BY Hard Steel. comes a magnet by indtuition. "What appears, then, to be a simple piece 'of steel attracting a magnet, is in reality one magnet attracting another. If, now, you substitute the iron bar in the place of the steel one, you will again have the needle deflected, but this time to a much greater angle than in the case of the steel bar. -This is just what you might have predicted from what you had been taught by several of the previous experiments, for the less magnetic resistance of the iron naturally permits the induction to take place more easily. In this way you can determine roughly whether a sample of iron, or steel, has great or small magnetic resistance; or whether a bar is made of steel or of iron. The iron bar when under the influence of the needle's magnetism is a much stronger magnet for the time being than the steel bar. In each case, how- INDUCTIVE EFFECTS OF LIKE AND UNLIKE POLES. 47 ever, you find both action and reaction taking place. First there is the inductive action by the magnetism of the needle upon the bar, and then follows (as a second and necessary consequence) the reaction from this induced magnetism upon the needle causing the latter to be deflected. And, just as the inductive action was greater upon the soft iron, so also was the reaction correspondingly greater. You thus learn the important fact, that in all cases of magnetic indtiction, action and reaction are equal and opposite. The tendency of the magnetic needle to attract the bar of steel or of iron was exactly equal and opposite to the attraction which they respectively exercised over the needle. The weight and position of the bars prevented the needle from moving them, but the force tending to do so was there all the same. The reaction of the magnetism evoked in the plate of soft iron by the needle in Experiment XIV. (last Lecture) explains why the latter did not quite resume its natural or zero position of rest, even after it was freed from the influence of the bar magnet. Seeing that there is a greater intensity of field, or a larger number of lines of force within a certain space in the immediate neighbourhood of a magnet than there is within an equal space further away, you conclude that the shorter the distance between the inducing body and that under induction, the greater will be the inductive action. This may be proved by moving the bars nearer to, or further away from, the compass-needle, and noting how the angle of deflection varies. An interesting variation of this experiment (as illustrated by the following figure) would "be to balance the efifect of magnetic resistance against dis- tance, by placing one or the bars on each | .„ :_- -ai — <-j!\. >_— ^^^ side of the same pole i Wanl steel PA Soft iron of the needle, and al- .^ tering their distances %--», \ i / -- v*'* therefrom until it was ■^*''''-- W — ,r/dii'«*'° found that the needle ^W-»-¥^- M'^ was not deflected. You would thus prove that Balancing a Needle between Habd there was as strong Steel and Soft Iron. an inductive action set lip by the magnetism of the needle in the soft iron bar at the greater distance as there was in the steel bar at the shorter dis- tance, on account of the greater magnetic resistance of the latter. Experiments XVII.— Inductive EflFeets of Like and Un- like Poles. — Take a bar magnet and a few soft iron nails. Hang the nails in a chain, one below the other, say from the N-pole of 48 LECTURE VI. the magnet. Let us suppose Mia' Demagnetising Inductive Effect of an Unlike Pole.* K SI T 'f: in Demagnetising Inductive Effect of a Like Pole.* (=- N Iw Increased Magnetising In- ductive Effect of a Like Pole.* rs* HI Increased Magnetising In- ductive Effect op an Un- like Pole.* that it is strong enough to support a chain of three nails. Now gradu- ally slide along the top of the magnet the S'pole of another bar magnet (as illustrated by the accompanying figure), and you find that, one by one, the nails will fall away. The same thing happens if the If -pole of the second magnet is brought under the nails, as shown by the next figure. If, however, you bring the N-pole of the second magnet above the nails or its S-pole below them, you will be able to add a fourth, and possibly a fifth, nail to the chain, as represented by the two following figures. By induction, the N-pole of the first magnet polarises the first nail. It in turn magnetises the second nail; and the second, the third ; and so on. When the S-pole of the second magnet is brought along the top of the first magnet, it tends to induce a N-pole in the nails, where already there is a S- pole, and a S, where there is a IT. This neutralises so much of the efiect of the first magnet, that the weight of the nails causes them to drop ofi". The very same thing happens when the N"-pole is brought near the nails from below. When, however, we bring the N-pole of the second magnet over the nails, the existing polarity in them is con- firmed and strengthened ; the force of attraction among themselves and between them and the magnet is increased; and, hence, we can add to the length of the chain. Whilst the two magnets (repre- * It will be a useful exercise for the student to draw these four figures to a larger scale, and to show by dotted lines and arrows the directions of the magrletlc lines of force under the different circumstances in accord- ance with previous explanations. POLARITY REVERSED BY INDUCTION. 49 sented by the third of the liwt yet oi' Jigurew) are very materially assisting each other's inductive action on the nails, they are, at the same time, as it were, fighting against one another. Each N-pole, while it i.s strengthening the polarity of the nails, is weakening the inductive action of the other N-pole by tending to convert it into a S-pole, or by repelling its Unes away from the direction of the nails. This explains why it is, that two magnets placed to- gether with like poles adjacent, will not attract and support twice as long a chain, or as great a weight of nails as one magnet will. It also explains why compound magnets (referred to and illustrated in Lecture II.) are not so strong as the sum of the strengths of their separate plates. Polarity Reversed or Consequent Poles produced by Induction. — If a pole of a strong magnet be gradually brought up to a lihe pole of a weaker magnet, repulsion will take place when they are within a, certain distance of each other; but if the distance is diminished so that the two magnets are close together, attraction will result; from the fact, that the natural polarity of the weaker magnet has been reversed. This is sure to be the case whenever a weak magnetic needle is suddenly placed in a strong magnetic field, wherein the Unes of force are flowing in the opposite direction to those through the needle, and the needle from any cause is not free to instantly turn round so as to set its polarity in the same direction as that of the field. Great care should, therefore, be taken not to subject magnets or needles to this reversing action. It may happen, however, that the weaker needle or magnet, especially if it be a long one, in- stead of having its polarity entirely reversed, has merely an unlike pole induced at the end nearer the stronger magnet, while still retaining its former unlike pole at the further end. In this case, unlike poles, or what are termed omisequent poles, are sure to be found existing somewhere along its length. In either case, be- fore the magnet or needle can be effectively made use of again for experimental purposes it must be freshly and properly mag- netised. 50 LECTUKE VI.-^ QUESTIONS. LECTUHE VI. — QUESTIOKB. 1. What is the magnetic condition of a bar of soft iron held horizontally above, and parallel to, a permanent magnet of the same size, resting hori- zontally on a table ? Give a sketch indicating the polarity of the bar and the direction of the lines of force. 2. Near a ball of perfectly annealed soft iron, the north end of a strong steel magnet is placed. What is the action of the magnet upon the ball ? What change occurs in the ball when the magnet is withdrawn ? And what occurs when the south pole of the magnet, instead of the north, is placed near the ball ? Illustrate your answers by diagrams. 3. A compass-needle and a straight strip of soft iron of the same length as the compass-needle ai'e fastened together so as to be in contact with each other at both ends. Will the force which tends to make the combi- nation point north and south be the same as that which would act on the compass-needle alone ? Give reasons for answer. (S. and A. Exam., 1887.) 4. A rod of iron and a rod of steel are stroked in succession with one of the poles of a bar magnet. How do the iron and steel rods respectively all'eot a compass-needle when brought near it 1 (S. and A. Exam., 1889.) 5. Two bars of soft iron are so placed to the east and west of the north pole of a compass-needle that the needle still points north and south. If the iron to the east of the needle be replaced by a bar of hard steel of exactly the same size and shape as itself, will the direction in which the magnet ;^oints be altered ? If so, in which direction will it move, and why ? (S. and A. Exam., 1888.) 6. When a fiiece of soft iron . is attracted by a magnet, the iron is said to be magnetised, so that the attraction really takes place between two magnets, the original one and the one produced by magnetisation. What proof can you give of the truth of this statement ? 7. A bar magnet is laid on a table with its N-pole projecting over the edge. A soft iron ball clings to the under-side of the projecting end. State and explain what happens when the S-pole of a second magnet is brought above and near to the N-pole of the first. Give sketches. 8. Two similar rods of very soft iron have each of them a long thread fastened to one end, by which they hang vertically side by side. On bring- ing near the iron rods, from below, one pole of a strong bar magnet, the rods separate from each other. Explain this. (S. and A. Exam., 1885.) 9. A dozen sewing-needles are hung in a bunch by threads through their eyes. How will they behave when hung over the pole of a strong magnet ? 10. A long magnet and a piece of soft iron of the same size and shape are placed parallel to each other underneath a sheet of paper upon which iron filings are strewed. How will the filings arrange themselves 1 (S. and A. Exam. 1891.) ^ 11. A bar magnet is laid upon a table, and a soft iron bar of about the same length as the magnet is hung horizontally just above it by a flexible string. What will be the effect {if any) on the soft iron bar if a second bar magnet be laid on the table and brought near the first, at right angles to It, and with its north-seeking pole pointing to the middle of the first mag;uet ? (S. and A. Exam., 1885.) Give sketches and explain your ideas of the actions which take place, by di-awing the lines of force as thev wiU occur- bolore, and when, the second bar magnet is brought near the first. ( 51 ) LECTURE VII. Contents. — The Earth Regarded as a Magnet — Geographical and Mag- netic Poles and Meridians — True Polarity of the Earth — Declination or Variation — Inclination or Dip— Earth's Magnetic Axis and Equator — Questions. ExPEKiMENTs XVIII. — The Earth Regarded as a Magnet, — Lay a strong bar magnet on a pile of books or on a ilat board raised above the level of the table, and carry a compass-needle round the magnet on a level with it, exactly in the manner described and illustrated in Lecture III. Or, hang the bar magnet up by one end in a vertical position and pass a dipping- needle round it, following the outlying lines of force. You tvfll find that in whatever position the needle is placed one of its poles will invariably tend to point towards one pole of the magnet, and its other pole to the 6ther pole of the / ^^^' X / I \ \ \ v.. »^ ..X* / / magnet. In fact, the magnetic ajds of the needle in each experi- ment always forms a tangent to the bar magnet's curved lines of force at the position where the needle happens to come to rest. Precisely the same effect is observed if a dipping-needle be carried round anywhere on the surface of the Earth from pole to pole as illustrated by the figure on the next page. This proves that the Earth possesses magnetic force, and that a magnetic field surrounds it on all sides. It is due in a large measure to the di- rective action of the Earth's mag- netism upon the mariner's compass-needle that the navigator is enabled to steer his course from place to place when beyond the sight of land, and that the African e.xplorer finds his way across the " Dark Continent." We may therefore fairly assume for the purposes of elementary explanation that the Eaith Dipping-needle Tangential TO Magnet's Curved Lines OF FOECE. S2 Lecture vti. acts magnetically as if it had a great magnet inside it, and we consequently illustrate it here as if such were the case. For a more advanced and exact understanding of all the phenomena at- tending the Earth's magnetism other views of the case have to be considered and discussed, but an explanation of these views would be out of place at present. We shall therefore now proceed tu explain different terms and particulars relating to " the Ewrth as a magnet " by aid of a few diagrams and experiments. Diagram of the Earth as ajj Imaginary Magnet, etc. % Not % to Sg N/B to Sill 6E ME d Index to Kigube. means North geographical pole. South geographical pole. North magnetic pole (true). South magnetic pole (true). Earth's axis (if a straight line). A geographical meridian (if a circumferential line). Magnetic axis (if a straight line). A magnetic meridian (if a circumferential line). Geographical equator (circumferential hne). Magnetic equator (circumferential line). Declination or variation angle (=17° at London, 1889^). Inchnation or dip angle ( = 67° at London, 1889}. GEOGRAPHICAL AND MAGNETIC POLES AND MERIDIANS. 5 3 Experiments XIX.— Geographical and Magnetic Poles and Meridians. — Note the direction of the sun at mid-day* and lay down a stmight line on a level board or table ; such that, if produced or extended into a great vertical plane it would exactly bisect the sun. Mark the end of this line towards the sun, Sgr (for South geographical direction), the nearer end Ng (for North geographical direction), and then take a point, O, midway between them. Upon W f- this line place a compass-needle with its centre exactly over the point, O. After the ^^ needle has come to rest under the influence ^ S* of the Earth's magnetism, mark on the board the position of the needle's N and S poles.f Bemove the needle and draw a straight line through O, joining N and S, and extend this line in each direction to Sni, and to Nvji, as shown by the accompanying figure. An extended vertical plane passing in the direction, ON^, would pass through the North geographical pole of the Earth, and if extended in the opposite direction, OSgr, it would pass through the South geograph- * According to Greenwich mean time, the sun is dne south at mid-day four times every year (about April 14, June 14, Sept. I, and Deo 25), con- sequently if Greenwich time be adopted it will be necessary to consult the " Nautical Almanac " in order to find how many minutes before or after mid-day the sun is exactly south, and the observa- tion to be correct must be taken accordingly. By solar time the sun is due south every solar mid-day. Another plan, often adopted by sailors, of finding the geographical and magnetic meridians, and from these the declination or variation angle, is to observe the direction of the North Star, then the direction of the magnetic axis of the mariner's compass, and to note the angle between them ; making the necessary plus or minus allowance shown by the " Nautical Almanac " for the time and place that the observed direction of the North Star may be to the East or to the West of the true direction of the geographical North Pole. t It may happen that the poles of the compass-needle are not situated exactly in the straight line joining the ends of the needle. In order to check this possible error, the needle should be fitted with two centres, tluis. — y— > so as to admit of tinning it upside down. If any differ- enrp should now occur in the positions of N and S, then take the mean im-itiiins between the first and last observatioa for the poles of the mag- ftytie a.\is of the needle,. Finding the graphical Magnetic dians, and Declination. Gbo- and Mbki- the 54 LECTURE vn. icalpole; consequently, the line, Sg—Ng, lies in a Geographical Meridian ; for — . . . . BY DEFINITION. — A geographical Meridian is an imagi- nary line drawn on the Earth's swface in ihe plcme which passes through the geographical poles of the Earth and a given place. Again, an extended vei'tical plane passing ip the direction, OW or OSm, may be assumed to pass through the true* " Magnetic South Pole " of the Earth ; and if in the direction, OS, or ONw, to pass through the true " Magnetic North Pole " of the Earth ; consequently, the magnetic axis of tho compass-needle, SN, or the line Not— Sm lies in a Magnetic Meridian ; for by— DEFINITION. — A Magnetic Meridian may be taken as an imaginary line drawn on the Earth's ?urfafi? in the plane . wMch passes through the magnetic poles of ihe Ea/rth and a given plage ; or more accurately as a line ill the vertical plane containing ilie magnetic axis of a cornpass-rieedle frf a given place. Declination or Variation. — Referring again to the twc) last figures, you observe that the line, Ng — 8.9-, coincides with the Qeo- graphical Meridian of the position, O (or place of observation), and that the line. Not— Swi, coinpides with the Magnetic Meridian * True Polarity of the Earth. — The student must carefully observe and remember that since unlike poles attract and like poles repel ewh other, the IT -pole of the magnetic needle can only be attractedhy a S-pole, of another magnet, and the S-pole of the needle by a N-pole. He must therefore regard and always mark the polarity of the Earth by Sm, for the irm south magnetic pole situated in the Nortlwrn hemi- sphere, and by JUm, for the true North magnetic pole situated in the iSouthern hemisphere. Con- siderable ambiguity and vexatious explanations will be avoided if he at first adopts, and then adheres to, this notation and view of the case. The accom- panying small figure (in addition to the previous larger " Diagram of the Eart.h as an Imaginary Mag- net ") will still further impress this upon him. The figure illustrates a bar magnet («., «) upon the Earth's surface, and the true TSm and Sm poles of the Earth with the lines of force as they actuolhi circulate throvgh the magnet, round the same, and to the Eartii's poles, as well as the Earth's external field (or surface Earth's lines between the Nm a\id Sot. poles of the Earth). The lines which pass through the bar magnet are in the same direction as the Earth's ]ines, but the extenial field lines of the bar magnet circulate or act in the ojyponte direction to the Earth's field lines. We have in the text used the WQrds "may be assumed to pass through llie true magnetic poles," because, in reality, the magr\etism of the Earth is so irregular that we cannot say th^t the meridian -vyould pass through them in all cases. Tj:arth'i/s\Fole I « M I < Earth' "ilraU I'RUE Polarity op THE Earth, and OP A jlAGNET ON ITS Surface. DECLTNATION AND INCLINATION OR DIP. 5 5 of the same place. These two lines make an angle, d°, with each other, and this angle is termed the Declination or Variation angle for that place. Hence we have the following — DEFINITION.— r/je Declination of a place is the value of the angle in degrees betireen the magnetic meridian and the geographical meridian of that place. The Declination is termed (E.) Easterly or (W.) Westerly Declination, according as the magnetic meridian lies to the East or to the West of the geographical meridian when looking from the north pole of the compass towards the Northern Hemisphere. The declination is different for different places on the Earth's surface ; and it also varies from time to time. Thus, at London, in 1800 it was 24° 6' W. ; in 1880, 18° 40' W. ; by 1888 it fell to 17° 40' W., and is still decreasing; at Sydney, in 1880, it was 9° 30' E.; and at Glasgow at present, in 1889, it is about 21° W. Experiments XX. — Inclination or Dip. — Take a large wooden ball (10'' or 12" in diameter — an old globe will do very well) pivoted on a wooden stand. Fix a cylindrical magnet 8" or 10" long inside it and diametrically opposite to the axis upon which the ball rotates. See that the poles of this magnet are equi- distant from the surface of the ball. Let this ball represent the Earth. Now suspend a dipping-needle above the centre of the ball, as shown by the following figures :-^ At Poles. At London. At Magnetic Equatok. Wooden Model with Eae Magnet to Rbpebsent the Eaeth. Then turn the ball round into each of the above positions and watch the effect upon the dipping-needle. I . When the bar magnet is vertical with its S-pole (representing the true, Sm, or South Magnetic pole of the Earth) next the needle, the latter hangs perpendicular with its N-pole pointing downwards. Upon turning the ball half round, the needle again 56 "LECTUKE VII. assumes a vertical position, but its S-pole is downwards, being attracted by the, N»i, pole of the magnet. These two positions represent what would take place if you could take a dipping- needle to places on the Earth's surface fairly opposite the magnetic poles of the Earth. 2. Turn the ball round through an angle so as to approximate roughly to the angle which a line drawn from London to the centre of the Earth would make with the Earth's magnetic axis, and observe that the needle dips or inclines about 67° to the horizon. This represents the inclination of a dipping-needle at Greenwich Observatory. 3. Turn the ball still further round until the magnet lies hori- zontal. You observe that the needle also becomes horizontal or parallel with the bar magnet. This represents the position of a dipping-needle at the Earth's magnetic equator. Now refer back to the two first figures in this Lectvxre, and observe the several positions of the small dipping and compass needles ( >) drawn around and upon those diagrams ; and you cannot fail to imagine the actual condition of magnetism as it emanates from and encom- passes the Eartli. From observations made with dipping-needles at different lati- tudes it has been proved that the magnetic poles are situated some distance inward from the surface of the Earth. Also (as may be illustrated by the wooden model), the nearer a compass is brought to the magnetic poles the less sensitive does it become as a director of position; in fact, if it was taken right opposite the poles it would have no directive action, for it would simply tend to stand vertically, like the needle in the first case in the last experiment. Again, on the contrary, the nearer a compass-needle is taken to the equator the more free and sensitive is its action. ExPEEiMBNT XXI.— Take a flat board (a drawing-board will do very well), and set it up vertically in the plane of the magnetic meridian. Draw a horizontal or level Una, SgO^g, along the middle of the board, and hang from a copper nail by a thread without twist, or hold up, a dipping-needle* so that its axis Ls just oppo.site the point, O. When the needle comes to rest, mark on the board the positions of the poles N and S of the needle, and, upon removing * Like that in the above figure. See Appendix to "Magnetism " for how to make and use a Laboratory Dipping-needle. THE earth's magnetic AXIS AND MAGNETIC EQUATOR. 57 the needle, draw the line, 'NniOSm, through the points, S, O, and N. The needle (as previously explained) takes up a tangential FiNBING THE DiF, OR INCLINATION ANGLE. position to the magnetic curved lines of force at the place; and, since it is free to lie exactly in the magnetic meridian, the angle, lUgOSm (formed between the magnetic axis of the needle and the horizontal Hne), constitutes what is termed the Inclination or Dip Angle. DEFINITION. — The Inclination or Dip at « place is the angle in degrees between the magnetic axis of a dipping-needle {free to move in the plane of the magnetic meridian) and a horizontal line in the same plane. The angle of inclination (i), or the dip, is gradually decreas- ing at present in this country ; for, as registered at London, in 1800 it was = 72° 8' N., in i868 = 68° 2' N., and in 1888 = 67° 25'. In Glasgow at present it is nearly 72°. The Earth's Magnetic Axis and Equator (see second figure in this Lecture). — Like a bar magnet, the Earth has both a mag- netic axis and a magnetic equator. DEFINITIONS.— r/ie Magnetic Axis of tlie Earth is the straight line ivhich joins its magnetic poles. The Magnetic Equator is an imaginary irregular line drawn round the Earth, and joining all places where the dipping-needle lies horizontal v;hen placed with its magnetic axis in the plane of a magnetic meridian. Local circumstances, svich as great deposits of iron with other magnetic ores, local magnets of various kinds, and magnetic stoi-ms, considerably affect the regularity of the magnetic equator 5 8 LECTFRTi: vn. line, as well as the declination and the inclination at different places on the Earth's surface. Charts giving the average local horizontal intensity of the Earth's magnetism, the local declination and incli- nation at different important parts of the Earth (with separate curved lines connecting, (i) places which have the same intensity, (2) the same declination, (3) the same inclination), are procurable by mariners and others, to whom such knowledge is of importance. Daily and continuous most accurate observations are made at Greenwich and Kew Observatories, as well as at several other similar stations of every variation and circumstance connected with the Earth's magnetism. LKGTURK VII. QUTISTIONS. 59 Lecture VII. — QtrESTioNS. 1. Sketch a section of the Earth through its magnetic poles, showing the true polarity, magnetic meridian, and magnetic equator ; also geographical meridian, geographical equator, magnetic axis, and magnetic equator. 2. What is the meaning of the term " geographical meridian " ? What is the meaning of " magnetic meridian " ? What name is given to the angle between the two meridians, and what is its present value at London ! 3. Explain concisely and clearly why the magnetic pole of the Earth, situated in the Northern Hemisphere, is termed a true South pole 1 If you called it a North pole, then what would you call the pole of a compass- needle which pointed towards the North, and why 1 4. You are asked to illustrate the declination and the inclination of the magnetic needle ; how will you proceed ? Point out the full analogy between the Earth's action and that of a bar magnet. 5. How does the position of a " dipping-needle " change when it is taken from London (a) towards the North pole, or (6) towards the Equator ? (S. and A. Exam., 1885.) 6. What is meant by saying that the magnetic dip at London is 67° 30' ? State in general terms at what places on the Earth's surface the magnetic dip is least. (S. and A. Exam., 1888.) 7. How do (i) a dipping-needle, (2) a compass-needle, behave at the mag- netic poles of the Earth 1 (S. and A. Exam., 1889.) 8. If a compass were carried round the Earth's equator, would it point in the same direction at all places ? If not, state, as nearly as you can, what changes would be observed in its behaviour during the journey. (S. and A. Exam., 1887.) 9. If you wish to support a uniform bar magnet horizontally on a pivot, how is it that the pivot must be placed nearer to one end than to the other ? To which end must it be nearer in this country 1 10. A bar magnet suspended horizontally sets in the magnetic meridian. Supposing a second bar magnet to be suspended by the side of the first, how will they act upon each other ? Make your answer clear by a diagram. 11. A large soft iron rod lies on a table in the magnetic meridian, and a dipping needle is placed at some distance and at about the same level, (i) due south, (2) due north of it. How will the magnitude of the angle of dip be affected in each case? (Neglect any inductive action between the needle and the bar. ) (S. and A. Exam. 1890.) 12. A compass needle is deflected 15° from the meridian, when a bar magnet is placed on the table some distance away. Will the deflection be altered if the poles of the magnet are connected by a bent iron rod ! Give reasons. (S. and A. Exam. 1890.) 13. Given a magnet and the means of suspending it. How will you determine (i) the magnetic meridian, (2) in which direction North lies? It is assumed that you do not know which end of your magnet is a north and which a south pole. (S. and A. Exam. 1891.) ( 6o > LECTURE VIII. Contents.— The Mariner's Compass — Magnetisation by the Inductive Effect of the Earth's Magnetism — Magnetisation of Iron and Steel Ships — The Earth's Influence on a Magnet is Directive, but not Trans- lative — A Compass-needle always obeys the Stronger Force — Astatic Pair — Questions. The Mariner's Compass. — We have more than once referred to this, the oldest and the most useful, application of the magnet, DiiEcult and dangerous as the navigator's occupation often is, it would be very considerably increased had he not his reliable friend, the compass, to guide him. This fact is now so thoroughly re- cognised that every registered ship is bound to be fitted with a tested and certified mariner's compass before leaving port. We shall here explain a ship's compass of the simplest kind, leaving a descriptioii of Sir William Thomson's improved form to our Advanced Treatise, as it involves the examination of sevei'al points somewhat beyond the range of an elementary text-book. Index to Figure. CO represents CompasB card. B represents Binnacle case, inside which the con- trolling magnets are fixed to compensate for semicircular and heel- ing eiTors. Gh represents Chain boxes where wi'onght - iron chains are kept to com- pensate for quadrantal eiTOr. BC represents Binnacle cover. LL represents Lamps to light up compass card at night. M'Gregoe's Patent Mahineb's Compass. Referring to the two following figures and explanatory index, you wiU observe that the instrument consists of a carefnlly-made magnetic needle, so fastened to the under-side of a light card that THE MAEINEK S COMPASS. 61 Sectional Elevation through Binnacle 'Bowl. the centre of the needle coincides with the centre of the card, and its magnetic axis is fairly in line with the north and south points of the same, the north pole of the needle being next the north point or " head " of the card. The centre of the needle is fitted with a small A cup of agate or sapphire stone, in order to minimise the friction and avoid rusting between it and the fine pointed steel support upon which it rests and turns. The combined card and needle are accu- rately balanced on this steel point, which is centrally , fixed to the inside of a brass or copper bowl. The bowl is provided with a glass cover to protect the card from damp and dust, with- out concealing from view the various degrees and in- dications printed upon the surface of the card. The bowl is not rigidly fixed to its binnacle or case, but is supported on a " gimbal," which permits the bowl and card to maintain a level position when the ship pitches or rolls. The gim- bal ring has two bearings diametrically opposite each other, which carry the out ■ standing knife edges or round journals fixed to the bowl, and at right angles to these are two other journals, which fit into the bearings secured to the binnacle bowl. Plan op Compass Caed* and Bowls. M'Gbegoe's Oedinaey Compass. Index to above Figdees. CB repi-esents Compass Bowl and Card. J,J„ „ „ Journals. GR „ Gimbal King. -: J3J, „ „ Journals. BB ,, .Binnacle Bowl. I „ Lubber Line (top fig.). ■ * The compass card is shown on the plan (by mistake) as turned through 90° to the right of its position on the sectional elevation. 62 LECTDEE Till. The steering compass is placed in front of the "wheel," so that the " man at the wheel" may easily see the compass card which is divided off into thirty-two " points," as illustrated by the fol- lowine figure. When he wishes to steer any particular course by compLhe turns the helm so that the desired pomt on the com- pass card is faii-ly opposite to the vertical black hne (techmcally Messrs. Beown & Son's Maeinee's Compass Card. termed the " lubber line "), which is drawn on the forward inside of the bowl, in line with the direction of the ship's motion. In order to make the mariner's compass a really useful and reliable guide, every ship which makes long voyages should be pro- vided with a chart showing the latest declination or variation at various places on the Earth's surface. The declination has to be added to, or subtracted fioui, the ship's true course before giving MAGNETISATION BY EARTH's MAGNETISM. 63 the steersman the compasa course. For Example, — If you wish to make a true North (geographical) course, and the declination at the position of the ship at the time is say 20° W., then the steering course will be 20° E. ; or in nautical language you steer exactly North 20° East, or roughly NNE. (North-North-East), more accu- rately NNE. I N. (North-North-East by i point North). If you had steered Noiih by compass card, the ship's course would have been N. 20° W., or you would have gone continuously 20° to the westward of the intended course. So you see with what watch- ful care the captain of a vessel has to guard against errors liable to occur through a misapplication of declination. There are other local errors to be guarded against, which will be referred to shortly. Experiments XXII. — Magnetisation by the Inductive Eflfect of the Earth's Magnetism. — Another very conclusive proof that the Earth acts as a magnet is found in the fact that iron or steel bars when they lie in a northerly and southerly direction for some time become magnetised, more especially if they are sub- jected to vibration. To illustrate this, take a bar of unmagnetised steel or hard iron (an ordinary poker will do very well), and hold it level with its length pointing east and west. Test its ends by means of an ordinary compass-needle, and you find that it exhibits no polarity,each end equally showing a slight attrac- tion indifferently for either end of the needle. Now hold it in the magnetic meridian and at the angle of dip for the place you are situated in (say 67° at London, or 72° at Glasgow), as shown by the figure, and hit it several smait blows on the head with a hammer or mallet. Again test it by the needle, and you find that the point has had a north pole induced in it, for it repels the N-pole of the needle, and the head has had a S-pole induced in it, for it repels the S-pole of the needle. Finally hold the bar or poker about level in an east and west direc- tion as at first, and hit it hard several times. Test it now by Test-needle. Eakth's Magnetism inducing Polaeity IN A POKEE OE StEBL BAE 64 LECTUEE vm. the needle, and (unless it was composed of very hard steel) the whole of the magnetism will have disappeared from it. When held in the second position, the bar was fairly in line with the magnetic Unes of force of the Earth, and the Earth's polarity induced opposite poles in it, the blows which you gave it merely facilitating the molecules turning round to obey the directive action of the inductive force of the Earth's magnetism. When in the last position, there was the least possible chance of the Earth's force acting inductively upon the molecules of the bar, so that your blows relieved the inter-molecular tension and per- mitted the polarity of the molecules to become short circuited and self-sufficing amongst themselves in a similar manner to the steel filings when shaken in the glass tube (as illustrated and explained at the beginning of Lecture IV.). Take a compa-ss-needle to any place where iron or steel has been racked for some time in a vertical position, Or to the iron raiUngs surrounding a house or graveyard, and you will find, in this country, that the lower ends of the bars are all N-poles, and the upper ends S-poles, for the reason that the lower ends point towards the true south magnetic pole of the Earth. Of course their magnetism will not be quite so strong as if they had pointed directly in line with the Earth's magnetic force, and, in the case of railings, if they happen to have horizontal bars of iron securing their top and bottom ends, these bars will act as keepers and pre- vent your observing the full extent of free magnetism which they would exhibit if not so fastened. Magnetism of Iron and Steel Ships. — It may safely be said that every iron and steel ship is a huge floating magnet. First, when a ship is being built, the hammering and riveting to which she is subjected naturally assists the magnetic inductive action of the Earth's magnetism in producing sub-permanent magnetism in the harder and steely particles of her frames and plates. This effect is all the greater if the " ways " upon which she is built lie north and south. When the slup is sent to sea some of this magnetism may be knocked out of her, due to buffet- ing the waves, especially if the direction of the ship's head is on a course exactly opposite to that upon which she lay when being built. Still, there is always a certain amount of the originally induced sub-permanent magnetism left, and this produces what has been termed the " setnicircular " error of the compass, or an error felt most when the ship is on a H", E, S, or W course. This error is approximately cancelled by fixing a permanent mag- net or magnets to the deck, or in the binnacle below the com- pass, in a Kne with the keel, so that the polarity of the magnet is opposite to the polai-ity of the ship. But neither the COMPASS EEEORS IN IRON AND STEEL SHIPS. 6$ magnetism of the ship nor that of the counteracting magnets is constant, and, consequently, navigators require to be continually on the watch for any serious alteration of this error. Second, when a ship is afloat, the Earth's magnetism induces jnagnetism in her softer particles of iron or steel, and so produces what has been termed the " quadrcmtai " error of the compass, or an error most observable when the ship is on an IfE, SE, SW, or NW course. This error is approximately cancelled by placing masses of soft iron or magnets athwartships, in a line with, and on each side of, the compass. Third, there is another error, termed the " heeling " error, due to the heeling or rolling of the vessel, which is approximately corrected by placing magnets directly underneath the compass in a vertical position. Fourth, there is the error due to carrying an iron or steel cargo, which may be termed the " cargo " error. In Telegraph steamers, which have several tanks for holding iron or steel-sheathed sub- marine cable, this error is very observable and very variable, since the cable is being more or less continually altered in quantity or position, due to paying it out, or picking it up, or shifting it from tank to tank. Fifth, in steamers which are fitted with the electric light, more especially if fitted upon the " single-wire system " — i.e., with the hull of the ship forming a return circuit for the electric currents — great care and caution has to be observed that the wires are so run and the dynamos so placed that the magnetism arising from their currents and poles does not afiect the mariner's com- All these five kinds of local errors combined, form what sailors term the deviation of the compass, and the deviation combined with the declination forms the total error of the compass. The deviation error may act with, or against, the declination of the place, according to circumstances or the course the ship is steering ; and, consequently, before a new ship sets out upon a voyage she is taken to some retired and convenient spot — such as the Gareloch, on the Clyde, for Clyde-built ships — and has her compasses ad- justed by a competent and authorised compass adjuster. A Table of the deviation errors of the ship's standard compass t is given by * In the steamer Bombay electrically fitted out in Glasgow under the author's supervision, he noticed a maximum error of 9° in the steering compass, and he had to get double wires run wherever the leads came near to the compasses before the eiTor became inappreciable. Sir William Thomson drew particular attention to this kind of error in a paper read by him upon this subject before the Institution of Electrical Engineers in May 1889. — See Proceedinge of this Institution. t The standard compass should be a specially weH-made compass, fitted E 66 LECTURE VIII. Magnet on Cork in Basin OF Water Turned Bound BY Earth's Force only. him to the captain before the ship starts on her voyage ; but (as has already been pointed out) the circumstances affecting the mariner's compass are so variable that this Table is only true for the place and condition of affairs where and when the ship was swung for compass adjustment. He has, therefore, to avail him- self of every convenient opportunity of taking observations of the sun, or the north star, or some well-known headland, in order that he may guard against making mistakes in his navigation. Experiments XXIII. — The Earth's Influence on a Magnet is Directive, but not Translative. — The statement is often made that the Earth's effect upon a pivoted or suspended needle is merely directive, and is not translative. A simple experiment will make the meaning of this clear to you. Float a very small, light magnet on a cork in a basin of water, and place it pointing east and west. Immediately on being released, the cork with its magnet swings round as if on a pivot, and comes to rest with the needle in the magnetic meridian ; but neither cork nor needle moves towards the north or towards the south. The direction changes, but not the position. The Earth's true magnetic south pole is, at the same time, attracting the north and repelling the south pole of the needle ; but, seeing that the attractive effect is acting at a shorter distance than is the repulsive effect, the student might expect to find the needle moving as a whole towards the former pole. The poles of the Earth are, how- ever, so far away, that the short distance between the poles of the needle ceases to be of any account when com- pared with their distances from the Earth's poles. Thus there are forces of attraction, and of with all the latest improvements for observing, and, if need be, balancing, the several local errors, or so placed upon an elevated position (such as the top of a wooden pole or mast) that it is practically beyond the local field of tlie ship s magnetism. This compass is sometimes termed the pilot com- pass, and IS used by the captain and officers as a standard or reference compass, whereby the readings of the ordinary and commoner steering compasses may be checked. Magnet on Cork in Basin op Water turned and attracted towards the Bah Magnet. A COMPASS-NEEDLE OBEYS THE STRONGER FOECE. 67 repulsion, acting on the needle at the same time, and at prac- tically the same distance ; but as these forces are equal and oppo- site to each other, the needle, as a whole, is neither attracted nor repelled. If, however, we bring the pole of a magnet up towards the needle at such a distance that the length of the latter is con- siderable as compared with the distance between them, we shall find that the needle will be both directed and attracted. ExpEEiMENT XXIV. — A Compass-needle always obeys the Stronger Force. — Place a small compass-needle on the table, and hold above the needle, and parallel with it, a bar mag- net, the poles of which point in the same direction as those of the needle. At first hold the magnet at such a distance that it does not apparently afiect the lie of the needle, as shown by the first case in the accompanying figure. ITow bring it down slowly, and In sI Needle beyond the Astatic Position. Nbeule undee the Reach op Bah Earth's Magnet- Influence op the Magnet's Field op ism and Bar Mag- Stkongbb Force. Force. net Balance each other. when at a certain distance the needle will be observed to waver, and jerk about. If the magnet be fixed here, it will be found that the needle will set or stand still in any position that is given to it. If the magnet is brought down still further, the needle will turn sharply half round, and come to rest with its poles as shown by the third case in the above figure. In the first position, the mag- net was so far away that very few of its lines of force passed through the needle, and, consequently, the latter was not appre- ciably afiected. As it approached closer, more lines of force passed through the needle, until, in position number two, it was able just to counteract the directive force of the Earth's mag- netism. As a consequence, the needle was free to set in any position. Brought nearer stiU, the influence of the magnet 68 LECTURE VIII. overcame that of the Earth, and the needle, obeying the stronger of the directive forces, turned round and came to rest with its S-pole towards the N-pole of the magnet. Astatic Pair. — With the magnet in the second position, the needle was said to be astatic (Greek, a and etrrrnu, I stand) because it ■was found standitig still, or resting, in whatever position it was mechanically placed. If, then, two magnets be taken equal in length and in strength of pole, and fixed together by a stout piece of wire or other rigid connection, with their magnetic axes paral- lel and their similar poles pointing opposite ways, they will form what is variously termed 9 an astatic pair, astatic combination, or astatic needles. For if such a com- bination is suspended by the middle, as shown by the figure, so as to be free to move in a horizontal plane, the two magnets satisfy or neutralise one Astatic Needles. another. The Earth's at- tractive force on the N- pole of one needle is exactly counterbalanced by its repelling force on the S-pole of the other needle, and consequently there is no directive force to cause the combination to set in any particular position. We shall have occasion afterwards not only to show that it is practically impossible to make a perfectly-astatic pair, but also of explaining how this arrangement of two needles when combined with one or with two coils of wire forms a most useful and reliable kind of sensitive galvanometer for indicating the ;presence and direction as well as for measuring the strength of electric currents. i .ql- . IN A Nl 1.«5 LECTURE VIII. — QUESTIONS. 69 Lecture VIII.— Questions. 1. Sketch and describe the simplest form of mariner's compass, and give an " Index of Parts," stating of what each part is made, and why 1 2. Why are iron and steel ships said to be magnets 1 3. What are the several kinds of errors to which a mariner's compass is liable ? What are the causes which produce these several errors 1 4. Explain in your own words why the mariner's compass has sometimes a steel magnet or magnets fixed near it, on to the deck in front of it, and at each side of it. Why are lumps of soft iron sometimes fixed athwart- ships on each side of the standard compass ? What do you mean by the standard compass ? 5. If a long bar of very soft iron is held upright, how is it that its upper end repels the south-seeking end of a compass-needle, and that its lower end repels the north- seeking end of a compass-needle ? What would be the precise effect of hitting the end of a bar of steel when held in the mag- netic meridian, and at the angle of dip (in this country) ? What would be the effect if the bar was held level east and west, and then hammered ? 6. Two equal bars of steel, after having been equally magnetised, are kept for some years in a vertical position, one (a) with its south-seeking pole upwards, the other (5) with its north-seeking pole upwards. The bars are so far apart that they do not act on one another ; which of the two bars would you expect to find had kept its magnetism better, and why ? (S. and A. Exam., 1886.) 7. A bar of soft iron AB is placed horizontally east and west, the east end A being about 4 inches to the west of the north-seeking pole of a com- pass-needle. The end A being fixed, B is raised until the bar is vertical. How is the needle affected by the bar when in its original and final posi- tions ? (S. and A. Exam., 1889.) 8. A small magnet is placed upon a flat cork which floats in a basin of water, and it is fastened to the cork with a little wax. Describe and ex- plain the behaviour of the magnet (i) when under the influence of the Earth's magnetism alone, (2) when an artiScial steel magnet is brought near to it. (S. and A. Exam., 1887.) 9. It is sometimes said that the Earth has no tendency to impart to a magnetic needle a motion of translation, but that it has under certain cir- cumstances a tendency to impress upon it a motion of rotation. What is the meaning of these statements ? 10. A'strong bar magnet is placed on a table with its axis lying in the magnetic meridian, and with its north-seeking pole towards the north. State in what direction a compass-needle points (i) when placed imme- diately over the centre of the bar magnet, (2) when gradually raised verti- cally upwards. N.B. — The compass-needle can only turn about its pivot in a horizontal plane. (S. and A. Exam., 1888.) 11. How would you construct an astatic needle out of a uniformly mag- netised strip of watch-spring, which you are aDowed to bend or break as you please ! (S. and A. Exam., 1887.) 12. Two equal and equaDy-magnetised bar magnets are fastened together 70 LECTURE Vm. — QUESTIONS. at their centres at right angles to each other, so as to form an equal-armed cross. How wOl the cross set itself when balanced at the middle upon a point ? 13. An astatic combination of two magnets is injured so that the mag- nets are at right angles instead of parallel to each other. If it be sus- pended as usual, what position wiU it assume with regard to the magnetic meridian ? Illustrate your answer with a diagram showing the forces which act upon the magnets. (S. and A. Szam., 1888.) 14. The beam of a balance is made of soft iron. When it is placed at right angles to the magnetic meridian, two equal weights placed in the opposite pans just balance. WiU the weights still appear to be equal when the balance is turned so that the beam swings in the magnetic meridian? Give reasons, with sketches, for your answer. (S. and A. Exam. 1889.) 15. Apiece of steel wire, bent so as to form two sides of a square, is magnetised in such a way that each of its free ends is a North pole, and the bend a South pole. When placed upon a cork floating in water, how will it set ? (S. and A. Exam. 1890.) 16. A rod of iron, AB, held vertical with the end B downwards, is smartly tapped with a mallet. When turned into a horizontal position and brought near to a compass needle, the end B repels the North pole of the needle at a distance of four inches, but attracts it when the distance is reduced to one inch. Explain this. (S. and A. Exam. 1890.) 17. A tall iron mast is situated a little in front of the compass in a wooden ship. Explain the nature of the compass error when the ship is sailing in an easterly direction (i) in the northern, (2) in the southern hemisphere. (S. and A. Exam. 1891.) 18. A rod of iron when brought near to a compass needle attracts one pole and repels the other. How will you ascertain whether its magnetism is permanent or is due to temporary induction from the earth J (S. and A. Exam. 1891.) ( 71 ) K.B, — Along with this Appendix the more advanced student should refer to a-paper read by W. H. Preece, F.R.S, before the British Association Meeting, 18901 on Experiments with different brands of Steel and their Magnetic Intensities. Also to Prof. S. P. Thompson's Cantor Lectures on the " Klectio Magnet," delivered before the Society of Arts in 1890. APPENDIX. PEAOTICAL NOTES ON MAKING EXPERIMENTAL APPARATUS FOR STUDYING MAGNETISM. Preliminary Note. — Every student of Magnetiam and Electricity has naturally a great desire to make and to experiment with the apparatus illustrated in his text-book, or shown and described to him by his Teacher in the Lecture-Koom. This desire is often thwarted by the idea that the materials and necessary tools are beyond his means ; and even should he have these at his command, unless he is a trained mechanician, or has a natural aptitude for such work, he probably becomes disheartened by a failure in his very first attempt to make some simple piece of apparatus — or, having made it, to get the desired results from it. Now, the materials and tools are of the cheapest and simplest kind, and may be procured gradually, as he requires them, from any optician and a tool shop or second-hand shop that deals in hardware implements. Failure in making the apparatus, or in getting the desired results from it when made, generally arises from not following some prescribed method which has been found successful by others. In The Glasgow and West of Scotland Technical College, the Author has conducted for some years Practical Electrical Instrument-Making Glasses, where the Day Students are each provided with a bench and drawer to hold their own tools, and they have in addition the use of the College engine, dynamos, lathes, grindstones, and drilling-machine, &o. Under such cir- cumstances, he finds that the Electrical Students not only take a very deep interest in their practical work, but that they obtain a much more thorough and lasting knowledge of the lea ding principles of Magnetism and Electricity than can be gained from merely attending lectures. He also finds that Elementary Evening-Class Science and Art Students, who make apparatus and experiments at home for the purpose of more thoroughly understanding the Lectures and the Class questions, are thus better prepared for the Advanced Class and the Electrical Engineering Laboratory than if they merely studied text-books The following notes are but a few examples of what the Elementary or ■First-year Students are in the habit of making in the Magnetism Section. Further examples of Electrical Apparatus will be g' ven under the Appen- dixes to the sections on Voltaic and Frictional Electricity. If these notes are appreciated, they will be extended in future editions. To Make a Permanent Magnet. — (i) Read carefully over the following instructions, and then make a full-size or scale drawing of the 72 APPENDIX. — PRACTICAL NOTES. shape and size of magnet required. For private experiments a convenient size is 6" X i" x i" for a bar magnet, tut for most lecture-room experiments it should be not leas than 12" x i" x |", or if cylindrical, 12" x |" diameter. Bar Magnets. (2) Procure a piece of good close-grained, rolled steel that has not been heated since it was iirst made, of the shape 'and size required.* (3) Trim the ends neat and square by a file, and put on a file-mark (as shown in the above figure), or type one end with the letter N. (4) Put the steel into a clean bright fire, and let it become gradually heated until it has reached a moderately bright red ; take it out by the tongs, and dip it level and sideways into a can or dish of cold water, moving it about until it is quite cold, thus tempering it glass-hard. Care must be taken that the steel is never made a bright red or white-hot, and that it is uniformly red all over its length and on all sides just before being put into the water. A coke-fire ia naturally better than a coal-fire, but best of all for heating the steel is one of Fletcher's Metallurgical Laboratory Gas-Ovens. Magnetising Bae Magnet. The hardening must be done promptly and, if possible, at a first heat, for we find that steel will not make such a good magnet if heated a second or third time with the view of getting better results from it. (5) Magnetise the tempered bar with the N or marked end as a North * Messrs. Spagnoletti and Crookes, Adelaide Works, Uxbridge Eoad London make a speciaUty of supplying the best magnetisable steel at very moderate prices'. HOW TO MAKE A BAR MAGNET AND A COMPASS-NEEDLE. 73 pole by any one or other of the processes described in Lecture I., remem- bering, however, that if you have a good electro-magnet and strong battery or dynamo-ourreat at your disposal, wiis method -will yield the best results ; and do not rest satisfied unless your magnet will easily lift another piece of hard steel of its own weight, or from three to four times its own weight of soft wrought iron. (6) With two such magnets, a sheet of paper, and some para£Sne wax, &c., you are in a position to perfoi-m the experiments illustrated and explained in Lecture IIL, re making magnetic curves, &o. To Make a Compass-needle and Stand. — (i) Read over the followinsr instructions, make a full-size drawing of side view and plan, and procure the materials required. (2) Chisel or turn the base from a piece of mahogany, or other bit of hard wood, and after boring a small hole fair through the centre to &t a large darning- needle, fix the latter tightly and vertically into the hole with the pointed end upwards. (3) Uijou a piece of thin strip steel 4" x i" x-gij", or a piece of strong clock-spring or crinoline steel, describe the outline plan of the needle; keeping the ends rounded, unless the needle should be required for taking deflection readings on a graduated card. The advantage of rounded ends when the needle is simply used for indicating the presence or polarity of magnets or currents is, that for the same length of needle you get more mass, and, therefore, greater magnetic turning effect ; moreover, junior students are liable to break the fine points. (4) Soften the strip by first putting it into the fire or into the flame of a Bunsen burner or spirit-lamp until it gets dull red, and then letting it cool slowly in fire-ash. Bore a tapered hole, about ^" or J" diameter, at the centre of the steel, and with a chisel or a metal-shears cut the needle roughly into shape, just leaving sufficient margin to file or grind it true to the drawing. (5) Make a glass or hard brass, ^, centre to fit the hole in the needle. A glass centre is constructed from thin soft English-made glass tube, about i" outside diameter, by taking a piece of it about 6" or 8" in length, and holding the middle of it in the flame of a Bunsen burner, or blow-pipe, or spirit-lamp, and turning it round and round between the fingers and thumb of each hand (applied to each end of the tube) until it gets uniformly soft at the part held in the flame ; then suddenly pulling fair and straight with each hand in a line with the axis of the glass tube until it is separated into Compass-needle. 74 APPENDIX. — PRACTICAL NOTES. two pieces with conical ends. Now take one of these pieces in the right hand and turn the fine conical point in the flame until a neat rounded end is formed thereat Try the cone into the hole in the needle, and mark on the glass close to the under-side of the needle. Take it out, and with a A file cut a groove fair round the tube. Put the groove line into the flame, and applying a little pressure to the point of the cone, break it off from the rest of the tube. Grind the rough edge of the base of the cone on the tile until it is smooth. A brass, /\, centre may be turned in the lathe from a piece of hard brass rod. This requires some skill in brass-turning to make a neat, light, deeply bored centre. (6) Temper the needle in the manner described in the previous and the next set of notes. (7) Fix the, /\, centre evenly into the hole with a little melted shellac, or balsam-cement, or glue. (8) Magnetise the needle by one or other of the methods explained in Lecture I. (9) Now try the needle upon the pointed suppart, and you will probably find that the W-end dips. You will consequently have to grind a little off that end i^ntil it balances evenly, and again magnetise it to saturation. (loj Paint the If -end red with vermilion paint, and the S-end blue with Prussian blue. You can now perform all the experiments mentioned in Lectures III. , V., &c., wherein the compass-needle bears a part in testing the polarity of magnets, &c. To Make a Dipping-needle and. Dip Circle. — A simple and cheap form of this instructive piece of apparatus may be easily made by students themselves, as explained by the accompanying figure and descrip- tion. The form illustrated below wiU be found sufficiently delicate for the laboratory work of junior students, or for class lecture purposes. Operations. — (i) Kead carefully over the following instructions, and then make a full-size drawing of the apparatus similar to the above figures.* Mark thereon the more important dimensions, make out a list with sizes, of the various things required, and procure them. (2) From a well-seasoned plank of yellow pine i' thick, make the base, B, and the upright board, UB. Secure them together truly at right angles to each other by three or four brass screws. If of large dimensions it may be necessary to affix four neat brass (L) corner pieces between, B, and, UB, about 2" from the ends of the latter. (3) From hard wood (mahogany or birch) shape out the horizontal bracket, HB, and turn or chisel the back central nipple for, UB. Fit neat brass pinching screws into the parts indicated by, PS, on the above sketch. (4) Glue on the hard-wood centre piece to the back of, UB. Dowel pin, and then screw on, HB, with brass wire and brass screws. (5) Procure two pieces of soft English glass rod about 6" long by J" diameter, and indent sharp, V, grooveacentres in one end of each by holding the rods in turn with the left hand in the flame of a blowpipe or Bunsen burner, whilst you press fairly and evenly forward in a line with the axis of the glass rod a heated fine steel pricker, draw point, or stout darning-needle. (6; Level the base board, B, by an ordinary spirit-level, and by means of The next drawing to a scale of ij" to the foot is a suitable size for lecture purposes. HOW TO MAKE- A DIPPING-NEEDLE. 75 a surface-gauge or a set-square mark oflf centres on the front and back of, HB, and, UB, all on a level line. Then bore out a true, fair, and level hole right through to fit the shanks of the glass centre rods, GC, and insert them into the holes. Db Ml J^^^^MlB* 1 ^f^y/^Pay##4gy#j ^^^^m Dipping-needle and Dip Circle. Kote to Figure. — la the plan of the above figure the needle has been intention- ally drawn full length, as if it lay horizontal, to show that it clears the insides of the horizontal bracket. Index to Side View and Plan. NS represents K and S poles of the dip needle. A CW GC PS B UB HB Oto90 LG Axis of needle. Central washers to needle. Glass, V, centres. Pinching screws. Base (of wood). Upright board. Horizontal bracket (of hard wood or brass). Scale on cardboard or on paper. Looking-glass (to assist in avoi^jng paraMax error). 76 APPENDIX. — PRACTICAL NOTES. (7) MaUng ike Dipping-needle.— {a) Procure a piece of good magnetisable steel atrip 8" to 12" long, according to the size of the dip circle, f " to J' broad, and about ^V" thick. Special steel strip may be easily secured from the magnet makers, or a strong clock-spring or the blade of a worn-out metal saw may be ground and dressed up to suit. (6) Shape the < > ends by a cliipping-chisel or metal-shears, and by grinding. Soften the middle, and bore a hole truly at right angles to the strip to fit tightly a darning-needle about :" along from the point. Grind the other end of this darning-needle to a correspondingly fine, true smooth point, and force it into the hole in the steel strip, (c) Insert this axle. A, between the glass centres, GO, and revolve the dipping- needle so as to test whether the axis is trulg at right angles to the strip, and whether the glass centres are fair and level with each other. If the dipping-needle should ^ U, be " off the truth," or square, as it is technically termed, SZ?^ it=£3 youmayhave to widen the hole and press tight on to. A, — ^^^X| PI two ebonite or cardboard washers, CW, and fix them to, NS, with balsam or glue, taking care that these washers dry to the dipping-needle with the needle truly at right angles to its axis.* If the glass centres are not fair and level to each other, turn one or both of them round until they are so, mark the position opposite to the pinching screws, PS, and grind a short flat piece on each of them, so that the points of these pinching screws may always naturally fix the glass centres in the adjusted position, [d) Now balance the dip- ping-needle very accurately by grinding off a little from one end or the other until, when set horizontal, it will remain so, although the axle may he perfectly free between its centres, {e) Next take the needle out, and carefully temper the ends glass hard until within an inch or so of the centre. There is considerable liability of such a length of thin steel strip becoming warped or twisted in this tempering process, but if you take a length of soft fine iron wire and bind it round the strip from the middle to the end before heating it to a dull red, and then dip the strip verticaUy into the cold water, you should succeed without bending the needle. (/) Finally, magnetise the dipping-needle by one or other of the methods described in Lecture I. If you have not got an electro-magnet or a long solenoid capable of taking in the whole length of the needle, then you should use the divided touch method. (8) From a piece of stout white cardboard (fully ^" thick) cut out the complete circle for the scale, with indents, so as to let it be curved and slipped in between the inside ends of, HB. Cut out a central hole the size of, GC, and two quadrants or sectors or segments to fit two pieces of looking-glass, L6, of the same thickness as the cardboard. Glue on the cardboard and the two pieces of looking-glass. (9) Thoroughly clean the points of the axle and the V centres, and re- place the dipping-needle between the glass centres so that it is perfectly free to move round with a minimum of lateral movement. Stick on a weighted cork to the, N-end of the needle, turn the plane of, UB, east and west magnetically, carefully level the base with a spirit-level, and look fair upon the needle until its reflection in the looking-glass is exactly hidden. Mark the position where the point of the S-end comes to rest on the card. Mark this point 90°. Kemove the weighted cork, and see if the * Some mochaniciaus prefer to screw the axis and fit on two brass washers, as shown by the small figure above, but this plan is rather too difficult a lob for nn ordinary student. ' HOW TO MAKE A DIPPING-NEEDLE. "J "J needle again comes to rest exactly as before. Mark ia the same way as before a point go° opposite the IT-end of the needle. (lo) Kemove the needle (and, if necessary, the horizontal bracket), and with one point of a pair of compasses placed in the back, V, centre describe the various circles, and divide them off neatly, as shown by the above sketch, but with tentlis of degrees added in the full-size card. Students who are not skilled or tidy may find that they have so soiled the cardboard that they would prefer to draw the circles upon a sheet of paper, and after- wards paste the same upon the cardboard. They must be careful, however, that the two 90° coincide with the vertical positions just found under operation (9) for the needle, when it is at right angles to the magnetic meridiao, and that the paper circle is finally fixed dead true with the back, V, centre. Necessary Observations to Find Out the Mean Angle of Dip for tJte Place and Needle. — (1) Again place the apparatus so that the needle hangs vertical — i.e., with I^ and S exactly opposite the lower and upper 90° re- spectively. When the base has been carefully levelled, run a pencil mark round the edge of the rectangular base. Now turn the base round through 90°, level the base, and take a reading of the needle. This tells us roughly the angle of dip, for the plane of the needle's motion is now in the magnetic meridian. Suppose the reading to have been 70°. If we accepted this as the true dip, we assume three improbable things about the needle — (a) That its magnetic axis coincides with the line joining the < > points. (6) That its centre of mass coincides with its centre of motion both in regard to its breadth and its length. (c) That there is no frictional error or no friction between the axle and the glass centre bearings. We have, therefore, to take the mean of eight sets of readings (of which the following series is an example) before we can say with any degree of accuracy what is the true dip. Inclination Test, made in The Electrical Engineering Laboratory of The Glasgow and West of Scotland Technical College, by A. H. Allen, Oct. 1889. ist. — To find the position at right angles to the Magnetic Meridian : — Circle facing North Pointer on horizontal scale at 100°. I^eedle at 90°. „ „ South „ „ „ i36°.3 Mean position of pointer „ „ ii8°.is )■ „ Circle then turned through 90=, painter at 28". 15 on scale. 2ud. — To find angle of dip : — • Marked end North seeking, KT-poIe S-pole „ Beading'. Beading. ^'^"• I. Circle facing West, Face of Needle towards Circle 73°. 10 72°.5o 72°, 80 2- „ „ „ ,. from „ 79°.S2 79°.52 790.52 3. „ „ East, „ „ „ 73 .25 72 -So 72''.92 4. „ „ „ „ towards „ 79°.70 79°.70 79°-70 Marked end South-seeking, 5. Circle facing West, Face of Needle towards Circle 67°. 80 68". 15 67''-9S 6. ,. „ „ „ from „ 53°. 7S 53° IS S3°-4S 7- „ ., East „ „ „ 70°.9S 70'>.9S 700.95 8- I. „ .. ., towards,, 58°. 15 57°. 55 57°. 85 Mean of the Means of all readings 68°.39 PRINTED BY BALLANTYNE, HANSON AND CO. LOUDOIT AND EDINBURGH CONTENTS TO PART II. LECTURE IX. PAGES Electro-Magnetism — Supply of Current for our Experiments may be Derived from Batteries or Dynamos — Magnetic Field of a Straight Current — Direction of the Magnetic Eield of a Straight Current — Direction of Currents in Conducting Wires — Specimen Question and Answer — Questions . . . 79-S7 LECTURE X. Simple Apparatus for Studying the Magnetic Action and Direction of Electric Currents — Simple Galvanoscopes, or Simple Ver- tical and Horizontal Current Detectors— Multipliers or De- tector Galvanometers — Specimen Question and Answer — Questions 88-95 LECTURE XI. Magnetic Eield and its Direction as Due to a Circular Current — Intensity or Strength of the Magnetic Eield at the Centre of a Circular Current — Simple Tangent Galvanometer— Sine Galvanometer — Table of Natural Sines and Tangents — Questions ... 96-104 LECTURE XXL Electro-Magnetic Solenoid — Magnetic Field Inside a Solenoid and its Direction — Magnetic Field Outside a Solenoid and its Direction — Combined Effect of the Magnetic Fields Due to a Permanent Magnet and an Electro-Magnetic Solenoid — Sir William Thomson's Graded Tangent Galvanometers — Sir William Thomson's Mirror Galvanometer — Simple Astatic Galvanometer — Questions . lOJ-Il LECTURE Xin. Magnetic Polarity Due to a Straight Current — Magnetic Polarity Due to a Circular Current — Magnetic Polarity of an Electro- Magnetio Solenoid — Given the Direction of the Current in a Solenoid, to Find the N and S Poles of the Solenoid, and ficc vend — Specimen Question and Answer— Questions . . 116 -121 c CONTENTS. LECTURE XIV. PAGES Magnetisation of Iron and Steel by an Electric Current— Definition of an Electro-Magnet— Magnetic Field of an Electro-Mag- net—Attractive Force of an Electro-Magnetic Solenoid towards an Iron Core— Blyth's Current Meter— Horseshoe Electro-Magnets, with Practical Examples— Alteration in the Length of Iron when Magnetised— Questions . . 122-131 LECTURE XV. Action of a Force and the Reaction against it are always Equal and Opposite in Direction— Rotation of a Magnetic Pole Round a Current, and of a Current Round a Pole — Faraday's Apparatus for Exhibiting the Rotation of a Current-Carrying Conductor Round One Pole of a Magnet— The Automatic Twisting of a Current- Carrying Wire Round » Magnet — Questions 132-138 LECTURE XVI. Electro-Dynamics — Ampere's Laws — Action between Parallel and Inclined Currents— Ampere's Stand — The Jumping Spiral, and other Apparatus for Illustrating Ampere's Laws — Questions .... 139-146 LECTURE XVII. Electro-Magnetic Induction — Currents Induced in a Closed Circuit by the Motion of a Magnet in its Vicinity, or vice versd — Currents Induced in a Closed Circuit by the Motion of a Current- Carrying Coil in its Vicinity, or vice versd — Diffe- rent Directions of the Induced Currents on Approach and Withdrawal of a Secondary Circuit Moving in the Primary Field — Induced Currents in a Closed Secondary Circuit on Making or Increasing, and on Breaking or Diminishing, the Primary Current — Table of Induction Currents — Faraday's Law — Lenz's Law — Electro-Motive Force, Resistance, and Current— Comparative Statements of the Forces, Resistances, and Currents, Illustrated by Hydraulic and Electrical Cir- cuits — Ohm's Law — Questions 147-157 LECTURE XVIII. Historical Note on the Discoveries of Galvani and Volta, &c. — Volta's Pile— Origin of the Terms Voltage and Volt, &o. — Simple Voltaic Cell and Its Chemical Action— Polarisation — Local Action — Amalg.imation of Zinc Plates — Daniell's Cell and its Chemical Action — Finding the Fall of Potential through a Cell, and Measuring its Internal Resistance — Different Forms of Daniell's Cell- Grove's and Bunsen's Cells and their Chemical Action, &c. — Questions . , , 158-171 CONTENTS. LECTURE XIX. Heat is Developed when a Force Overcomes a Resistance — Table of Good Conductors, Partial Conductors, and Non-Conductors — Illustrations of the Conversion of Electric Energy into Heat — Heat is Developed by a Current in every Part of its Cir- cuit — Heat Developed by a Current in any Part of a Circuit is Proportional to the Resistance of that Part, and to the Square of the Current— The Resistance of a Conductor is Inversely Proportional to the area of its Cross Section — Questions 172-179 LECTURE XX. Polarisation Inside a Single Fluid Cell, Illustrated by the Magic- Lantern — Electro-Chemistry, or the Decomposition of Liquids by Electric Currents — Definition of Electrolysis, Electrolyte, Electrodes, Anode, Cathode, Ions, Cathion, Anion, Migration of Ions, Velocity of Ions, Voltameter — Electrolysis of Water — Electroplating — Electrotyping— Determining the Direc- tion of a Current in a Circuit and the Poles of a Battery or Dynamo by Electrolysis — Questions 180-191 APPENDIX TO PART II. Practical Notes on Making Experimental Apparatus for Studying Voltaic Electricity . • 192-200 ( 79 ) ELEMENTARY MANUAL MAGNETISM AND ELECTRICITY. PART II. BLBCTEO-MAGNETISM. LECTURE IX. Contents. — Electro-Magnetism — Supply of Current for our Experiments may be Derived from Batteries or Dynamos — Magnetic Field of a Straight Current — Direction of the Magnetic Field of a Straight Current — Direc- tion of Currents in Conducting Wires — Specimen Question and Answer — Questions. Electro-Magnetism. — Part I. of this Manual was chiefly devoted to illustrating and describing experiments with a view to proving the primary laws of Magnetism and the fundamental principles underlying their practical applications. In Part II. we shall now treat in a similar manner of Electro-Magnetism and Electro- Kinetics or Current Electricity. The study of Electro-Magnetism naturally follows directly after that of Magnetism. For, when- ever and wherever an electric current exists, for however short or long a time, a magnetic field is always created along and around the whole path of the current. Moreover, some of the most im- portant practical applications of Magnetism, as in the case of Telegraphy, Telephony, Electric Lighting, &c., necessitate the use of electric currents for their action. Supply of Current for our Experiments may be Derived from Batteries or Dynamos. — In the first place, we shall take for granted that we can derive Electric Currents from batteries or dynamos without expecting that the student knows anything more about these appliances than the fact that they are sotirces of electrical energy, from which we can obtain at pleasure any F 8o LEOTUEE IX. desired supply of current for the purposes of our experiments. Later on, we shall describe in detail the construction and erection of several forms of batteries and the principles upon which Electric Currents are produced by dynamos. In most of our diagrams we shall adopt the method now commonly employed by practical Electricians of illustrating a Battery by alternating long thin and short thick parallel lines thus, — | with the IronFilinga , + , sign to indicate the positive end or terminal where the current leaves, and a , — , sign to indicate where the current returns to this store or source of electrical energy. Also, when the " circuit " or path for a current through a conductor is " closed " or complete, we shall, when necessary, indicate the assumed direction of the current by arrows placed at intervals along the circuit. This simple arrangement will instantly convey considerable informa- tion to the student without the necessity for any further explana- tion in the text. If a dynamo should be substituted for a battery, then the electrician's symbol, -h Q- , for the same will be used, with , + , and , - , signs to indicate the positive and negative ter- minals, or where the current leaves and returns to the dynamo. Magnetic Field of a Straight Current. — Experiment I. — Take a battery, a copper wire, and some soft iron filings. Join the , -I- , and the , — , ends of the battery by means of the copper wire, and then dip the middle of the wire into the iron filings. We find that the iron filings adhere to the wire and form a cluster round it not vmlike what a fresh swarm of bees are in the habit of doing round the first branch of a tree upon which they happen to alight, or like a crowd of ants gathered together upon a sweet- ened stick. In fact, the filings are attracted and magnetised just as if the wire was surrounded by innumerable magnets with their axes forming tangents to it all along the wire. In Lecture I., we saw that when we dipped a bar magnet into iron filings, the filings adhered to the ends only ; but in the present experiment we find the filings clinging equally well to the wire whenever we dip it amongst them, and along its entire length. Experiment II. — ^Take a stout copper wire ^ inch diameter (No. 12 Standard Wire-Gauge), a piece of wood or thick card- board (8 inches square), a sheet of parafllned paper (like that used in Experiment YIL, Lecture III., Part I.), and some iron filings contained in a pepper-pot or muslin bag. Fix the wire centrally .Bau&ry Current in Wire ATTRAOTma Ikon FlMNGB. MAGNETIC FIELD OE A STKAIGHT CUEllENT. aud vertically upwards through the board covered with the paraf- fined paper, as shown by the first three of the following figures. Connect the lower end of this wire to the , + , and the upper end to the , — , pole of a battery or dynamo by flexible wires, and thus pass a strong current (30 to 40 amperes) upwards through the vertical stout copper Then, whilst the current is flow- ing through the wire, shower down upon the parafiined paper from the pepper-pot or muslin bag (held about two feet above the paper) a quantity of soft iron filings, at the same time tapping the board gently with the other hand, and you will produce a graphic representa^ tion of the magnetic field set up by the current in the plane of the paper or at right angles to the direction of the current. In order to fix the filings in their places, you have only to BATTE/iY FlOTIKES ILLUSTBATXNG THE MaSNETIO FiELD DnE TO A STRAIGHT CnBMtfT-CAKBTIKQ WlBB. 82 LECTURE IX. pass a red-hot copper bolt or the flame from a Bunsen burner over (close to but not touching) them, so as to melt the paraffin wax (as explained at page 23, Part I.), when you obtain a diagram like that illustrated by the plan and lower left-hand figures. The right-hand figure is an imaginary picture of the magnetic field surrounding the whole length of the vertical wire, which is sup- posed to be conveying a current in the direction of the arrows. The magnetic lines of force follow circular paths or swirls around the current-carrying wire, for we observe that the iron filings and small magnetic needles brought within the range of the current's magnetic field are induced to place themselves as tangents to circles of which the wire forms the common centre, whether we pass the current up or down the wire. Direction of the Magnetic Field of a Straight Current.— -We shall now investigate the directions of the magnetic lines of force evoked by a current according as the current flows in one direc- tion or the other. Experiment III. — Place a freely -supported horizontal test- needle upon a table. You observe that its magnetic axis takes up a position in a line with the magnetic meridian, and that the North pole points northwards. Hold a straight copper wire fair above or below, and parallel to the magnetic axis of the needle. No deflection of the needle is observed ; but get some one to con- nect the ends of the copper wire to the poles of a battery whilst it is in one or other of these positions, and immediately the needle turns round from its natural position to one with its N-pole pointing towards the East or towards the West, according to the direction in which the current flows along the wire; i.e., according as one end or the other of the wire is connected to the , -I- , pole of the battery. This clearly indicates that there is a diflPerence in the direction of the magnetic lines of force produced by the current according as the latter is flowing in one direction or the other. To observe and to remember this precise difference, place the wire and connect it to the battery in the exact manner indicated by the figure on page 83. You observe that the N-pole of the needle turns from you. Now get some one to place his Bight Hand above the wire vidth the palm of the hand toioards the wire, and with the fingers pointing in the same direction as the current is flowing, and ask him to extend his thumb at right angles to his fingers, and consequently at right angles to the direction of the cur- rent. You observe that his thumb points in the same direction as the N-pole of the needle placed under the wire. Reverse the direction of the current by connecting the ends of the straight copper wire to the opposite ends of the battery, or reverse the MAGNETIC FIELD OF A STRAIGHT CUKEENT. 83 wire, and again holding it parallel to the magnetic needle, get your assistant to place his Right Hand as before (viz., with the palm towards the wire, and with the fingers pointing in the same direction as the current). His outstretched thumb will be found to point in the same direction as the N-pole of the compass or test needle. Place the wire below the compass-needle and pass the current in eiflier direction, and you will find that if the palm of the Right Hand be placed towards and on the opposite side of the wire to the compass-needle, the outstretched thumb will always indicate the direction in which the N-pole turns. This Satterl/ Dotted Cireleg and Arrows , theretm indicate direction ^iisJ of Current's Field of Force. Testing fob the Dibection op the Magnetic Field Due to the Knows DiEEOTIOK OP CHRBENT IN A STBAIGHT WiRE, OB VICE TBBSA. rule (devised by the author) is so easily remembered after a few trials that it can never be forgotten, and, moreover, it is far simpler than Ampere's rule or any of the many devices men- tioned in other text -books. Besides which, as we shall show later on, it can be applied to ascertaining the polarity of a solenoid, and with a dight modification to Professor Fleming's rule for finding the direction of a current in a Dynamo Gene- rator Armature, or the direction of rotation of an Electric Motor Armature. ExPEMMBNT rV. — First, Take the stiff copper wire and pass it vertically through the centre of the flat horizontal wooden board, a-s in Experiment II. Connect it to your battery and pass a strong current upwards through the wire as before. Whilst the current is flowing place a horizontal test-needle at some little dis- tance from the wire, and move it round the latter. You observe that it always lies with its magnetic axis as a tangent to the circle of which the wire forms the centre, and with its N-pole directed as indicated by the Bight-Hand Bnle. Now, if you draw a plan of this experiment as illustrated by the accompanying §4 LECTURE IX. figure, plotting out the directions of the magnetic Hnes of force as they naturally flow from and around the test-needle, due to its own ' inherent magnetism, as well as the circular magnetic lines of force due to the current, you cannot help noticing that the direction of the latter is the same as the needle's ovm lines through Hself. The needle, therefore, naturally places it- self so as to accommodate and assist the magnetic lines of force due to the current. Second, Pass the current downwards through the wire, and immediately your test- needle turns round so as to CDBEENTFlO^Q Up THROUGH f^jgj ^j^g ^^^ ^^_ J^pply the Right-Hand test and plot down a plan of the experiment, when you have the following figure : — ., ^ If we could produce a Free jifj N-pole unattached to a S-pole ^/""^ (which experimentally we can- X^ not do), we should find that this N-pole would revolve round the wire in a circle in the direction of the current's magnetic lines of force so long as the current lasted ; but see- ing that our N-pole is unavoid- ably attached rigidly to a S- pole (which is naturally equally impressed in the opposite direc- tion), the axis between them CmnENT FLOWING Down thhouoh "^^^t? ,"P f^fi^?'^ Vosit^?^ tan- Papbk. gential to the circular direction of the current's magnetic field. Hence The Eule. — The direction of the magnetic lines due to an electric current is the same as the natural direction of the magnetic lines through the body of a freely-suspended and otherwise unaffected test-needle brought within the range of the currents field. The converse of this rule is also most convenient, for it is very often of great importance to an electrician to know in which direction a current will flow or is flowing from a battery. DIRECTION OF CURRENTS IN CONDUCTING WIRES. 85 dynamo, or other source of electrical energy, without having to feel his way along the whole length of conducting wire to the source. Direction of Currents in Conducting Wires (Jamieson's Rule). — Experiment V. — i. Move the conductor (if possible) into the magnetic meridian. 2. Place a freely-suspended compass-needle below or above the wire. The current will deflect the N-pointing end of the needle to the left or to the right. 3. Place the right hand as it were in the wire with the palm next to the needle so that the outstretched thumb Eight Hand Above thb COKDUOTOB. Needle Below the Conductob. Right Hand Below the Conductor. Needle Above the Conddotob. coincides in direction with the deflected JT-pointing pole of needle. Then the current flows along the wire in the direction indicated by the arrows to the negative ( - ) pole of the dynamo or batterv. Specimen Question and Answer. Question. — A current flows through a telegraph wire between Edinburgh and London, but you do not know whether it conies from Edinburgh or from London. Supposing this knowledge desired, how would you obtain it ? Answer. — Since the telegraph wire joining Edinburgh and London is almost in a line with the magnetic meridian, a com- pass-needle placed close to and above the wire would naturally lie parallel to the wire if no current were passing in either direction. Suppose, however, we find that the N-pole of the needle is deflected to the East ; then if the Right Hand be 86 LECTURE IX. held tinder the wire and needle with the outstretched thumb in a line with the N-poIe of the needle, this indicates that the current is flowing Northwards, or from London to Edinburgh. If the needle should be deflected to the West, then by a simi- lar proof-test the current must be flowing from Edinburgh to London. [The student should make two sketches for himself to illustrate this answer to the question.] LECTURE IX. — QUESTIONS. 87 LeCTDEE IX. — QUBSTIONB. 1. A strong current is passed through a straight copper wire, and the wire 13 then dipped into soft iron filings. What is the result, and what do you infer from it 1 2. A strong current is passed through a wire. What is the condition of the medium surrounding the wire ? How would you prove your statements by experiments ? Give a sketch of how the lines of force arrange themselves in the vicinity of the wire. 3. A strong current is passed down a vertical wire. How would yon arrange and carry out an experiment whereby you would obtain a permanent record of the configuration of the magnetic field set up around the wire by the current ? If a test-needle were brought near and carried round the wire, what position would it take up, and why ? 4. State generally how the lines of magnetic force dae to an electric current passing through a wire arrange themselves. Illustrate your answer by a, sketch of the experiment, in which your Right Hand, aided by a compass- needle, will always enable you to determine their direction. 5. A vertical wire, down which an electric current is flowing, is held fl) due east, (2) due south of a small compass-needle. How is the needle affected in each case ? (S. and A. Exam., 1889.) Give two sketches. 6. Two compass-needles are arranged near each other so that both point along the same straight line. A wire connecting the ( -f- ) or platinum and ( — ) or zinc ends of a battery is stretched vertically half-way between the needles. How will the current in the wire affect the needles, and how will the result depend upon whether the ( -I- ) or platinum terminal is connected with the upper or lower end of the wire respectively ? (S. and A. Exam., 1887.) Give a neat sketch to illustrate each answer. 7. A wire lies east and west (magnetic) immediately over a compass-needle. How is the direction in which the needle points affected when a strong current flows through the wire (l) from west to east ; (2) from east to west 1 (S. and A. Exam., 1889.) Give two sketches. 8. A telegraph wire runs north and south along the magnetic meridian. A magnetic needle free to turn in aU directions is placed beside the wire, and on a level with it. How will this needle act when a current is sent through the wire from south to north ? 9. A current is flowing through a rigid copper rod. How would you place a small piece of iron wire with respect to it so that the iron m.ay be magnet- ised in the direction of its length ? Assuming the direction of the current, state which end of the iron will be a north pole. (S. and A. Exam., 1891.) ( 88 ) LECTTJEE X. Contents. — Simple Apparatus for Studying the Magnetic Action and Direc- tion of Electric Currents — Simple Gralvanoscopes, or Simple Vertical and Horizontal Current Detectors — Multipliers or Detector G-alvanometera — Specimen Question and Answer — Questions. Simple Apparatus for Studying the Magnetic Action and the Direction of Electric Currents. — Expektment VI. — ^Take a piece of apparatus of the following shape and construction,* and place it upon a table, so that the magnetic needle lies parallel with the wires Wj, Wg. First, Pass a current from a battery Apparatus for Testing the Direction op CtrERENTS. through the upper wire, Wj. Observe the direction in which the N-pole of the test-needle turns, and by applying the Eight Hand ■ in the manner described by Experiments III. and V. in Lecture IX., determine the direction in which the current flows along the wire Wj, and whether terminal Tj or Tg is connected to the , + , pole of the battery. Eeverse the position of the battery leading wires and again perform the experiment. Second, Pass a current through the lower wire, Wj, and then reverse the connecting wires, and in each case determine the direc- tion of the current, and whether terminal Tg or T^ is connected to the , + , pole of the battery or other source of electrical energy. * See Appendix to Part II. for a description of how to make this form of Oersted Apparatus. ELECTRIC CUEEENTS. 89 In each of these four cases,* when the current is flowing along one or other of the wires, it is evident that the Earth's magnetic force (by its action on the needle's magnetic force) is constantly tending to turn the needle back to the normal position {i.e., in a line with the plane of the magnetic meridian of the place), whilst at the same time the current's magnetic force is deflecting the needle (by its action on the needle's magnetic force) away from the magnetic meridian. The position which the needle ultimately takes up (if the current is kept constant) is the resultant direction due to these two forces acting simultaneously on the magnetic force of the needle. In other words, the needle is deflected from its normal position by an amount depending upon the strength of the current — the Earth's field and the distance of the needle's poles from the wire being considered constant. Experiment VII. — Connect terminals Tg and T3 by a short copper wire, and terminals Tj and T4 to the battery. Tou now observe that the deflection of the needle is greater than when the current flowed along only one of the wires. Apply the Right- Hand test to each of the wires, and thereby determine the direc- tion of the flow of current, and reason out from this, by aid of a sketch, that the directions of the current's magnetic field in Wj and Wg both act upon the needle's field so as to assist each other in overcoming the restraining action of the Earth's field. Reverse the battery connecting wires between T^^ and T^, and again you observe that the deflection is greater than in Experi- ments VI., but in the opposite direction to that of the above experiment. The usual form in which this apparatus is made and supplied by electrical instrument-makers to teachers is illustrated by the accompanying figure. It is termed the " Simple Oersted Appa- ratus," from the fact that the first published account of the direc- tion in which a magnetic needle turns in obedience to (the mag- netic force evoked by) an electric current flowing parallel to the needle's magnetic axis, was described by Oersted of Copenhagen * The studeut should make four separate sketches explanatory of each of those four experiments. He should show by dotted lines and arrows — First, The direction of the Earth's magnetic lines of force. Second, The direction of the Needle's magnetic lines of force. Third, The direction of the Current's magnetic lines of force. By so doing he will most forcibly bring to view the direction in which the N-pole of the needle must turn in obedience to direction of the current's mag- netic field, &c. If in each case he draws a Eight Hand properly placed with respect to the flow of current and the test-needle, he will find that the direc- tion of the outstretched thumb will agree with the direction in which the N-pole turns. 90 LECTUEB X. Simple Oersted Apparatus. on July 21, 1820.* It consists of a stout copper wire, bent into the form shown, and fixed by its ends to a flat wooden base. The ends of this bent wire are each connected by a copper strap or wire to a binding screw or terminal also fixed to the same wooden base. A central brass, copper, or wooden pillar, end- ing in a sharp-pointed steel needle, so sup- ports the horizontal test-needle that it lies midway between the upper and lower horizontal portions of the thick copper wire. This form of the apparatus does not, however, permit of the following instructive modification of the above experiments, which we shall now describe by aid of the former diagram. Con- nect terminals Tj and T^ by a fairly long copper wire laid along the table, so that a current passing through this wire will not affect the test-needle. See that the needle lies midway between Wj and Wj, and shift round the apparatus until the wires lie parallel with the magnetic axis of the needle. Now connect ter- minals Tj and T3 to your battery, and you should find that no deflection of the needle is produced by the current as it flows from Tj to T^, and also in the same direction from T^ to Tg (or vice versa, as a whole), for the direction of the current's field around Wj is contrary in its effect upon the needle's field to that around Wj. These two current fields, therefore, cancel each other's effects upon the needle's field. Should the needle, how- ever, be ever so little nearer to one of the wires (Wj or W^) than to the other, then the needle will be deflected by a small amount, due to the difference of the intensity of the two current fields at the position of the needle. Simple Galvanoscopes or Simple Vertical and Horizontal Current Detectors. — When it is desired to note, for lecture pur- poses, the direction and, roughly, the relative strengths of strong currents passing in a circuit, we may employ either a vertical or a horizontal galvanoscope or detector of the forms shown by the accompanying figures. The deflections of the needle are read off * See Journal of the Socicly of Telegraph Engineers for 1876, pp. 459-469, for a verbatim copy, in Latin, and translation of Professor Oersted's original communicatiun on his discovery of electro-magnetism, dated Copenhagen, 2 1 St July 1820. CURRENT DETECTORS. 91 Vertical Galvanoscope ok Current Detector. upon a graduated scale placed parallel to the plane in which the needle turns, and if made of sufficient size, with the N-pole of the needle painted bright red and the S-pole bright blue, the action of the needle when under the in- fluence of the current may be clearly observed by a large audience. The current in each of these two detectors is conveyed but once along the /roni and returns but once along the back of the needle. As we have already pointed out, the mag- netic force exerted by each of these portions of current acts in the same direction upon the needle. If we should, however, require a more sensitive instrument so as to produce considerable deflections of the needle with weaker currents, then all we have to do is to pass the current through an insulated conductor wound many times around the needle. Multipliers or Detector Galvano- meters. — Such an instrument as we are about to describe is sometimes called a " Multiplier," since the curi-ent's effect upon the needle is thus increased by a certain value depending directly on the number of times that the current circu- lates round the needle.* It is, however, generally termed a " Detector Galvano- meter," the single term Galvanometer being reserved for a still more delicate and accurate instrument for measuring current strength in electro-magnetic measure. ■niere are many kinds of Galvanometers, horizontalGalvanoscope such as Astatic, Tangent, Sine, Differen- or Cdeeent Detector. tial, Ballistic, Sir William Thomson's Mirror, Marine and Graded Galvanometers, Deprez-Darsonval Permanent Magnet and Movable Coil Galvanometer, (fee. We shall explain in an elementary manner a few of these instru- * The magnetic field set up by a current flowing once along parallel to the magnetic axis of a freely- suspended needle has just half the moment or turning effect or " torque " upon the needle that the current has if it returns parallel to the magnetic axis on the opposite side of the needle and at the same distance therefrom. 92 LEOTUKE X. Detisctob Galvanometer fob Class Illustration. ments in this Manual, and the whole of them fully in our Advanced Text-Book. The term Galvanometer is derived from the name of an eminent physician of Bologna named Galvani (who first discovered in 1 786 the presence of electric currents when experimenting with a frog's leg), and the Greek word /ier^m (metron), a measure. Hence — By Definition. — A Galvanometer is an instrument for measur- ing the strength of galvanic {or electric) currents. A Detector Galvanometer for class illustration purposes, as usually made and sold by electrical instrument-makers, is shown in perspective by the accompanying figure. It consists of a narrow 1 — 1 shaped wooden bob- bin, upon which are wound about 100 turns of No. 20 copper wire insu- lated with cotton to prevent the current short circuiting froin one turn to another, the two ends of the wire being also connected to terminals fixed into the wooden base. The card is divided off like the , ^ ^^~g" J\ >A-.^>, manners compass, and the needle |y\ is supported upon a fine pin-point between the upper and lower set of layers of insulated conductors. The two opposite figures show a sectional elevation and plan of the same form of instrument, which students should make for themselves.* Here not only the ■i i\ -^«i^^ •■■«' jK^ ah instrument itself is shown, but /( \^^i£r°:^S.^^/\ ^^° *^® battery, leading wires, and the direction of the current as it flows through the insulated copper conductor, together with the corresponding direction in which the needle is thereby de- flected. In the next figure we have purposely given an outside view of the form which this instrument takes, when supplied to the * See Appendix to Part II. for index to parts and instructions how to make a Detootor Galvanometer. Battery Detector Galvanometer. MULTIPLIERS OR DETECTOR GALVANOMETERS. 93 Tolegi-aph or Telephone Line-Man, or to the Electrical Engineer, as a simple portable detector or rough-and-ready measurer of the presence, direction, and strength of electric currents ; for it is advisable that elementary students should be able to recognise at a glance the names and the uses not only of the instruments adopted in the class-room^ and laboratory, but also of the correspond- ing instruments employed in the practical applications of electricity. All that thj operator has to do in order to use such apparatus is to join the outstanding ter- minals to the two ends of a circuit (or one end of the leading wire, or the telegraph line to one terminal, and the other terminal to the " Earth " t) in order to detect whether a current is passing or the circuit is complete. If he observes any deflection of the needle (whether to the right or to the left), he thereby understands from a previous test the direction of the current, and from the number of degrees deflec- tion which the needle gives he obtains an approximate idea of the strength of the current. WOODHOnSE, Rawson, & Co.'s Lnre-MAM's Detectok.* Specimen Qdestion and Ans'rter. Question. — Suppose that you were the only person at a very out-of-the-way Telegraph St-ation, and that you had no proper testing apparatus for localising the position of faults; in fact, nothing but the compass-needle attached to your watch-chain and a length of insulated fine copper wire. Suppose that the single telegraph line between your office and the next one gets broken, and that upon permanently connecting the , -I- , pole of your battery to the line, and the , — , pole to the eai-th, you observe that no current passes to line. How would you proceed to find the fault ? Answer. — Taking your compass-needle and the insulated copper wire with you, walk or ride alongside of the line for some dis- tance, keeping a sharp look-out for the break. Suppose that 3'ou cannot find it in this way. Then bare one end of the fine wire, and * The three terminals on this instmment are connected up with two sets of coils, the one a thick wire of short length, and the other a finer wire of long length, to suit different lengths of circuits. t The technical sense in which the word " Earth " is used by electricians will be fully explained later on. Here it may be taken to mean a connection between the galvanometer and a water or gas pipe or a plate sunk in damp soil or in a pond. 94 LECTUEE X. climbing one of the telegraph poles, firmly twist that end of the fine wire to the telegraph wire. Returning to the ground, con- nect the other bared end of the fine wire to the Earth wire which runs down the side of the pole. Now coil round two, or three, fingers of your left hand the middle portion of the fine insulated wire, so as to make up a temporary bobbin or coil of a " Detector Galvanometer." * Place this coil lengthwise in the magnetic meridian with the fiat parts horizontal or the plane of the coil vertical, then insert your compass-needle inside the coil. If you get a deflection of the needle, you know that you have not yet reached the fault or break, for your station battery sends a cur- rent through the line, and your temporary detector or multiplier to Earth at the pole. You may therefore repeat this simple experiment farther along the line until you get no deflection, when you know that you have passed the break, and can conse- quently cautiously return along the line until you spot the break. * The coiling of the fine wire into the form of a, bobbin is only necessary should you find that no deflection is produced upon your compass-needle when you hold a single length of the connected wire in the magnetic meridian, and close over or under the compass-needle. LECTURE X.— QUESTIONS. 95 Leotuhb X.— Questions. 1. Siippose that you have a gal vanio battery, or any other source of electrical energy capable of giving forth electric currents, locked up in a room, and that you have only access to the free ends of the two leading wires connected to the + and — poles of the battery or other generator. Sketch and explain con- cisely how you would ascertain by aid of a compass-needle which wire was connected to the + pole, and which to the — pole of, say, the battery. 2. Sketch and describe Oersted's simple apparatus for illustrating the mag- netic action of a current upon a magnetic needle. Sliow by arrows upon yout sketch, not only the direction of the current, but also the directions of the magnetic lines of force produced by the current in each part of the wire, and the direction of tlie needle's field before and when deflection takes place. Also show the + and - poles of the battery. 3. A current passing through a long wire is so weak that, when the wire is stretched over and parallel to a suspended magnetic needle, the needle is not perceptibly deflected. Describe and explain any arrangement which would enable you to obtain a movement of the needle by the action of the current. (S. and A. Exam., 1880.) 4. \Vires from two separate voltaic batteries are stretched one above the otlier from north to south (magnetic), and equal currents pass through both wires. If a magnetic needle, free to turn horizontally, but not vertically, is liung half-way between the wires, how will it be affected — (ot.) If the cuiTents are in the same direction ? (J.) If the currents are in opposite directions ? (S. and A. Exam., 1885.) 5. Sketch and describe concisely by an " index of parts," the construction and action of a lecture-room Multiplier or Detector Galvanometer ? 6. Sketch and describe by an index a telegraph lineman's vertical " Detec- tor Galvanometer." How, and for what purpose is it used ? 7. The conductor of a coil or bobbin of insulated fine wire has become broken, but you cannot feel the break when unwinding it. How would you search for the position of the break with a small compass-needle and a battery ? 8. Two long wires are placed parallel to each other in the same horizontal plane and in the magnetic meridian. A magnetic needle capable of turning in any direction about its point of suspension is placed exactly half-way between them. How will it behave if the same electric current ilows through the easterly wire from south to north and through the westerly wire from north to south ? [The action of the earth on the magnetic needle may be neglected.] (S. and A. Exam., 1890.) ( 96 ) LBCTTJEB XL Contents.— Magnetic Field and its Direction as Due to a Circular Current — Intensity or Strength of the Magnetic Field at the Centre of a Circular Current— Simple Tangent Galvanometer — Sine Galvanometer — Table of Natural Sinea and Tangents — Questions. Magnetic Field and its Direction as Due to a Circular Current. — Experiment VIII. — i. Take a stout copper wire about J^ inch in diameter, and bend it into a circle of, say, lo inches diameter. 2. Fix this wire circle in a vertical position, and in the plane of the magnetic meridian. BATTERy ^ "^ -^^ — ~ — Gkaphio Eepeesentation, bt Aid op Iron FttiNGS, of the Magnetic FffitD Due to a CmcDiiAB CtniKENT.* 3. Place a piece of stout cardboard or wood covered with paraffin-waxed paper in a horizontal plane containing the hori- zontal diameter of the wire circle, as shown by the accompanying figure. * The above figure, as well as those at pages 81 (lower left), 98, lo5 (lower two), 1077 108, and the upper one at page 125, have been kindly supplied for this Manual, from Mr. H. D. Wilkinson's " Letters for Learners and Un- professional Readers," which appeared in the 1889 numbers of The Electrician. These excellent articles have since been published in book form, CIECULAK CURRENT MAGNETIC FIELD. 97 4. Connect the free ends of the circular wire to the battery or its terminal board, so as to pass a strong current through the wire. 5. Whilst the current is flowing through the wire shower fine soft iron filings upon the paraffined paper. 6. Fix the filings in position whilst the current is flowing by the method explained in Lecture III., Part I. You observe that the direction of the filings near the centre of the wire circle lie straight along and parallel with its axis, whereas to the right and left of this axis or centre line the filings lie in curves around each side of the wire. Take a small freely-suspended test-needle,* and whilst the current is flowing through the wire move the needle into different positions inside and outside the wire circle, and you will find that the several positions taken up by the needle corroborate the direc- tions of the magnetic field just obtained by aid of the iron filings. Apply the right-hand test (as explained in Lectures IX. and X.), and it will confirm the direction in which the N-pole of the needle turns wherever it is placed with respect to the current- cari-ying wire. Reverse the direction of the current through the wire, and note the immediate reversal of the direction of the test-needle, wherever it may be placed, within the range of the current's field. Intensity or Strength of the Magnetic Field at the Centre of a Circular Current. — In the last Lecture we proved that when a current flowed once above or in front of a magnetic needle, and then returned below or behind the same, that the strength of the field set up midway between these two currents was double that due to the current through one of them, and consequently the deflection of the needle placed midway between these two long and oppositely directed currents was greater than that due to either of these currents acting singly upon the needle. Now, compacing this case with that of a circular current flowing right round a needle, you cannot help observing that the strength of the field, and consequently the deflection of the needle, is still further increased, since every portion of the current flowing round the needle tends to set up a magnetic field in the same direction at the centre of the circle. In the following right-hand figure, with the current flowing as depicted, the N-pole of a magnet placed in front of the paper, and in the axis of the circle, would be attracted forward from the * A short piece of magnetised steel wire suspended at its centre by a fine thread, or a double-pivoted test-needle held with its axis parallel to each part of the wire in turn, will do very well. g8 LECTUEE XI. observer into and forced through the point p, with a force due to the magnetic field set up by every poiiion of the circular current ; whereas, in the two left-hand figures, a magnet held midway '• li -^ ' ' k Magnbtio Field Due to Up and Down Stbaiqht CnEEENTS, Magnetic Field Due to a CmouLAn OOBEENT. between the straight wires would be impressed forward by the straight portions only of the current in each wire. It has been proved by mathematics, and confirmed by experiment, that — If r = radius of the current-carrying wire circle, And if 2r = the distance between the two very long straight wires carrying the same strength of current, Then— The strength of field at centre of the circular cuvrent The strength of field midway between the straight currents " 2-irr* 6.28 I-S7 47-47 = T' Hence, we see that by bending a wire into the circular form and passing a current through it, we obtain an effect upon a mag- netic needle placed at the centre of the circle of fully i J times the strength that we should obtain by merely passing the same current strength fair above and helow or fair in front of and behind the same needle placed at the same distance therefrom. Con- sequently we find that accurate and sensitive galvanometers, wherein the needle moves, have their coils of wire made in the circular form. We shall now proceed to describe two forms of galvanometer, whose constancy and accuracy for measuring * 2Tr is the circumfereiice of a circle whose radius is r, and ir is the ratio of the oiroumferenee of a circle to its diameter, or tt = 3-1416 ■,.\2t = 6-2832, or roughly = 6-28. SIMPLE TAIfGENT GALVANOMETER. 99 Simple Tangent Gal- vanombtee. current strengths depend upon their having a circular coil of current-carrying wire. Simple Tangent Galvanometer. — As will be seen from the accompanying figure, the coil of this instrument consists of a single turn of thick wire or narrow copper ribbon, bent into a circle of ten or more inches in diameter. The free ends of the wire are secured to two terminals, Tj, Tg, fixed to a central round upright piece of dry wood, such as polished mahogany. The lower end of this upright terminates in three stiff arms supported by levelling screws, and the upper end carries a circular case of wood or brass, with a horizontal scale divided into degrees and covered by a plate of glass. In the centre of this circular case is fixed a fine steel point, which supports a very short, thick magnetic needle (one inch or less in length), svith a light aluminium or glass pointer cemented at right angles to it by shellac, so that the ends of the pointer extend to the divisions of the graduated scale. The needle being small and the radius of the coil great, the poles of the needle move in a part of the field (produced by the current flowing round the circle) where its direction and strength are toleinbl}' uniform or constant. Now, by placing the plane of the coil in the magnetic meridian the magnetic axis of the needle will be in the plane of the coil, and consequently the magnetic force produced by the current at the centre of the coil will act at light angles to the polar axis of the needle, and deflect it from this position against the restraining horizontal force of the earth's magnetism. Since this horizontal force of the earth may be considered constant for any particular place (as far as the junior student is concerned), all he has to do is to pass currents of different strengths from his battery through the wire circle by joining the terminals Tj^ and Tg to the + and - battery poles, and to note the deflections of the needle by reading the number of degrees through which the pointer moves to the right or left from its zero or original position. ExPEEiiiENT IX. — To ascertain the relative strengths of these currents, look at the following table of tangents * for the tangents * See the last few pages of Munro and Jamieson's " Pocket-Book of Elec- trical Rules and Tables " for more complete tables, giving minutes in addition to degrees. 100 LECTUEB XI. of these angles, when, if dj and d^ represent the deflections in degrees corresponding to the currents e^ and Cg, Cj tan dj ta,ndo or tan d^ : tan d^ : : e^ : c^ That is, the tangents of the angles of deflection are directly proportional to the strengths of the currents flowing through the coil. Table of Natural Sines * and Tangents. ^ Sin. Tan z Sin. Tan. Z Sin. Tan. z Sn. Tan. ^ Sin. Tan. o" •OCXM i8° 3090 •3249 36" 5878 •726s 54- 8090 1-3764 72- 95" 3.0777 19 3256 •3443 8192 i-428i 73 9563 3-2709 1 0175 •0175 37 6018 •7536 56 829a 1-4826 2 0349 •0349 20' 94=0 •3640 38 6157 •7813 74 9613 3-4874 3 0523 KJ524 39 6293 -8098 57 8387 I-S399 75 9659 3-73=1 21 3584 ■3839 58 8480 1-6003 76 9703 4-010S 4 0698 •0699 22 3746 •4040 40 6428 •8391 59 8572 1-6643 0871 •0875 23 3907 •4245 77 9744 4-3315 6 1045 •losi , 41 6561 -8693 60 8660 1-7321 78 9781 4-7046 24 4067 •4452 42 6691 •9004 79 9816 5-1446 7 Z2Z9 •1228 25- 4226 .4663 43 6820 •932s 61 8746 1-8040 8 ■1405 26 4384 ■4877 62 8829 1-8807 80 9848 S-6713 9 Z564 •1564 44 6947 •9657 63 8910 T.962& 27 454° ■5095 45 7071 I-OOOO 81 9877 6-3138 lO 1736 •1763 28 4695 •5317 46 7193 1-0355 64 89S8 2-0503 82 99"3 7-1154 29 4848 •5543 f§ 9063 2-1445 83 9925 8-1443 IZ 1908 ■1944 47 7314 1-0724 66 913s 2-2460 12 2079 .2126 30 5000 •S774 48 7431 Z.II06 84 9945 9-5144 13 2250 •2309 49 7547 1-1504 67 9205 =■3559 ?l 9962 11-43 31 5150 •6009 68 9272 2-4751 86 9976 14-30 14 2419 •2493 32 5299 ■6249 SO 7660 Z.1918 69 9339 2-605X 15 2588 •2679 33 5446 ■6494 ll 9986 19-oS i6 2756 ■2867 51 7771 1-2349 1 70 9397 2-7475 88 9994 28-64 34 5S92 •6745 52 7880 1-2799 1 89 9998 57-29 17 2924 •3057 35 573* •7002 53 7986 1-3270 1 71 9455 2-9042 Example. — Let the deflection produced hy one battery when joined up with the tangent galvanometer be c?j = 17°, and by another battery c?2 = 3i°. Then, the currents Cj and Cj are not proportional to 17° and 31°, but to their respective tangents. For or tan di : tan lig tan 17° : tan 31° •3 : -6 •6 "1 "2 : Co and consequently Cj = Cj — =2 Cj, or the current is twice as strong in the second case as in the first. Suppose we so arrange the size of the coU of our tangent galvanometer, &c., that when we get a deflection of 45° the * Each of the sine values is a decimal quantity. SINE GALTAKOMBTEE. lOI current shall be one practical unit of current, or, as it is called, one ampere. Then, since tan 45° =i, the natural tangents of all the other deflections will represent the respective values in amperes, such as in the above case, e.g., tan 17° = -3 ampere, tan 31° = -6 ampere, and tan 64° = 2 amperes, and so on, as may be seen from the preceding tables.* Hence, — By Definition. — A tangent galvanometer is one [having a venj small magnetic needle, so placed in the field of a coil or coils 0/ large radius that the magnetic field produced by a current in them in approximately uniform at the place where the needle is sztspendeil'], wherein the tangents of the angles of defiection are proportional to the strengths of the currents producing the deflec- tions. Sine Galvanometer. — One disadvantage of the tangent galva- nometer is its want of sensitiveness, due to the necessity of having a coil of large internal radius, in order that it may give accurate results. This implies a comparatively small deflection of the needle for any particular strength of current flowing in the coil. If, however, we mount a coil that it can be turned round upon its vertical axis, so as to follow up the deflected needle until the deflection is a maximum when the coil and the needle are in the same plane, then we may use a much smaller coil and a much longer needle, or the coil as close to the needle as we please, and yet find that the current values of the various * The student may check his arithmetical results obtained by aid of the " Table of Natural Tangents " in the following manner ; — (l.) Draw a, circle upon a sheet of paper of the same diameter as the galvanometer scale. (2.) Draw any radius to this circle, and consider the point where it touches the circumference as corresponding to the zero or starting-point of the scale. (3.) From this zero point draw a tangent line to the circle, (4.) Plot out the various angles of deflection of the needle on the circle from the zero point, and draw extended radii through each of these points until they cut the tangent line. (5.) Then measure with any convenient scale or rule from zero along the tangent line the several distances to where the several angles of deflection cut the tangent line, and these distances will be directly proportional to the tan- gents of the angles, and therefore to the currents which produced the same angular deflections of the magnetic needle. We intentionally avoid treating the student at this stage to the more advanced explanations and formula required for finding by means of a tan- gent galvanometer the strength of currents in absolute measure, or in practical units of current, viz., amperes (see page 74, Munro and Jamieson's " Pocket- Book of Electrical Kules and Tables "]. We also intentionally leave over to our more advanced treatise the method of constructing a scale of tangents, and a description of other arrangements of tangent galvanometers such as those of Graugain and Helmholtz, which furnish a more uniform field where the needle is poised. 102 LECTURE XL deflections follow a regular law. Such a galvanometer is repre- sented by the accompanying IJgure, and is termed a Sine Galva- nometer, because tJie cutrents j^assed through the circular coil are propm-- tional to the sines of the angles through which the coil has to he turned from the magnetic meridian in order that a balance may he ohtained helioeen the cwrenffs field ami the earth's field upon the needle when coil and needle are in one plane. This galvanometer (as illustrated) may, of course, be used as a tangent galvanometer by simply fixing the coil in the magnetic meridian, but, as may be inferred from our previous statements, the deflections will not be strictly proportional to the tan- gents unless the needle is very short. When used as a sine galvanometer, the vertical coil is first fixed in the magnetic meridian, «.e., the coil is moved round until it is strictly parallel to the magnetic axis of the needle. Then the pointer on the lower horizontal scale should stand at zero. When the terminals Tj and T^ are at- tached to the battery, and the current flows through the coil, the magnetic needle is deflected to the right or to the left. The vertical coil is then turned round in the same direction as the needle is deflected, until it fairly overtakes the needle, and again lies parallel with it. The angle through which the coil has been turned is read off on the lower scale, and noted. In this posi- tion the current's field has a maximum turning effect upon the needle, and very weak currents may thus be accurately measured by an instrument made out of the simplest and cheapest materials by the student himself. If Jj° and d^ be the angles through which the coil has been turned from the meridian position when a balance is obtained due to the respective currents, Cj, Cj, Sine Galvanometek. then sin d-^ sin dn sin r?j° : sin d^ SINE GALVANOMETER. IO3 Example. — Let the coil of the galvanometer be turned through dj^ =18° when a current, Cj, is flowing through the coil from one battery, and through d^ = ^']° when a current, e^, flows through it from another battery. Then the currents are not proportional to these angles, but to their respective sines (see last Table). For sin d{ : sin d,° : :ci : C, sin 18° t sin 37° ::ci: «! ■3 : • 6 : : Ci :C2 "2 = Cy ■6 _ •3 2Ci, or the current in the second case is twice as strong as in the first. 104 LECTURE XI. — QUESTIONS. Lkotubb XL— Questions. 1. Sketch and describe an expeiiment whereby you can prove the directiona of the magnetic field set up inside and outside of a circular current-carrying wire. 2. A short horizontal freely-suspended magnetic needle is placed midway between two long (vertical) straight parallel wires through which the same current is passing, up one and down the other. How does the needle behave ? The same needle is now placed at the centre of a single turn of wire formed into a circle placed vertically, and if the same strength of current be passed through this wire as through the straight wires, how does the needle now behave ? Compare the strengths of the magnetic fields midway between the parallel wires and at the centre of the wire circle due to the current in the two cases. 3. Sketch the direction of the magnetic lines of force surrounding a circular current-carrying wire joined to a battery when the current flows, rst, from light to left ; 2nd, when from left to right. Introduce a right hand, properly placed outside and inside the wire, with the outstretched thumb and fingers in the proper direction. 4. Sketch and describe a simple tangent galvanometer with a single coil of wire. Why is such an instrument called a tangent galvanometer ? 5. Compare the relative strengths of the currents passing through the coil of a tangent galvanometer when the observed deflections of the needle are 27° and 45° respectively. 6. Why is it necessary to have a comparatively small needle and a coil of large diameter in a tangent galvanometer 1 7. Sketch and describe a simple sine galvanometer. Why is such a gal- vanometer called a sine galvanometer? How is it used for comparing the strengths of currents ? 8. Why can a long needle be used with a sine galvanometer and not with a tangent galvanometer ? 9. Compare the relative strengths of the currents passing through the coil of a sine galvanometer when the angles through which the coil has been rotated before the needle again stands at zero are 30° and 45° respectively. ( los ) LECTURE XII.* Contents. — Electro-Magnetic Solenoid — Magnetic Field Inside a Solenoid and its Direction — Magnetic Field Outside a Solenoid and its Direction — Combined Effect of the Magnetic Fields Due to a Permanent Magnet and an Electro-Magnetic Solenoid — Sir William Thomson's Graded Tan- gent Galvanometers— Sir William Thomson's Mirror Galvanometer — Simple Ajstatic Galvanometer — Questions. Electro-Magnetic Solenoid. — In the last Lecture we explained the general configuration and the direction of the lines of force of the magnetic field produced by a circular current of one or more turns where the width of the coil was small relatively to its diameter, and we gave two practical examples of its applica- tion in the form of tangent and sine galvanometers. We shall now treat of the properties of Electro-Magnetic Solenoids and their practical application. Definition of a Solenoid.— Tjf an insulated wire is coiled upon a cylindrical bobbin (just as cotton-thread is wound upon a pirn or reel, so that the several turns of wire are closely adjacent and of two or more ; /^^■y-pT'^'^s. layers deep), such an arrangement is called ^^^J]^_L_^f ' a solenoid when the length of the coil is not ^^^^^^^K ' small compared to its diameter. The accom- ^^B^^^^B * panying figure illustrates a solenoid, and the ^^^^^^^m ■ same becomes an Electro-Magnetic Solenoid ^^-^^^ ^^^^^ y- when an electric current is passing through ' -S T ~^ J^ its coils of wire.t Solenoid ok Bobbin Magnetic Field Inside a Solenoid and ^X^™ ^^°^- its Direction. — From what we stated in the last Lecture regarding the direction of the magnetic field due to * If this Lecture should be found too long, or in some parts too advanced, then the teacher or student may omit the description of Sir William Thom- son's graded and mirror galvanometers. + See Rankine's "Useful Rules and Tables," 7th edition, p. 437, or Munro and Jamieson's " Poclcet-Book of Electrical Rules and Tables," 6th edition, p. 402, for how to calculate the length of any particular size of wire that can be coiled upon a circular bobbin. Also see Appendix to Part II. io6 LECTURE XII. a circular current, the student will at once -understand by an inspection of the three following figures that when a current circulates through a long length of conductor coiled into the form of circles placed close alongside of each other, the direction of the magnetic field along and parallel to the axis of the solenoid (inside) is straight and tolerably uniform in strength to within a short distance of the ends of the solenoid. The field due to each current-turn of wire assists the neighbouring cur- rent-turn, so that, upon the whole, we have (as illustrated by the first figure *) a collection of magnetic lines of force arranged not unlike that of bowmen's arrows placed within their hide-bound sheath. Experiment X. — The student may easily prove the truth of the above statement (or the experiment may be shown to a class) Maonetic Field within a Helix or Solenoid. Sir W. Thomson's Graded Galvanometer Coil. Maonetic Field Inside and Outside a Solenoid and a Galvanometer Coil along the Planes of their Axes as Depicted by Iron Filings. bj' placing a piece of cardboard covered with paraffined paper inside and surrounding the outside of the solenoid and scattering soft iron filings over the paper whilst the current flows through the wire.t The direction of the field is indicated by the arrows in each case. The Magnetic Field Outside a Solenoid and its Direction. — The last two figures also illustrate the direction of the magnetic field outside the solenoids. * We have purposely drawn only an open spiral or helix of wire in the . ubove figure, and a single layer in the next figure, in order to be able to show more clearly the directions of the several magnetic lines of force. f See Part I., Lecture III., page 21, for how to fix the iron filings, and thus obtain a permanent record of the magnetic field. THE MAGNETIC FIELD AND ITS DIEECTION. 107 Experiment XI. — In order to bring home to the mind of the student ^hat the contour of the field outside an electro- magnetic solenoid is identical with that of a permanent cylind- rical bar magnet of the same length and width, we would ad- vise him to place a piece of cardboard covered with para- ffined paper outside the solenoid only, and thereby produce by means of iron filings, in the manner already described, a perma- nent picture similar to the accompanying figure. To do so e£fe^ " BATTEfiTI tiValTr +V,u =r.loT,rM'rl MAGfNETic Field Outside a Solenoid as Depicted tively tne solenoid by Ikon Filings. must be fixed with its axis horizontal, the plane of the card should pass through the horizontal axis of the sole- noid. Experiment XII.— Sufficient proof of these two last state- ments may be obtained by sim- ply taking a small compass test-needle and placing it in various positions inside and out- side the solenoid, having first ob- served that the planes of the coils are coincident with that of the magnetic meridian. TO BATTERY TESTIN8 THE Direction of the Cureent's Field Inside AND Odtside a Solenoid bt a Compass-Needle, io8 LECTURE XII. Combined Effect of the Magnetic Fields Due to a Permanent Magnet and an Electro-Magnetic Solenoid. — Experiment XIII. — I. Place a semicircular permanent magnet over the short sole- noid illustrated by the third figure in this Lecture, and see that the line joining the poles of the magnet lies fair in line with the magnetic meridian. Magnetic Field Dub Solely to CnEVBD Magnet, as Depicted BT Aid of Ieon Filings. Magnetic Field Dde to the Com- bined Effects of Cueved Mag- net and Cubeent theodgh Coil, AS Depicted et Ieon Filings. 2. Place a piece of cardboard with paraffined paper inside and outside the coil as before. 3. Before passing a current through the coil, take an impres- sion of the magnetic field by means of iron filings, and also note the direction of the lines of force by your small compass test-needle. The direction of the field is straight across be- tween the poles of the magnet, as illustrated by the accompany- ing figure. 4. Having substituted a fresh piece of paraffined paper, turn on the current through the coU, and again take an impression. The contour of the field is now considerably changed, not only from what it was in the last figure, but also from what it was as seen by the third figure in this Lecture. In fact, the field is now a combination of these two fields, the one due to the permanent magnet and the other due to the current. The filings take up a slanting position from left to right. If you were to substitute a fresh card and to reverse the current through the coil, then the slant of the direction of the filings would be from right to left. A freely-suspended magnetic needle moved along the axis or centre line of the coil would take up a position coincident with that of the iron filings. The farther the needle is removed from the coil the less will the deflection be from the magnetic meridian or plane of the coil. GEADED TANGENT GALVANOMETERS. 109 Geaded to Amperes, foe Stbong Cdeebnts, Sir William Thomson's Graded Tangent Galvanometers. — From the fore- going experiments and remarks the student will now have no difficulty in understanding the principle and action of these two prac- tical instruments. The first figure illustrates the form nsed for measuring strong currents, and the second that for weak currents or poten- tials, i.e. , the electrical pres- sure or difference of poten- tial between any two points in an electric lighting cir- cuit, such as the pressure be- tween the + and — poles of the battery or the dynamo, or the pressure between the terminals of an electric lamp. The only difference that may be mentioned here between the two instru- ments is this, that the first is fitted with a coil, 0, consisting of a few turns, or . of a single turn of thick copper strip, whereas in the o second instrument the coil, ^jBu. S Ar 0, consists of many turns "^"^ »- sw - - (6000 to 10,000) of fine German - silver wire insu- lated with silk. In each instrument the coil may be regarded as a very short solenoid fixed to one end of a wooden platform, P. A V groove is formed in the centre line of this platform {i.e., truly below the axis of the coil, C), so that the quadrant-shaped case, or little brass box marked M, for Magnetometer (containing the freely-poised magnet, with its attached index or light aluminium pointer, and scale under- neath the same), maybe moved to any desired distance from the centre of the coil, C. The semicircular permanent magnet marked N and S is suitably supported by outstanding projections from the magnetometer case, M, and consequently this magnet moves along with the needle and its case. From what we said under the previous heading, the student will see that the greatest deflection of the needle is obtained for any particular current strength when the magnetometer needle is exactly at the centre of the coil, C, and that the further the needle is moved therefrom the less will be the deflection, because the current-carrying coil's field gets weaker the farther away you get from the coils. The V groove is so graded or graduated that for each position of the needle along the same, the exact value of the deflection * obtained with either instru- ment is found by the simple arithmetical operation of dividimg the deflection in degrees by the number marked immediately under the front of the magneto- Geaded to Tolts, foe Weak Cueeents oE Potentials, * The first instrument has its V groove so graded tliat the result of the above rule produces practical units of current in amperes, and for the second instrument practical units of electro-motive force, or electrical pressure, or potential in volts. no LECTUKE XII. meter ease on the platform scale, and then multiplying this result hij the intmsity of the magnetic fidd on the needle* This graded scale consequently affords with these instruments a much wider range of observation than if the position of the magnetic needle had been fixed. There are two points of importance which it is necessary to observe when aiming at accurate measurements with these instruments — 1. See that the instrument is so placed (when no cuneiit is passing and i/eo magnet is near the magnetometer) that the aluminium index points lu zero. 2. Make certain that you know the precise value of the strength of the semicircular magnet at the time, for bar magnets (as we saw by experiments in ]?art I.) lose their strength. Sir William Thomson's Mirror Galvanometer. — This useful and well- known instrument is one of the most delicate and accurate appliances used by electricians for determining the Resistance , to the flow or passage of currents through conductors and insulators, the Current Strength through a conductor, the Electrical Pressure or Potential supplied by a battery nr other source of electrical energy, and the capacity of an insulated conductor or the charge gi ven to it. It was also applied by its inventor to the production at the receiving end of Telegraphic Signals sent in at the sending end of long sub- marine cables, such as those crossing the Atlantic Ocean. It is still used for Cboss Section of Brass Tube, showing Back of MlRUOIl. i^ongitudinal section of Brass Tube, showing Mirror in Dead-Beat Chamber. Back View of Mirror Galvanometer, show- ing Winding, &o. that purpose whenever there takes place a break-down or failure of his Siphon Recorder, the only other instrument yet devised which is capable of such delicate and accurate work. Aswillbe observed fromthetwo accompanying sets of figures, it consists of a solenoid or hollow brass bobbin filled with copper wire covered with a double layer of silk thread. One end of this wire is soldered to one outstanding terminal, Ti, whilst the other end is soldered to the other termi- nal, T2. Under certain circumstances, such as for testing and telegraphic signalling purposes, the galvanometer may have two or more separate coils of * I.e., the number marked by the maker on the semicircular magnet, plus the Earth's field, which is generally taken as = -17 in the British Islnnds. Sea Ra,nkine's " Useful Rules and Tables," 7th edit., pp. 396, 397. MIEROE GALVANOMETEE. Ill wire of different lengths and number of turns wound upon the saitie bobbin, the ends of each ooU being suitably attached to their respective terminals, fixed outside the galvanometer bobbin ; in fact, the coils of this instrument are wound to suit the particular class of work for which it is intended. For strong currents or the measurement of low resistances, thick wire with a few turns is employed, whereas for weak currents or the measurement of great resistances many thousand turns of fine wire are necessary. Inside the central hole of the bobbin is placed a brass tube, BT, containing a very light, truly ground, curved, microscopic glass silvered mirror, M, suspended from a small hole in the upper side of this tube by means of a very fine cocoon silk fibre, SF, attached to BT by shellac. Upon the back of the mirror are fixed (by shellac varnish, previous to suspending the mirror inside the tube) three or four tiny magnets with their axes parallel, and their N-poles pointing in one Sib William Thomson's Mikhob Galvanomiteb and Connbotions, &o., Joined dp foe Testing Insolation Resistance. direction. The three or four little magnets thus form a compound magnet of greater strength than a single one of the same or even greater weight. These thin magnets, when the galvanometer is in use, hang horizontally, and they are about -^V inch °r 1^^^ i" thickness, and from J to J inch in length for mirrors varying from | to I inch in diameter, the smaller sizes of mirror, which, to- gether with the magnets, do not weigh more than 2 grains, being used for tha most delicate testing galvanometers, while the larger sizes are used for class demonstration instruments. To render the mirror "dead-beat" in its action, i.e., so that it may come to rest almost instantly, without swinging to and fro for a number of times when deflected by a constant current, or when the cur- rent ceases, the mirror and compound magnet are not only made as light as possible, but they are encased between two plain glass faces, 6F1, GF2, fitted to the ends of two screwed tubes, STi,ST2. These tubes maybe screwed towards or from the mirror, M, until any desired range of movement and " dead-beat" II 112 LECTURE XII. action is given to the latter. When the mirror is deflected from rest, it forces the pent-up air within the central chamber of the brass tube, BT, against each of these plain glass faces, GFi and GFa, and the air reacting therefrom as a " couple " force upon the mirror, causes the mirror to take up a position of rest more or less quickly according as the glass faces are near or far from the same, This gives each movement of the mirror greater definiteness without detracting from its sensitiveness, and thus saves an immense amount of time and annoyance to the electrician ; for there is nothing more tantalising than to work with an instrument whose magnet keeps on swinging to and fro for a number of times before a definite reading of its position can be ascertained. As will be readily understood from the perspective figure, the deflections of the mirror are rendered visible by aid of a parafSn-oil lamp and a scale. The combined lamp and scale are placed (if necessary) within a dark screen, and at about three feet distant from the mirror. The direct rays from the lamp to the mirror pass through a vertical slit in the wooden frame (support- ing the lamp and scale), and are reflected back upon the scale by the mirror. A thin vertical wire is often fixed in this slit, with a lens in front of the same, so that when the proper focus of the lens and scale has been found, a definite vertical black line (of the image of this fine wire) is reflected by the mirror upon the scale, whereby the precise deflection of the "spot" to the right or to the left of the centre or zero division of the scale can be read off with ease and accuracy. Above the galvanometer coil is fixed a controlling magnet, NS, which may be turned round horizontally (with or without the aid of the tangent screw on the top of the galvanometer), so as to act. by magnetic induction upon the compound magnet on back of the mirror, and thereby turn the latter, and therefore the mirror, to any desired position. For example, it may be used to bring the " spot " of light to zero on the scale, or to any division to the right or left of zero. This magnet, NS, may also be elevated or depressed along its vertical central guide-rod, so as to cause its magnetic field to act with more or less restraining force upon the mirror's magnet, and thus render the latter more or less directly sensitive to the current's field. The most sensitive position for the whole system is when the coil of the galvanometer is placed in the magnetic meridian, and when the controlling magnet, NS, is elevated so that its force upon the mirror's magnet just balances the action of Earth's field upon the same,* i.e. , when the mirror's magnet is rendered astatic, and therefore free to be acted upon solely by the current's field. Since the angle of deflection of the spot of light upon the scale is double the deflection of the mirror (owing to the incident and reflected rays of light), it has been foimd that the deflections of the spot on the scale are practically proportional to the strengths of the currents passing through the galvanometer coil within the limits of the range of the scale as supplied with these instruments. This is an immense advantage in favour of this instrument, as it saves much calculation. We shall not attempt here to explain the precise arrangement of the rest of the testing apparatus illustrated by the last figure further than to observe that the battery, key, switch, galvanometer, combined Wheatstone-bridge resist- ance box, and the cable which is the unknown resistance to be found by aid of these appliances, are joined up, by means of the various wires shown, for the purpose of measuring the Insulation Resistance by the Leakage Current which takes place through the dielectric or insulating material of a submarine or an underground cable, or over the insulators of an aerial line employed for electric lighting or transmission of power. We must defer this pleasure, owing to the necessarily more complex nature of the problem, to our Advanced ♦ The student should here refer back to Experiment XXIV,, Part I., Lecture VIII. and note the second figure on page 67, wherein the Earth's magnetism and a bar magnet balance each other with respect to the magnetic needle. SIMPLE ASTATIC GALVANOMETER. 113 B-^^ Theobetioal Fighkb of a Simple Astatic Galvanometeb. Treatise on Magnetism and Electricity. At present, students who may desire to know more about the matter than has just been given, are referred to page 146 of our Electrical Pooket-Book, where they will find a complete explanation with an example. Simple Astatic Galvanometer. — We have just described in connection with the mirror galvanometer how the directive action of the Earth's mag- netism upon a sus- .,, , pended magnet or needle may be, les- sened, or even exactly counteracted, by the employment of a com- pensating or control- ling magnet properly placed at a certain distance vertically above (or below) the needle. There is, however, another method for effecting the same object, and one which is more frequently adopted in practice, viz., by using an " astatic pair " of magnetic needles like that described and illustrated at page 68 of Lecture VIII. (Part I.), in combination with one or two coils of current-carrying wire. This com- bination is termed an Astatic Galva- nometer. The following three figures serve to illustrate this galvanometer in its simplest form. The iirst figure is a theoretical sketch of the coil, astatic pair with silk fibre suspension and battery. It shows the direction of the current from the + pole of the same through the several turns of the coils and back, to the — pole of the battery. The second figure indicates the style in which it is made for lecture-table demonstrations ; and the third figure the practical instrument as it is used in the laboratory or testing- room. The galvanometer consists of one coil of insulated wire wound upon a hollow [ZZI-shaped bobbin. The lower needle can move freely within the hollow space of the coil, whilst the upper needle moves parallel to and near the uppermost layer of wire. Attached to the upper end of the connecting wire binding the two needles rigidly together is fixed a light glass or alu- minium pointer parallel to the magnetic axis of the needles — .1 ■■■ffi 1 OOM FOBMOF A SIMPLE ASTATIO GALVANOMETEK. 114 LECTunE xn. Between this pointer and the upper needle is fixed a horizontal circular card graduated in degrees. The base of the instrument is supported by three levelling screws, and the delicate parts are covered bya bell-shaped glass jar, which protects them from dust and damp, and the needle from air draughts. The upper end of the silk fibre suspension is attached to a small hook terminating in an ad- justing and elevating screw and nut, whereby the pointer may be brought to zero, and the needles raised or lowered at pleasure. When not in use, the fibre suspension is lowered until the poin- _ ^" ter rests upon the card, K^ and thus all stress (due to the weight of the Peactioal Testikg-Room Fobm ov Simple needles, &c.) is removed ASTATIC Galvanometer. ^^^^ ^^^ 4^^^ ^j^j^j^ should be very fine, so as to make the torsional error a mini- mum. It will be readily seen from an inspection of the first figure how each portion of the current flowing in the coil acts with the same directive sense upon the lower needle, and also how the portions of the current in the upper layers of the coil act similarly upon the lower and upper needles. The instrument is consequently not only more sensitive (and therefore better adapted for the measuring of weak currents than the single-needle one of the same general shape), owing to the principle of the astatic pair diminishing the effect of the Earth's field upon the needles, but also owing to this extra action upon the upper needle of one side of the current-carrying coil. In th3 finer, more accurate, and expensive forms of astatic testing galvanometers, there are two circular coils or solenoids, just like those employed in mirror galvanometers, with fine com- pound magnets at the centre of each, to the upper set of which is attached a mirror ; but we must leave the detailed description of this more complicated form to our Advanced Text-Book. LECTURE Xn. — QUESTIONS. llj Lectube XII. — Questions. 1. What is a solenoid? When does a solenoid become an electro-magnetio solenoid ? Sketch a solenoid joined up to a battery, and show by signs and arrows not only the polarity of the battery, but also the direction of the current through the wire. 2. Give a sketch showing the condition and direction of the magnetic field inside and outside an electro-magnetic solenoid, as indicated by the aid of soft iron filings and a compass-needle. 3. Give a sketch showing the combined effects of the magnetic field due to an electro-magnetic field and a superimposed permanent semicircular bar magnet. 4. Sketch and describe Sir William Thomson's graded tangent galvano- meters for measuring strong currents and electrical pressures. Having obtained a deflection with one or other of these instruments, how do you find its current or the electrical pressure value in practical units ? 5. Sketch in section and describe fully the construction and action of every part of Sic William Thomson's mirror galvanometer. How are the deflections of the mirror observed ? Are the deflections as observed proportional to the current strength passing through the galvanometer coil, and if 50, why 1 6. Describe the construction, action, and use of a common or simple astatic galvanometer. 7. How is it that a galvanometer with astatic needles is more sensitive than the same instrument would be if furnished with only a single needle ! (S. and A. Exam., 1886.) 8. Why is an astatic galvanometer better adapted for the measurement of weak currents than a galvanometer with a single needle 1 (S. and A. Exam., 1889.) C ii6 ) LECTURE XIII. N S s N s s N s s N s s N + s s Contents. — ^Magnetic Polarity Due to a Straight Current — Magnetic Polarity Due to a Circular Current — Magnetic Polarity of an Electro-Magnetic Solenoid — Given the Direction of the Current in a Solenoid, to Find the N and S Poles of the Solenoid, and viae versa — Specimen Question and Answer — Questions. Magnetic Polarity Due to a Straight Current. — In Lecture IX., Experiment III., we discussed the direction of the magnetic field due to a straight current. We there found by experiment the direction which a freely-suspended magnetic needle takes up in virtue of the action of the current's field upon the needle's field. Experiment XIV. — Looking at the two accom- panying figures, and bearing in mind what we have frequently proved by experiment, let a compass- needle be moved up or down close to and along the farther side (i.e., the side away from the reader) of the current-carrying wires, which are supposed to lie in the plane of the paper. Then the N and S poles of the needle take up the positions indicated by the N and S letters. Consequently we may regard an electric current as if it were producing -North and South polarity of that direction along that side of the whole circuit of the current-carrying wire.* Magnetic Polarity Due to a Circular Current. — Experiment XV.— Now bend the _§_ wire in the last ex- sS periment into the circular form, and pass a freely-sus- pended dipping- needle along the far- ther side of the cur- rent-carrying wires, m the plane of the n D o supposed, ss before, to lie which are paper. We observe that the N and S poles of the needle take up the positions indicated by the N and S letters. Consequently • Although this is an a,rbitrary expedient for facilitating the explanation in regard to the above and similar oases, the student must not lose sight of the true closed curved nature of the current's field, already frequently referred to. MAGNETIC POLARITY DUE TO A CIRCULAR CURRENT. 117 we may regard the current in the circular wire as producing North and South polarity along each circuit, as shown by the two figures. Experiment XVI. — To prove this important fact in another way :— (i.^ Take a stiff copper wire and bend it into a circle. (2.) Connect the free ends of this wire to the terminals, T + and T-, of a floating bat- ^JlrM"< tery cell, i.e., connect one ••*^' « end of the wire to one end of a strip of sheet-copper about I inch broad, bent into the form of a (J \\ (marked Gu on the annexed X^-;,-;jig— >_-r-3i figure, for Ci«prMm, the Latin *'" ' for copper), and the other end of the wire to a piece of zinc of the same breadth (marked Zn for zinc) ; both plates being suspended in a bath of dilute sulphuric acid from a cork or wooden float. (3.) Now hold, say, the S- pole of a bar magnet to- wards the centre of the North side of the current-carrying wire. Immediately the wire will be attracted towards the magnet (carry- ing the float and cell along with it), and then it will move along the magnet until it reaches the middle or equator of the same. Pre- sent the N-pole of the magnet to this North side of the wire, and immediately repulsion takes place between the two. The wire will move away for a short distance, and then turn bodily round with the float and cell, until the South side of the wire presents itself to the N-pole of the magnet, when the effect of attraction will take place just as in the previous case, until the wire moves along the magnet to the middle of the same. In the figure we have shown by arrows, not only the direction of the current along the wire, but also the direction of the current's circular field around the same, and the bar magnet's field, with a right hand so placed on the wire that the outstanding thumb indicates the N-side or polarity of the current!s magnetic lines of force. From these indications you cannot fail to observe that the direction of the current's field is in the same direction as the magnet's lines of force through itself. These two forces therefore act in sympathy, as it were, with each other; and since the poiaeittof a cirodlae ooebbnt, showinq Attkaction bbtweek the same and a Bab Magnet. Ii8 LECTDEE XIII. Bireetiott of ilotion POLABiTr OP A Solenoid, showing Attraction BETWEEN IT AND A BAR MAGNET. magnet is fixed, the current-carrying wire, which is free to move, must be urged towards and along the line of the magnet's axis until it reaches the equator of the same, or the position of equili- brium between the two forces in action. Magnetic Polarity of an Electro-Magnetic Solenoid.— Ex- PBHIMENT XVII. — Adopting precisely the same iloating device and battery cell as in the last experi- ment, but substitut- ing a short solenoid or coil of insulated wire for the single turn, you observe the same effects, viz., when the N- pole of the bar mag- net is presented towards the centre of the S-side of the solenoid, the latter is attractedjwhereas when the BT-pole is presented to the N-side of the coil, repulsion takes place. Or, generally speaking, we prove that a solenoid through which a current is passing possesses polarity in the same way as if it were a magnet, and that unlike poles attract each other and like poles repel each other. Experiment XVII I. — i. You may vary the demonstration by substituting for the short coil in the last experiment a long spiral or helix of wire, either suspended from a thread (as shown by the first of the two following figures), or supported by a float- ing battery cell (as shown by the second figure). These spirals are equivalent in their magnetic action to as many circular currents as there are number of turns of wire, since their axes lie in the same straight line. If you present the N-pole of a bar magnet to a certain end of each spiral, you observe that it is repelled. You accordingly conclude that this end must be the N-end of the spiral. If you present the same pole of the bar magnet to the other end of the spiral, you get attraction, whereas if you hold the S-pole of the bar magnet towards it, you find the spiral repelled, and you again conclude that this must be the S-end of the spiral. 2. Suppose you remove aU magnets and iron from the vicinity of these solenoids or spu-als, and still permit the current to flow through them, you will find that they very soon take up a posi- tion with their axes in the plane of the earth's magnetic meridian, POLAKITY OF ,iN ELECTRO -MAGNETIC SOLENOID. 119 just like that of a compass-needle or suspended bar magnet, owing to the mutual action between their current's field and the Earth's magnetic field. 3. If you place in the same glass bath or porcelain basin, con- taining sulphuric acid and water, a long floating spiral of small diameter (like that in the second figure), and a short open coil (like Hetlm of Suspended Wire Spibai or solbsoid. FtOATING WlEE SpIBAT. OK Sot.BNOID. Index to Parts. N,S, represents North and South poles of the solenoids. 1,0, „ Inner and Outer cups ountaining mercury. B „ Battery. C« „ Copper plate of battery cell. Zn ,, Zinc ,, ,, ,, T-I-,T — II Terminals (positive and negative) of cell. that on the previous page), these two current-carrying coils will act towards each other, in the first instance, like two floating magnets, for you will find that they turn round so as to present their unlike poles to and then attract each other. Einally, the long spiral will penetrate the open coU as far as the cork floats will permit. Electro-magnetic spirals or sole- noids, therefore, possess magnetic polarity, and act like magnets. Given the Direction of the Current in a Solenoid, to Find the North and South Poles of the Solenoid, or vice versl* * The two following rules, devised by the author, seem to be much more easily applied and remembered by students than the rules generally given in text-books, wherein the experimenter has to suppose himself swimming in the wire with the current, or looking normally at one end of the solenoid, and then think whether the current is moving in the same or in the opposite direc- tion to the moving hands of a clock or watch. See Appendix, Part TI. 120 LECTUKE Xin. Rule I. — If you know the Direction of the Current in the winding, then by placing your right hand, as shown, the thumb points in the direction of the N-pole of the solenoid or spiral. Ride 2. — Ascertain the N-pole of the spiral or solenoid by means of a compass-needle, then by placing your right hand on the solenoid (as shown by the figure) so that the outstretched thumb points in the direction of the N-pole (or where the magnetic lines of force leave the coils), the fingers will point in the direction of the current passing through the windings. DtRECTIOM OPy MAGNETIC FORCE Specimen Question and Answer. Question. — A guttapercha- covered copper wire is wound round a wooden cylinder, AB, from A to B. How would you wind it back from B to A (i) so as to increase, (2) so as to diminish, the magnetic effects which it produces when a current is passed through it ? Illustrate your answer by a diagram drawn on the assumption that you are looking at one end. (S. and A. Exam., 1889.) Answee. — (i.) Continue winding the wire from B back to A in the same way as you would wind thread upon a pirn, i.e., keep turning the cylinder in the same direction "+ as when winding on the wire from A to B. '7* (z.) Before commencing to wind hach the wire from B to A, bind the last turn to the wooden cylinder, and then revolve the cylin- der in the reverse direction to the way in which it was turned when winding on the wire from A to B. N.B. — The two figures illustrated on page lig show left-handed and right- handed spirals. Since the current is thus sent through the windings in the different directions shown, although starting from the same end, it produces opposite polarity in the two coils. Consequently, in Case (2), if we put on the mm.e number of turns of wire from B to A, but in the reverie direction to that from A to B, there would be no electro-magnetic efiect whatever, for the magnetic field due to the one layer of current-carrying spiral would cancel that of the other, seeing that the current doubles back on itself, as shown by the above small figure. LECTURE XIII. — QUESTIONS. 121 Leotuke XIII. — Questions. 1. What do you understand by the expression, "Magnetic polarity of a straight current " ? 2. A. current flows through a circular wire laid on the table, in the direction ot the motion of the hands of a watch. Give a sketch illustrating the polarity of the faces of the wire circle. 3. You present the N-pole of a bar magnet towards the centre of a floating copper ring through which an electric current is flowing in the direction of the hands of a watch as viewed from the side to which the bar is presented. Sketch and explain what will happen. Show the current's and the magnet's fields, and place a right hand on the wire to show the direction of the current's field. What will happen if you present the S-pole instead of the N-pole of the bar magnet to the same side of the wire ? 4. Sketch and describe by an index the floating solenoid and battery. Show how it behaves when the pole of a magnet is presented to it. What occurs when another floating solenoid with battery is placed beside it ? 5. Are solenoids similar to magnets, and if so, in what respects ? Give sketches showing how you would find out which was the N-pole of a solenoid, (l) by aid of a compass-needle, (2) by aid of a bar magnet, (3) by aid of nothing else than your right hand, if you know which way the current flows through the coils. 6. A piece of copper wire is wrapped spirally round a ruler from end to end, and the ruler is hung horizontally, so that it can turn about its centre while a current is passing through the wire. How can you tell in which direction the current is passing, (l) by using a bar magnet { (2) without using anything but your right hand ? ( 122 ) LBCTUEE XIV. CoNTENTts. — Magnetisation of Iron and Steel by an Eleotri o Current — Definition of an Electro-Magnet — Magnetic Field of an Electro-Magnet — Attractive Force of an Electro-Magnetic Solenoid towards an Iron Core — Blyth's Current Meter — Horseshoe Electro-Magnets, with Practical Examples — Alteration in the Length of Iron when Magnetised — Questions. Magnetisation of Iron and Steel by an Electric Current. — ^In the year 1820, very soon after Oersted publislied his discovery of the magnetic influence of an electric current upon a magnetic needle, Sir Humphry Davy found out that pieces of iron and steel could be strongly magnetised by passing a current of elec- tricity over them, or still better, round them, several times. Experiment XIX. — i. Take a soft iron bar (a common poker will do very well). 2. Coil an insu- lated copper wire round the bar in the form of a spiral, as shown by the two following figures. 3. Suspendthebar by two copper wires or by twine. 4. Attachtheends of the insulated cop- per wire to a battery as shown, and thus pass a strong electric current through the spiral windings. 5. Bring soft iron nails near to the ends of the bar. They become strongly magnetised by electro-maguetic induction, and great clusters of these nails may be hung from each end of the bar, if the current is strong. 6. Bring a compass-needle near to each end of the bar in turn, and you at once observe strong repulsion between one end of the bar and the needle's N-pole. This end of the bar is therefore a N-pole. Between the other end of the bar and the N-pole of the needle you observe equally strong attraction. Magnetising a Pokeb by a Cubbbnt. MAGNETISATION BY AN ELECTRIC CURRENT. 123 7. Holding the compass-needle in your left hand close to the N-pole of the bar, place your right hand on the wire spiral as MAGNETISIfrG A BAR OP iKON OR StHEL BY A CURRENT. instructed by Rule 2 (given on the previous page 120), and }'ou thereby ascertain which way th e current is flowin g th rou gh ^ the current -carrying wire. 8. Eemove the bar from the spiral (see second figure) ■ and again apply this same test. Yon at once find out that the N and S poles of the spiral are situated at the same ends as the corresponding poles of the bar when placed within the same > or, in other words, the spiral and the iron bar which it surrounds are similarly magnetised by the current. You cannot help, however, observing that the strength of the bar's poles is many times greater than that of the spiral's poles without thebar.* 9. If you suspend the spiral with its bar (as in the first figure of * At this stage we cannot expect the student to enter into and understand the formulae which enable the electrician to calculate the exact strength of the magnetic field produced by a current passing through a solenoid, or the strength of the magnetism induced thereby in a piece of iron or steel inserted Balanced Electro-Magnet and BicHEOME Battery Cbll. 124 LECTURE XIV. Experiment XVIII.), you find that they turn round, and come to rest with their common magnetic axis in the plane of the earth's magnetic meridian. Or you may balance an electro- magnet, as shown by the last figure on page 123, and pass a current from the battery cell through the solenoid surrounding its soft iron core. The electro-magnet will act like a dipping-needle if its magnetic axis be placed in the plane of the magnetic meridian. 10. Take a strong bar magnet and hold it towards one pole of the poker, or towards one pole of the electro-magnet in the last two cases, and you get either strong repulsion or strong attraction between them, according as the poles presented towards each other are like or unlike. 11. If you substituted a hardened steel bar for the iron one within the spiral, you would find it converted into a strong per- manent magnet. This was one of the best methods mentioned in Lecture I. for magnetising hardened steel bars. A current of electricity therefore possesses the property of converting a bar of iron into a magnet, and the combination forms what is technically termed an Electro-Magnet ; therefore — By Definition. — If a bar of soft iron be introduced into a solenoid traversed by an electric current, that bar becomes strongly magnetic, and is called an Electro-magnet. If you refer back to Part I., Lecture IV., where we discussed the Molecular Theory of Magnetism, you will find that we con- sidered each molecule to be a natural and complete little magnet by itself. You are, however, only capable of detecting their com- bined magnetic properties when they are so arranged that their magnetic circuits are not short-circuited amongst themselves, and consequently produce an external field. Prom this point of view, then, there is nothing extraordinary in the results which you have just observed. For, a current's magnetic force naturally acts upon the several molecules of the iron bar by magnetic induction (so as to induce them to take up the stressed condition with respect to each other favourable to the exhibition of an external field), very much 11 the same way that a permanent magnet acts on them. In other words, the current's field simply renders evident the innate inagnetism in the molecules of an iron or sted bar. Magnetic Field of an Electro-Magnet. — Referring back to Lecture XII., Experiment XI., you observe that the contour of the magnetic field of a >jolenoid as depicted by aid of iron filings within the solenoid. Tor these formulse see Rankin's " Rules and Tables,'' 7th edition, pages 308 and 309; also Munro and Jamieson's "Pocket- Book of Electrical Rules and Tables," pages 379 to 380b, and pages 433 to 436. MAGNETIC FIELD OF AN ELEOTEO-MAGNET. 125 is very similar to that of a bar magnet's field, a soft iron bar into the solenoid ■when the current flows through its- coils, the combined fields would be of the same polarity but very much stronger than in the former case. The filings would thereby be at- tracted more closely together. In other words, the field would become more intense and defined. If you pulled out the iron bar (or core, as it is technically termed) to a certain distance from the solenoid bobbin, the If you introduce TO BATTERY Combined Magnetic Fields of a Solenoid and Pbojeoting Iron Core. Weight field would take the form represented by the accompanying figure. The bar would still retain its polarity, but the strength of , , „ 1 <, ,, , 1 ij 1 / \ Soft Iron its poles would be considerably dimi- nished. Attractive Force of an Electro-Mag- netic Solenoid for an Iron Core. — The pull which an iron bar would experi- ence under the at- tractive influence of the current's field may be tested by the following inte- resting experiment. Experiment XX. — Place a solenoid, with a current flowing gr?""'^ ^^^fi. ^/IWi s: ■Weighing the Inductive Foeob between an Eleotko-Magnetio Solenoid and a Soet Ikon Bab. through it, directly underneath a 126 LECTURE XIV. cylindrical bar of soft iron attached to one arm of an ordinary balance, and put weights into the scalerpan on the opposite side of the beam until they balance the attractive force of the current's field upon the iron. If you test the ends of the iron bar, when in this position, by means of a compass-needle brought near to them, you will find that the bar is strongly polarised, and that the end next to the solenoid is of opposite sign to the near end of the latter, as shown by the diagram. You may vary the ex- periment by placing the solenoid nearer to or farther from the iron bar ; also by lowering the bar more or less into the solenoid, and noting the different weights which are required to produce a balance for each position. The strongest pull will take place when the bar is nearly midway between the ends of the solenoid, for then the greatest number of the current's lines of force are concentrated within and by the bar of iron. The solenoids and their iron cores for. the regulation of Arc Lamps are frequently devised so as to act upon the above prin- ciple, or what has been termed "the sucking-coil method." Professor Blyth's Solenoid Current Meter. — A practical instrument for measuring the strength of electric currents has been devised by Professor Blyth upon the above prin- ciple. Instead of using a balance andweights,which take a considerable time to adjust, he simply em- ploys the natural and uni- form- resistance of a spiral spring to counterbalance the pull exerted by the current's field upon a thin hollow iron core. His reasons for using a thin hollow cylinder instead of a solid one are, that there shall be a minimum of residual magnetism left in the iron core after each test, and that the core may be practically mag- netised up to saturation point by even a very weak current. Hence the pull exerted by the current's field upon the thin iron core is rendered propor- tional tothestrengthof the current passing through Professor Blyth's SoLENoro Cobeent Meter. ^^^ solenoid, and is there- fore directly indicated by the extension given to the resisting spiral spring. In the figures, T 4- and T - are the + and - terminals which connect the HORSE-SHOE ELECTRO-MAGNETS. I27 instrument in circuit with a battery or dynamo. The current passes from T + to T — through SO, the solenoid coil, which sucks the iron core, 10, into it against the resistance of the spiral spring, SS, attached by the loop li to 10, and by the loop l^ to AN, a screw for adjusting the zero marked on the 10 by the arrow. A rack, R, and pinion, P, are fixed to the brass tube, BT, which slides freely inside the outer brass tube. A vernier, V, is fixed to BT, and indicates on the divided scalfe, S, fixed to the outer tube, the current strength or amperes flowing through SO. A mirror, M, shows simultaneously the zero on 10, and the position of the vernier on the divided scale S. L is a spirit level, and LS three levelling screws. I'o Take a Test. — [i.) Level the galvanometer. (2.) Free the core, 10, by releasing three set screws not shown. (3.) Adjust by AN, R, and P, until the zero mark on IC agrees with zero pointer on outer brass tube, and at the same time the zero of the vernier, V, agrees with the zero of the scale. {4. ) Then connect the instrument in circuit with the source of electric energy, and raise 10 by U and P, until the zero oh 10 is again opposite the zero pointer on the brass tube. The reading of the vernier on the scale, S, gives, by reference to a constant (or a table carefully determined and drawn up by experiments in the laboratory), the current in amperes. Horse-Shoe Electro-Magnets, with Practical Examples. — From what we have already stated regarding straight electro- rn^rn ^^J i*- Iron Care. 11 Hi m *~ Bobbins ■ of W'ire. ^^ W m '^'^~ 5^^^ fl ^^ "^■Bpa^^ ^y-J^ilillll lllimilllllN 1^^ Keeper. Keeper, SiMPiE Hokse-Shoe Elbctko-Magnets. T+ " magnets, the student will now have no difficulty in understand- ing the construction and action of the horse-shoe form.* As will be seen from the above figures, the simple forms of horse-shoe electro-magnets consist of a bar of soft wrought-iron bent into the (\ form, and then carefully annealed, in order that the magnetic resistance and the retentivity for magnetism may be as small as possible. The bar is either wound from end to end with one or more layers of insulated copper wire (as shown by the first figure) of the required size to carry the current to be passed through it without being unduly heated, or two solenoid bobbins of brass or wood are filled with the insulated wire, and * See page 6, Part I., Lecture I., and the Appendix to Part L, for figures of the workshop or laboratory pattern of horse-shoe electro-magnet, and how to use it for the purpose of magnetising steel bars and needles. See Appendix to Part II., How to Make an Electro-Magnet. I 128 LECTURE XIV. firmly fitted on to the ends of the iron core (as shown by the second figure). Brass terminals or binding screws are then fixed to the ends of the wire, for the purpose of connecting these to the leading wires coming from a battery or dynamo. The current, as it flows through the coils of insulated wire, evokes magnetism by induction in the iron core, and this in- duced magnetism reacts on the current's field by concentrating the magnetic lines of force within the core, so that we have a very strong field produced between the poles of the electro- magnet — a field which is equal to the sum of the current's field and the induced field in the core. If you desire to test the lift- ing force of the electro-magnet, you have ordy to place a soft iron keeper (of a cross section at least equal to the cross section of the core) below the magnet's poles, and fit the same with a hook from which you may hang weights. You thereby ascertain the force required to disengage the keeper from the poles of the core, due to different current strengths, as measured by Thomson's or Blyth's current meters previously described. Horse-shoe electro-magnets are used for a great variety of useful purposes, besides that of magnetising bars of steel, and they are made in a variety of shapes, according to the work to be done by them. When it is desired that the electro-magnet shall respond quickly to a current of short duration, and demagnetise quickly whenever the current has ceased to flow, as in the case of the Morse _^_^^_______ telegraph instrument, then the bobbin windings Eleoteo Magnet ^^^ *^® cores are made as short as possible. FOB THE MoESE When a long, steady continuous pull is re- Telegkaph In- quired from an electro- magnet, as in the case STBUMENT. r e c i. c ■ -i 01 one rorm ot apparatus tor moving railway signals, then long solenoids and cores are advantageous. In the case of Sir William Thomson's siphon recorder for repro- ducing upon a moving ribbon of paper the telegraphic signals sent through long submarine cables, the magnetic field concen- trated upon the moving coil requires to be constant and very intense ; hence long electro-magnets are used. Edison, when he first devised his well-known dynamo machine for generating electricity for electric lighting purposes, adopted very long electro- magnets, but this was proved by Dr. John Hopkinson to be a mistake, since the magnetic resistance of the magnetic circuit was thereby unduly increased. By shortening the cores of the electro-magnet to less than one-half their former length, and increasing their sectional area as well as the number of layers of the solenoid wire, he raised the efficiency of the Edison dynamo Electro- Magnets^ ALTEEATIOK IN LENGTH OF IRON WHEN MAGNETISED. 1 29 by fully 25 per cent. All good dynamos are now made with short, stumpy electro-magnets. The ordinary house electric trembling bell is another practical instance of the advantages derived from the use of short electro- magnets. Here the current from the battery through the push and the coils of the electro-magnet solenoids is broken whenever the armature is attracted close to the poles of the cores, by the contacts between the back of the armature and the flat curved spring becoming separated. If the electro-magnet did not almost instantly respond to the current, and quite as instantly lose its magnetism at the moment the current ceased, then it would keep the armature attracted to its poles Edison -Hopkinson Dynamo, show- n • i_i i- TTT iNG Shokt Bleotbo-Magnet. for an appreciable time. We should thus have a succession of claps from the hammer on the bell instead of the well known " dirl " due to the rapidly succeeding attractions on completing the circuit and disengagements on breaking the same. Alteration in the Length of Iron when Magnetised. — The late Dr. Joule of Man- chester, the discoverer of " the Mechanical Equivalent of Heat," was the first to observe that when a bar of iron is magnetised its length is increased. Mr. Shelf ord Bid well has, however, proved quite recently that if the magnetisation is increased towards saturation, the bar not only ceases to ex- pand, but actually diminishes in length below its normal size. He has also experi- mented upon other magnetisable substances, and in the case of the metal nickel he obtained greater contraction than in the case of iron, with even a comparatively weak magnetising force. To test this interesting phenomenon, arrange an iron rod vertically, and suiround it by an insvilated spiral conductor of copper wire connected up to a battery, through a key, as shown by the accompanying diagram. Fix the lower end of the iron rod into an adjustable footstep, and Common Form of Housb Eleoteic Tkembeing Bele, showing Eleo- tbo-Magnet, &o. 130 h^CTVEE XIV. JtfiFTor I^imi 10 ft'hen Lever ii elevated by exteniion of Iron Rod due to itt beinf nagnetUed. let the upper end bear on a light brass lever at a short distance from the fulcrum. Let the other end of this lever act upon a projection extending from the back of a horizontally-pivoted or tightly-suspended circular mirror. Place a lamp and scale in front of the mirror, or better still, project a beam of light from an electric arc lamp or a lime-light on to the mirror, and let the reflected spot of light be Scale ^^_ cast On the op- posite wall of the room or on to a Tamp white screen. If the iron bar is lengthened by the magnetising influence of the current flowing through the coil of wire, then it will lift the lever and turn the mir- ror so that the spot is deflected Shblfoed Bidwbll's Expebiment roR Exhibiting the downwards : but Altebations in the Length of Iuun and OTaEu -i; • Magnetisable Metals wHEif Magnetised. j increase the current con- siderably, then the bar will contract owing to its increased mag- netisation, thus lowering the lever and causing the mirror to deflect the reflected rays above the zero point of the scale. Physicists and electricians have not yet found out the esact reason for these expansions and contractions. All they do know or surmise is that the molecules of iron or other magnetisable metals are turned or twisted round on their axis when magnetised. Why the length of the bar should fir.st expand and then contract is at the present time a mystery. LECTUEE XIV. — QUESTIONS. I31 Leotuee: XIV. — QnEsmoNS. 1. What happens to a bar of soft iron if a Btrong current be passed through a wire held close above the bar and at right angles to it ? What will now happen if the direction of the current be reversed ? 2. Suppose you wind an insulated wire round a poker and send a strong electric current through the wire, what occurs ? What will be the diflference between the condition of the knob and of the other end of the poker ? Give a sketch to illustrate your answer, showing the direction of the current through the wire, &o. 3. If you were given any battery cell you chose, wire with an insulating cover- ing, and a. bar of soft iron, one end of which was marked, state exactly what - arrangements you would make in order to magnetise the iron so that the marked end might be a north-seeking pole. Give a diagram. (S. and A. Exam., 1887.) 4. A long copper wire covered with silk is wound several times round an iron rod. On connecting the ends of the wire, one with each terminal of a battery, the iron rod becomes a magnet. How does the direction of magnetisa- tion of the iron (or position of its north-seeking and south-seeking poles) depend upon how the copper wire is wound, and which end of it is connected with the copper or + end of the battery ? Give a drawing. (S. and A. Exam., 1885.) 5. An insulated copper wire is wound round a glass tube, AB, from end to end, and a current is sent through it, which to an observer looking at the end A, appears to go round in the same direction as the hands of a watch. A rod of soft iron is held (i) inside the tube; (2) outside but parallel to the tube. What will be the magnetic pole at that end of the bar which is nearest to the observer in each case 1 (S. and A. Exam., 1888.) 6. A bar of iron is held vertically above and fair in line with the axis of a hollow solenoid through which a current is passing. What happens and why ? Give a sketch marking the direction of the current through the solenoid, its . poles, and also those of the bar. If the bar is now introduced until its ends are equally within or without the ends of the solenoid, what changes take place, if any, in the polarity of the bar and in its magnetic strength ? 7. A rod of soft iron is placed upright on a table. Its upper end is sur- rounded by a coil of insulated wire which does not touch the rod. When a strong current goes through the wire, the iron rises in the coil. Explain this by sketches and index of parts. (S. and A. Exam., 1883.) 8. Sketch and describe by index how you would make a horse-shoe electro- magnet. Clearly indicate by arrows the direction of winding and direction of current in order to produce the required polarity. 9. Sketch aud explain the action of an ordinary house electric bell. Why should you prefer to use a short electro-magnet for the same it you wished it to ring rapidly ? 10. If you were asked to design an electro-magnet for a large gong which required to strike distinct separate signals, what form of electro-magnet would you employ, and why ? 11. One end of a coil of wire, through which a current passes, is found to attract the north pole of a compass needle when placed at a certain distance from it. Will the action be the same (i) in nature, (2) in amount, when a rod of soft unmagnetised iron is placed inside the coll ? (S. and A. Exam., 1890.) ( 132 ) LECTURE XV. Contents. — Action o£ a Force and the Reaction against it are always Equal and Oppcsite in Direction — Rotation of a Magnetic Pole Round a Cur- rent, and of a Current Round a Pole — Faraday's Apparatus for Exhibiting the Rotation of a Current-Carrying Conductor Round One Pole of !i Magnet — The Automatic Twisting of a Current-Carrying Wire Round a Magnet — Questions. Action of a Jorce and the Beaction against it are always Ec[ual and Opposite in Direction. — ^You know from ordinary- everyday experience that if you hang a weight to the hook of a spring-balance, the spring reacts equally hard against the force exerted by the weight and in exactly the opposite direction. If you place a weight on the table or upon a graduated spring- balance, or lloat a ship in the water, or, in fact, apply a force anywhere and in any direction, you find a universal law in nature, that "Action and reaction are always equal and opposite." We shall now apply this law to our previous experience regard- ing the action of a current on a magnet's pole. Botation of a Magnetic Pole Bound a Current, and of a Current Bound a Pole. — Referring back to the experiments in Lectures IX. and X., you there learned that the N-pole of a magnet always tends to rotate round a straight current-carrying wire in a certain definite direction, and that the S-pole equally tends to rotate round the same wire in the opposite direction. Now, reasoning from the axiom just stated, viz., that "Action and reaction are equal and opposite," you obviously conclude that whilst the magnet's poles tend to rotate round the current, the current on the other hand must of necessity tend to rotate round the magnet's poles ; and further, that it merely depends upon the arrangement of the apparatus which of them move (the magnet or the wire), or whether both move simultaneously. We shall first of all explain the construction and action of Faraday's apparatus for exhibiting the rotation of a current round a pole. Experiment XXI. — (i.) Arrange apparatus as shown by the arst of the two following figures, and pass a strong current from the battery through the circuit in the directions indicated by the arrows. ROTATION OF A MAGNETIC CUEEENT. 133 You immediately find that the vertical current-carrying wire rotates in the direction indicated by the circular curved arrow. If you apply a suflSciently strong current, and your bar magnet is also strong, you will have no difficulty in obtaining a speed of over one hundred revolutions per minute. Now reverse the direc- tion of the current, or reverse the position of the bar magnet's Faraday's Apparatus poe Exhibiting the Rotation op a Coeeenu- Careting Conductor Round One Pole op a Magnet. Index to Parts. NS represents N and S poles of a, vertical magnet. B MC T + ,T- TC BW A BC Battery arranged to give a strong current. Mercury cup of wood and of annular form. Terminals where current enters and leaves apparatus. Top centre with mercury contact. Balance-weight to current-carrying wire. Arm of copper wire soldered to the vertical magnet. It is fixed in the first case, and free to rotate with the magnet in the second case. Bottom centre (adjustable) in second figure. Direction of current, and curved arrows the direction of rota- tion in each case. poles, and the wire rotates in the opposite direction. You natu- rally ask, " But how am I to reason out and to remember the precise direction of the current's rotation for any particular direction of current with respect to the N or the S pole of the magnet ? " You will have no difficulty in doing so if you 134 LECTURE XV. will first make a large figure of the precise arrangement of the several parts, and then plot down with dotted lines and arrow- heads the direction of the bar magnet's field and of the current's circular magnetic field around the current-carrying wire. You will remember that we have frequently shown that if a magnet were free to move in obedience to a current's field, the magnet would tend to so place itself that a maximum number of the current's magnetic lines of force passed through the magnet in the same direction as the magnet's own lines naturally flow through itself. Consequently, if the magnet be fixed but the current's con- ductor be free to move, the latter always tends to move into a position so as to confirm, and if possible to increase, the strength of the magnet. You also remember the hight-hand test which we have so frequently urged you to apply when the magnet was free to move. Now since the case of the current being free to move is just the very reverse of the magnet moving due to the reaction being directly in the opposite way to the action, we now ask you to apply your left hand to the wire, with the palm of the hand 'placed thereon facing the fixed magnet, which is on the opposite side of the wire, and with the four fingers in the direction of the current's ■flow along the wire, then the outstretched thumb will indicate the direction of the rotation of the current-carrying wire. Sketch a left-hand, as just directed, in your figure of the apparatus, and you will find that you cannot fail to remember the result of the above conclusions, which have been arrived at from reason- ing out the directions of the two forces in action, viz., the force due to the magnetism of the bar and that due to the current's field. (2.) Arrange your apparatus as shown by the second of the last two figures. Here the vertical broad bar magnet, instead of being fixed, is free to rotate on its magnetic axis. The current is this time conveyed from the centre up the sides of the magnet and past each side of the N-pole. Apply your left hand to either edge of the magnet, with the fingers pointing in the direction of the current's flow and the palm facing the N-pole; then your outstretched thumb at once reveals the direction in which rotation will take place. Reverse the magnet, or reverse the current's direction, and immediately the rotation is reversed.* * Messrs. Eadie and Nicol, two of the author's day-students, have just made from the last figure a large working model to illustrate this experiment. With a magnet J inch thick, 2J inches broad, and 14 inches long, they ob- tained thirty-nine revolutions per minute, with about thirty amperes of current. They fixed a ^-inch metal V mercury cup to the top of the magnet, and a /\ brass centre for the bottom centre to bear against, -which obviated the neces- sity of boring holes in the ends of their magnet. ROTATION OF A MAGNETIC CURRENT. 135 The student will at once understand the reason for not passing the current (in the last experiment) throughout the whole length of the magnet when we remind him that the action of the two poles being equal and opposite upon the current's field, no motion could take place ; therefore the action and reaction must be con- fined between one pole and the current. The following figures illustrate the form in which apparatus is generally made and sold by electrical instrument-makers for exhibiting the effects which we have just discussed. The middle figure shows the ad- justable stand with annular mercury cup, and the vertical magnet with radial bent arm of copper wire, as in the last experiment. The current is, however, conveyed along the lower half of the magnet between the terminals Tj and Tj, instead of by the upper half. This magnet may be easily removed and replaced by the two bar magnets with central spindle shown by the left- hand figure, when it is desired to illus- trate the rotation of the poles of a magnet round a current. These two magnets rotate within the central hollow of the annular mer- cury cup, and they may be placed with both S-poles or with both N-poles downwards, or again with the polarity of the poles in opposite directions, in which case no motion will take place if the poles are of equal strength and the current flows equidistant from each of them. On the right hand is shown another form of apparatus to be placed in the fixed stand. Here you see a central fixed vertical bar magnet topped by a -free or movable semicircular conductor or bent wire, whose ends dip into the annular mercury cup when the latter is sufiiciently elevated and fixed on its support. The current in this case divides along the two forks of the bent wire, and thus produces a couple or double Foe Eota- Toe Botation of Cue- Foe Bota- TION OP EBNTS THEODGH A TION OF POLES Maqnbt Bound its Divided BOUHD A N-POLE. CTTEEENT CUEEENT. Bound a Pole. 136 LECTTJEE XV. force tending to cause the forks to rotate round one pole of the magnet. Such rotation arrangements may be varied in many ways. The production of different forms of them was a great source of interest and enjoyment to Faraday, since they enabled him to test the conclusions which he had arrived at after patient thought and much study. The most beneficial practical outcome of their application has been the dynamo-motor, by which means any good dynamo-generator may be used for the conversion of electrical into mechanical energy. Sources of power, such as waterfalls and coalpits situated far from busy centres of industry, may thus be utilised by, first, the conversion of mechanical power (through the agency of watei>wheels or steam-engines and dynamos) to that of electrical energy, this energy being conveyed for miles, if need be, along well-insulated conductors ; and at the very spot, and to the re- quired amount the electrical energy may be again distributed and converted into mechanical energy, thus enabling artisans of all kinds to work at their special calling in their own homes, instead of having to congregate in some large factory where power has been concentrated by the adoption of great steam- engines and long lengths of power- absorbing shafting. The extension of this form of the distribution of motive power is only in its infancy in this country, but in America and on the Conti- nent many wonderful applications and adaptations of its capa- bilities have already been put into action. The Automatic Twisting of a Ourrent-Carrjing Wire Round a Magnet. — Bxpektmbnt XXII. — There is one very interesting modification of the foregoing experiments which was suggested some time ago to the author, but which he has not noticed in any text-book. Upon trying it before his class, he found that it worked admirably, and it is a very striking experiment. Referring to Case i in the next figure, you will observe that a cylindrical bar magnet is suspended in a vertical position from a wooden beam or other convenient support. A long fine flexible tinsel wire (such as is frequently used to adorn the grates of fire- places in the summer-time) is suspended from a terminal screwed into the wood, and the lower end of the tinsel is connected to another terminal fixed near to the lower end of the vertical magnet. These terminals are connected up through any con- venient form of reversing key with a battery.* The tinsel wire in Case i is shown hanging quite loose alongside the magnet ; in * The reversing key may be dispensed with if one is not at hand, and the connecting wires simply held on to the terminals of the battery and reversed between its poles at pleasure. TWISTING OF A CURRENT ROUND A MAGNET. 137 fact, the wire is what is called "dead," since no current is pass- ing through it. Case 2. — Depress Kj, so as to send a current upwards through the tinsel wire; it instantly becomes "alive," and twists itself rapidly round the magnet in the form of a beautifully defined left-handed screw-thread. Case 3. — Now relieve Kj and depress K^; immediately the tinsel untwists from the magnet, and then twines itself on again in the form of a right-handed screw-thread. The whole operation takes place so rapidly that a few pieces of white tissue-paper should be fixed to the tinsel wire in order to render the movements mora apparent. The experiment may be enjoyed by a large audience. Case 3. Current Down. ji|K ^i|k ^i| B B B Automatic Twisting os a Cukrent-Cabeting wieb Eound a Magnet. N.B. — The lower end of the magnet is several inches clear from the table. To foretell the directions in which the flexible wire will twist around the magnet, all you have to do is to hold your left-hand palm on this wire, as previously directed, with the fingers in the same way as you intend to pass the current, when the outstretched thumb will point towards the direction ; or you may apply your right hand across the magnet, as directed at the end of Lecture XIII., and this will foretell the way in which the current must flow along the tinsel wire when it forms a right- or left-handed spiral around the magnet, for the current must flow so as to increase the magnetism of the magnet, i.e., the lines of force through the magnet and the current's magnetic lines always tend to flow in the same direction, and thus mutually assist each other 138 LECTURE XV. — QUESTIONS. Leotdbe XV. — QnESTiONS. 1. What is meant by the expression, "Action and reaction are equal and opposite," as applied to forces ? Give illustrations with sketches, 2. How could you prove that the north pole of a magnet will revolve in one direction, and the south pole in the opposite direction, round a current- carry- ing wire ? How could you predict the directions in each case ? Give com- plete sketches. 3. Sketch Faraday's apparatus for demonstrating that a continuous current will revolve round one pole of a bar magnet, and describe its action by arrows, &c. If the current is reversed, what takes place, and why ? 4. A vertical fixed magnet with its north pole pointing upwards, supports an P-shaped wire free to rotate around and parallel to the magnet, (a.) If the lower end of the wire dips into an annular mercury trough placed at the foot of the magnet, and a, current be sent straight up the magnet from the S to the N pole, and then down the vertical leg of the r> what will happen, and why ? (5.) If half of the vertical leg of the f be iiow cut away, and the mercury cup raised until its lower end again dips into the mercury and the current reapplied, what will happen, and why ? (c.) If the current is then reversed, what will happen, and why ? Give sketches illustrating your answers in each case. J. A very flexible wire is hung loosely alongside of a vertical bar magnet. A current is passed down this wire. What will happen, and why? The current is stopped and then reversed. What will happen, and why ? Give sketches. 6. A current is passed along a long, cylindrical, permanent magnet. What reasons can you give for supposing that the current will take a spiral course along the bar ? In making your sketch to illustrate your answer, mark the N and S poles of the magnet and the direction of the spiral, i.e., indicate whether it is the same as a right- or a left-hanrled screw-thread. 7. A circular coil is suspended in a vertical plane by two long, flexible con- ductors joined to the ends of the coil. A bar magnet is presented (a) hori- zontally, with one end towards the centre of the coil ; (5) horizontally, with the magnetic .axis of the m.agnet parallel to the horizontal diameter of the coil ; (c) vertically, with the magnetic axis of the magnet parallel to the vertical diameter of the coil. Sketch what will happen to the coil in each case when a strong current is passed through it. 8. Sketch and describe how a freely-suspended vertical coll of wire tra- versed by a current would place itself if suspended between the poles of a horse-shoe magnet. ( 139 ) LECTURE XVI. Contents.— Electro-Dynamics — Amp&re's Laws — Action between Parallel and Inclined Currents — Ainpfere's Stand. — The Jumping Spiral, and other Apparatus for Illustrating Ampfere's Tjaws — Questions. Electro-Dynamics. — You are now ia a position to study this part of the science which is concerned with the force which one current exerts upon another, termed Electm-dynamics. This section of electro-magnetism was first demonstrated and de- veloped by a French philosopher named Ampere in 1821, shortly after Oersted's discovery of the action of a current on a magnet. If you bear in mind that a current always produces around its wire path a circular magnetic field of definite polarity, you will have no difficulty in resolving Ampere's Laws into the simple manifestations of attraction between unlike poles, and repulsion between like poles. We shall only give two of Am- pere's Laws here in our own words, and then proceed to prove them by experiment. Ampere's Laws^ Action between Parallel and Inclined Cur- rents. — Law I. — Parallel currents, if in the same direction, attract one another ; and if in opposite directions, they repel one another. ■*"^zz <- zzz '^zz -<- zzz ^. CJg-" *~ "^ "^ "^D ^ zz -> zzz n.- ZZZZ2ZZ in • „ Directions of currents. ^--» „ Directions of current's fields. eS' „ Hand-test for polarity or direction of current's field. for demonstrating his First Law, since it permits of the current- carrying-wire rotating completely round about the central vertical support. Tn the above figure, if you place your eight hand as shown 142 LECTURE XVI. (with the palm in each case facing the reader), you cannot fail to observe that there exists between the two parallel current- carrying wires the same electro-magnetic polarity; or, in other words, ilie directions of the two fields oppose each other. However, reverse the direction of the current in Wj, and you at once get attraction, because the fields now offer N and S poles towards each other. Test this again, by aid of your eight hand pro- perly placed on each of the wires, and you again confirm your belief in our rule, thus obviating all necessity for committing to memoi'i/ Ampere's First Law. Experiment XXIV. — A very striking and simple experiment, which confirms the first part of the First Law, is illustrated by the following figure. The figui'e is self-explanatory. On closing the key, a current flows from the battery, B, to the positive terminal, T -f , then up the vertical stand rod and down the whole length of the open spiral of steel or hard copper wire, S, into a mercury cup, MC, returning by the horizontal base-board connection to the negative terminal, T — , and thence back to and through the battery. Each successive turn of the spiral carries a parallel current in the sam,e d irecl ion. The consequence of this naturally is, that opposite current poles, N and S, are induced between every two turns, causing such a strong combined attraction that the whole spiral contracts and the lower end of the wire is lifted out of the mercury. This breaks the circuit and cuts off the current. Gra- vity, now unopposed, reasserts herself, and pulls down the spiral to its normal length, thereby again completing the circuit. The ycire thus keeps on jumping automatically up and down, creating a big electric flash at the mercury cup each time the circuit is broken. This jumping action is considerably augmented by putting a soft iron rod, SI, down the centre of AMpfaE's First Law, Illustrated by Roget's Ju.mping Spiral. AMPilEE'S LAWS. 143 Apparatus foe Testing} Ampere's First Law. Index to Parts. Bi, B2, for Batteries. the spiral (as shown by the figure), since it concentrates and intensifies the magnetic field between the different wire circles. Experiments XXV. — Just one more proof of Ampere's First Law, and then we will turn to his Second Law. Arrange the apparatus as illustrated by the annexed figure. Hold the wooden arm Ag, and present the coils Cg, 0^, fairly opposite to Oj, Cj. Attraction takes place between C^ and O3, while at the same time repulsion takes place between Cj and C^. Sir William Thomson has devised a complete set of Standard direct-reading Elec- tric Balances upon this prin- ciple, which measure from j-^^ to 2500 amperes.* Referring to the first figure of Ampere's Stand (see Ex- periment XXIII.), remove the bar magnet from the base board, and hold the straight part of wire Wj horizontally under, but at any angle with the lower horizontal side of the rectangle W^. Immediately Wj swings round until it lies parallel to Wg (just as a freely-suspended horizontal bar magnet would do if placed over another horizontal bar magnet), thus proving Ampere's Second Law. Here, again, you have no necessity for committing this law to memory, for all you have to do in order to predict the result is to place your right HAND upon each of the wires in turn (as previously directed), and you cannot fail to realise how these two current-carrying wires will tend to become parallel owing to the natural attraction between the N and S poles, and at the same time the natural repulsion between the N-< — *-N and the S-< — >-S poles of the two fields. Many practical testing instruments have been devised upon this principle, such as Siemens' electro-dynamometer, &c. This Second Law may be very prettily demonstrated by the following cleverly-designed instrument. Its construction will at * For a description of these instruments and how to Use them, see Eankine's " Rules and Tables," 7th edition, 1889, p. 398, and Munro and Jamiesou's "Pocket-Book of Electrical Rules and Tables," 6th edition, 1889, p. 82. K Ci, Co, C3, C4, Ai, A„ WB, Coils in circuit with Bi. Coils in circuit with B2. Arms of wood. Wooden beam supporting Ci, C2, by two flexible wires. Direction of currents. UA LECTURE XVt. once be understood by comparing the drawing with the " Index to parts." Its action is as follows : — ArtAEATUs TO Illustrate AmpI;ee's Second Law. B represents Battery. F, R, represent Forward and return T + ,T-, „ Terminals. sides of MCi, or MC. MC2, „ Mercury contacts. vioe versA if B.'s 1, 0, Inner and outer cups current reversed. of MCi. IC, OC, „ Inner and outer coils, Division of wood to both being free to bottom of MC2, which acts as a rotate on their ver commutator * and changes the tical axes. direction of current in 10 every -> „ Directions of currents. time it passes D. Auy device which changes the direction of a current in a circuit is called a commutator. AMPiiEE'S LAWS. 1 45 The current from the battery, B, flows independently through each of the coils, 00 and 10. The ends of the outer coil, 00, dip down into the inner, I, and outer, 0, cups of the lower mercury trough, MOj, whereby the coil receives a continuous current. Two wires also dip into I and from the upper mer- cury contact trough, MO2, which is subdivided into two distinct parts by an insulating division, D. The current in this inner coil, 10, is thus broken and then reversed every time its dipping ends pass this division,* but the momentum of the coil carries it over this " dead point " or the position of equilibrium between the two current fields when the coils are in the same plane. You obtain in this way contimioiis rotation of both coils in opposite directions. The student should draw a plan of the above coils showing their positions for different stages of their revolutions, marking with arrows and bight hands (properly placed) the directions of the currents, their fields, and the attraction or repulsion (as the case may be) between the forces. By so doing he will render himself quite independent of requiring to remember Ampfere's Laws.t * The mercury is filled into the sides F and R of MO2 until it rises a little above the division D. In the drawing the division is shown a little above the mercury, to assist in understanding its construction. + The author has intentionally drawn the above figure to a large scale in order that it may serve as a working drawing to those teachers and students who may desire to construct the apparatus. Eight to ten times the above size would make a splendid model for lecture purposes. He is indebted for the idea and description of this interesting model to Professor George Forbes' Course of Lectures on Electricity, delivered before the Society of Arts, London, in 1886, and now printed in book form by Messrs. Longmans, Green, &Co. 146 LECTURE XVI. — QUESTIONS. LeCTDBE XVI. — QCBSMONS. 1. What is meant by the term " electro-dynamics " ? Who first investigated the action of currents upon each other, and when ! 2. State Ampfere's First Law ? Sketch in section an improved form of Amp&re'a Stand, and describe how you would use it in order to prove that parallel currents in the same direction attract, and in the opposite direction repel, one another. 3. A long spiral of stiff copper wire is hung vertically from a support, so that its lower end just dips into a pot of mercury. A strong current is sent through the spiral, what happens and why ? Give a complete sketch. 4. Will the action you describe in answering Question 3 be automatic and continuous so long as the current is applied to the circuit ? What would be the effect of reversing the current through the spiral ? What effect, if any, would there be on introducing an iron rod down the axis of the spiral ? Give complete sketches. 5. State Ampfere's Second Law. Sketch and describe auy form of apparatus known to you by which you could clearly demonstrate this law. 6. Show how by the simple application of "the kight-hand aid to memory " you can dispense with the committing to memory Ampfere's Laws, and instead reason out for yourself and predict the directions in which attrac- tion or repulsion between current-carrying wires must take place, however they may lie towards each other, and in whatever directions the currents may flow through them. 7. A coil of wire, free to turn on a vertical axis, is suspended within a fixed coil. A current is sent through both coils. How will the inner one turn, and why ? Give a sketch. ( 147 ) LECTUKE XVII. Contents. — Eleotro-Magnetio Induction — Currents Induced iu a Closed Circuit by the Motion of a Magnet in its Vicinity, or vice versd — Currents In- duced in a Closed Circuit by the Motion of a Current-Carrying Coil in its Vicinity, or vice versd — Different Directions of the Induced Currents on Approach and Withdrawal of a Secondai-y Circuit Moving in the Primary Field — Induced Currents in a Closed Secondary Circuit on Making or Increasing, and on Breaking or Diminishing, the Primary Current — Table of Induction Currents — Faraday's Law — Lenz's Law — Electro-Motive Force, Resistance, and Current — Comparative Statements of the Forces, Kesistances, and Currents, Illustrated by Hydraulic and Electrical Circuits — Ohm's Law — Questions. Electro-Magnetic Induction.— Just ten years after Ampk'e de- monstrated the mutual action of electric currents, Faraday (in 1831) discovered that the motion of a magnet, or the motion of a current-carrying wire, in the vicinity of a closed circuit (or vice versd), produced electric currents in the latter. This action of a magnet or current in inducing secondary currents has been termed Electro-Magnetic Induction.* The phenomena of the induction of currents by magnets and currents form perhaps the most interesting as well as the most valuable branch of the study of current electricity or electro- kinetics (electricity in motion), since not only the Telephone, the Dynamo, and the Transformer (or Induction Coil), but also many other useful inventions for the practical application of electricity, have lately been produced, which for the most part depend directly upon this discovery by Faraday. We shall first illustrate a few of Faraday's experiments, at the same time endeavouring to render the actions which take place clearer by the assistance of the knowledge already gained by the student in the previous Lectures ; then state Faraday's Law, and finally explain what is meant by "Difference of Potential," and * Eleetro-Magnetie Induction.— li any conductor forming a closed circuit is placed in a magnetic field, either wholly or in part, and if any part of that circuit is made to move so as to cut across or traverse lines of magnetic force, an electro-motive force is set up in that circuit. This action is called electro- magnetic indMCtion. If the originating cause of a current is the movement of a conducting circuit in a magnetic field, such current is called a magneto- electric current.— ~Dv- J. A, Fl^^minq 148 LECTURE XVII. JIfatioit of JUagmt / cnerent indtiobd in a closed coil by the Motion of a Masnbt. the " Electro-motive force," which causes the flow of electricity along a closed circuit. Currents Induced in a Closed Circuit by the Motion of a Magnet in its Vicinity, or vice versa. — Bxpeeiment XXVI. — Take a bar magnet in your right hand, and a coil of fine insulated wire (of many hundred turns) in your left hand. Let this coil be connected up in circuit with a sensi- tive galvanometer * placed at least ten feet away from the magnet, so that the motion of the latter may not directly affect the needle of the former. First, Approach and in- troduce one pole of the magnet suddenly through the central opening of the coil. You observe a momentary deflection of the galvanometer needle to one side, followed by an immediate return to zero, thus indicating that a transitory current of electricity passed through the coil and galvanometer circuit. Second, Withdraw the magnet quickly to a distance from the coil, and you observe that the galvanometer needle swings round to the other side of zero, and then at once comes back to zero again, thus indicating that a second current of short duration has been induced in the coil, but this time in the opposite direction to that when the magnet approached and passed into the coil. Now hold the magnet steady in your right hand, and suddenly bring forward the coil'ovier the magnet, or approach both of them towards each other, and you observe the same effect as in the first case. Withdraw the coil from the magnet, or withdraw them simultaneously from each other, and you observe precisely the same effect as in the second case. It does not, therefore, make the slightest difference to the direction of the induced current whether the magnet approaches the coil or the coil the magnet ; so long as their relative direction of motion is the same, the effect is the same. You cannot, however, help observ- * The style of galvanometer shown by the above figure is of the ordinary detector pattern, which we described in Lecture X. as not being sensitive ; yet by making it with a large number of turns of wire and a long needle deli- cately poised, the author finds that he can carry out this experiment so that it can be seen by a large class. Of course, a Thomson's mirror galvanometer of long range, with the spot projected on the wall of the class-room, and the gas-lights partially turned down, is more impressive, CURRENTS INDUCED IN A CLOSED CIRCUIT. 149 ing a marked difference in the deflection of the needle (to the right as well as to the left) when the coil and magnet are brought together or separated quickly or slowlij. In fact, if you bring them together very slowly, you will produce scarcely any per- ceptible deflection ; whereas, if you do it very quickly, you may send the needle spinning round on its axis. Consequently, ilie more rapidly the coil and magnet approach or recede from each other, the stronger loill he the current induced in the coil. Currents Induced in a Closed Circuit by the Motion of a Cur- rent-Carrying Coil in its Vicinity, or vice versa. — Experiment XXVII. — Arrange your apparatus as shown by the annexed figure, and close the primary circuit, P, by depressing the key. Watch the direction in which the compass-needle, ON, turns, so as to know the direction of the pri- mary current through its solenoid. Now remember- ing that an electro-mag- netic solenoid or current - carrying wire produces a magnetic effect equivalent to a magnet, you will have no difficulty in connecting in your mind the following results with those of the last experiment : — First, Suddenly ap^roac/i the secondary coil, S, towards one end of the primary coil, P, and you *- observe the galvanometer CN momentarily deflected (say) to the right or down. It then swings back to zero. Second, Quickly with- draio the secondary coil from the primary one, and you notice the needle de- flected to the left or up ; and here again it imme- diately returns to zero. Precisely the same results as in the last experiment. Test the polarity of your primary coil, and if you so arrange the direction of the primary cur- Hmvn OuEBENTs Induced in a Closed Coil bt its Motion neak Cukkent-Caebying Coil. Index to Paets. for Battery with key. Compass-needle under wire. Primary coil. Secondary coil. Galvanometer. Iron core. B CN P S G IC 150 LECTUEE XVII. rent that the upper end is a N-pole, the deflections will also coincide in direction with those of the previous experiment. Reverse the polarity of your primary coil by reversing the battery current, and the deflections of your galvanometer needle will be reversed from what they were before on approach and withdrawal respectively of the secondary coil. Test also the effect of bringing the secondary coil towards and away from the primary coil suddenly, and slowly, and you confirm the results previously obtained. Now insert a soft iron core, IC, into the secondary coil, and again try all the above experiments. The deflections are not altered in dii'ection, but they are very much increased for the same rate of approach and withdrawal. You know the reason of this, for you have learned from previous experiments with solenoids and electro-magnets that a soft iron core concentrates the magnetic lines of force, and renders the field more intense within and close around it. In any case, you will understand the reason better after you follow our next expe- riment and explanation. Different Directions of the Induced Currents on Approach and Withdrawal of a Secondary Circuit from the Primary Field. — Experiment XXVIII. — Looking at the two following figures, you observe (as you must have frequently observed before when examining the magnetic curves of magnets and solenoids) that FiGUBES IlLUSTBATING DIRECTION OF INDUCED CuBBENTS DUE TO ApPKOAOH. the lines of force are more closely packed together close to the end of a magnet or a current-carrying coil than they are at some distance from it. In other words, the field gets more and more intense the closer you get to the poles. First, For the sake of explanation, let the pole facing the movable or secondary coil (to the right of each of the two figures) be a N-pole,and let the secondary coil be brought sud- denly forward towards this N-pole in each case. Not only do you observe the deflections of your galvanometer needles are in the same direction, but you experience a gi-eater resistance to the motion of the coil than you would do if the circuit was not closed DIFFERENT DIRECTIONS OF INDUCED CURRENTS. ISI or if you moved it with the same rapidity through the air when no magnets or current- carrying coils were near it. There is an opposition to the force you exert when you move it in a strong magnetic field in the way represented by the above diagrams. Why is this ? you naturally ask. Well, you remember that in bring- ing up a N-pole of a magnet towards another Npole, or a S pole towards a S-pole, or when bringing towards each other similar or like poles of electro-magnetic solenoids, you experienced a similar repulsive foi'ce against your efforts to bring them together. Consequently, you conclude that the current generated by induc- tion in the secondary coil must be of such a direction as to pro- duce a North or like polarity on its leading side to that of the jjole it approaches. Therefore, by placing your eight hand on the secondary, with the outstretched thumb pointing in the direction of motion (that is, tmoards the locality of the N-pole or face to which your secondary coil approaches), your fingers naturally point in the direction of the induced current (as shown by the arrows on the dotted secondary coils). Second, After the secondary coils have been brought close up to the N-poles of the magnet and the primary coil respectively, suddenly pull them away therefrom. You again experience a greater resistance than you would do when moving these secon- dary coils as quickly through the air, or as quickly from these poles if the circuits were not closed or complete. But you have often before experienced a similar opposing force when with- drawing the S-pole of a magnet or an electro-magnetic solenoid from the N-pole of another magnet or electro-magnetic solenoid. Consequently you conclude that the current now generated by induction in the secondary coil must be of such a direction as to produce a South or unlike polarity on the side nearest the pole from which it is being withdraiim. Again, place your right hand on the secondary coils with the outstretched thumb pointing in the direction of motion (that is, away from the north face of the magnet or primary coil), and your fingers naturally point in the direction of the induced current. We need not here repeat the experiments with a S-pole of a magnet or of a primary coil facing the secondary coil, for the student will at once be able, from what we have just said, to determine for himself, by aid of his EIGHT HAND, the directions of the induced currents in cases of approach and withdrawal from the same. What you have to re- member is this, that when you move a coil so as to increase the number of magnetic lines of force through it, your induced current must be of such a direction as to produce a repelling {i.e., a like) pole to these lines, because you are forcing the coil against the field ; and when you move the coil so as to diminish the lines of 152 LECTURE XVII. force througli it, your induced current must he of such a direction as to produce an attracting {i.e., an unlilte) pole to these lines, because you are forcing the coil aioay from the field. Induced Currents in a Closed Secondary Circuit on Making or Increasing, and on Breaking or Diminishing, the Primary Current. Experiment XXIX. — First, Take the cases of two parallel straight wires joined up as shown in the two following figures.* B B Induced Cueebnts in Straight PAEALLEn Closed Ciecuits. (i.) Close the primary circuit, P, by putting doion the key, K. A momentary current is observed, but in the opposite direction in the secondary circuit, S. (2.) Suddenly increase the current in tYieprimary (by switching on more cells), and again a momentary inverse current is observed in the secondary. (3.) Stop the current in the primary by letting up the key. A momentary direct current is observed in the secondai-y, i.e., in the same direction. (4.) Close key, and suddenly diminish the current in the^ri- mary (by switching out some cells), and again a momentary direct current is observed in the secondary. In Oases (i) and (2), by placing your eight hand properly on the primary wire, P, you observe that the primary current induces, as it were, N-polarity all along the side facing the secondary wire, S. In order that an induced current may be generated in the secondary wire, S, the current must therefore be of such a direction through it as to also produce a like or N-pole on the side facing the primary wire, for energy must be spent in opposing the generat- ing of this secondary current. In Cases (3) and (4) energy has also to be spent in opposing the dying away or diminishing of * The necessary galvanometer or detecting compass-needle has unfortunately been left out of the previous, the above, and the following primary circuits. It is useful, in doing the experiments, to show when the current (or its deflec- tion) in the primary is in the same or in the opposite direction to th.-it in the secondary circuit. TABLE OP INDUCTION CURRENTS. 153 the primary current and its field ; consequently the current in- duced in the secondary must present an unlike or S-pole along the face next to the primary wire. Experiment XXX. — Instead of two straight parallel wires, take two flat or two cylindrical or solenoidal coils, as illustrated by the two following figures, and again perform the same four expe- FlGUKES IHUSTEATING INDUCED COEEBNTS IN PARALLEL FLAI COILS AND CYLINDEIOAL COILS. Note the direction of the Current in the Primarp, also the Induced Currents in the Secondary on making and breaking the Primary Circuit. riments with the right-hand proofs as you have just seen done with straight wires. You will obtain precisely the same results, only the induced currents will be stronger for the same current strength in the primary, and distance between it and the secondary circuit. Put a piece of soft iron into the heart of each coil, and the induced currents will be still further increased, because the field is thereby increased in the coils. The results of the foregoing experiments in this Lecture may be put down in tabular form as follows : — Table of Induction Currents. MoMBNTART Inyeise Currents (or opposing Like Facing Poles) aee Induced in a Secondaey Circuit. By (i) Approach to Primary. „ (2) Starting Primary Current. „ (3) Increasing Primary Current. Momentary Direct Currents {or attracting Unlike Facing Poles) abx Induced in a Secondary Circuit. By (l) WUhd/rawal from Primary. „ (2) Stopping. Primary Current. „ (3) Decreasing Primary Current. 1 54 LECTURE XVII. We are now in a position to appreciate and (so far) to under- stand Faraday's Law and Lenz's Law. (iJe«d P- 156 Wore pp. .54 <»»<2 ^ss) FPeM EARTH ERBIB The Flow or a AVateb Cuekent Under Aqua-Motive Fokoe Theough a Pipe. The Flow of an Electeig Ctjkeekt Unpee an Eleoteo-Motive Force Tnnoran A Coppee 'Wire. electro-motive force, resistance, and current. 1 55 Comparative Statements of the ForceSj Resistances, and Currents, Illustrated by the Two Previous Figures.* Mef erring to the First Figure. 1. We show a Water System whereby on the turning of a tap or cock the circuit is closed, and a Curtent of Water issues from a tank or mechanically elevated source of mechanical energy,! due to the "head " or pressure or aqua-motive force possessed by the same above the position where it leaves the system for the earth, after having done work in turning the water motor. 2. The difference of level between the free or far surface of the water in the tank and the position where it leaves the water motor determines the total difference of pressure or "aqua-motive force" (due to the natural action of gravitational energy). It is this total difference of pressui-e which causes the fluid to flow through the tank and along the pipe, and turn the water motor with a definite ewrrent strength. 3. Now, leaving out of the question the reaction due to the water motor, and sup- posing the water fluid to pass clear away from the lower or negative end of the pipe to the ground, then the Current Strength or flow of water is directly proportional to the " head " or Total Ditferbnoe of Pressure between the far end of the source of supply, and inversely propor- tional to the Fbiotional Resistance which this pressure overcomes in flowing through the tank and the pipe, or water- current conductor. 4. There is no pipe, however large or smooth, that it does not offer some fric- tional resistance to the flow of the water along it, and consequently reduce the total "head" or aqua-motive force. The rougher and smaller the pipe the greater the loss of head. Itefeiring to the Second Figure. 1. We show an Elbotrioal System whereby on the turning of a key or switch the circuit is closed, and a Current of Electricity issues from a dynamo or battery or electrically elevated source of electrical energy,! due to the " potential" or pressure or electro-motive force possessed by the same above the position where it leaves the system for the earth, after having done work in turning the electric motor. 2. The difference of electrical level be- tween the far end of the dynamo or battery and the position where it leaves tlie electri- cal motor determines the total difference of potential or " electro-motive force" (due to natural action of electrical energy). It is this total difference of pressure which causes the fluid to flow through the dynamo or battery along the copper wire, and turn the electrical motor with a definite current strength. 3. Now, leaving out of the question the reaction due to the electrical motor, and supposing the electrical fluid to pass clear away from the lower or negative end of the copper wire to the ground, then the Current Strength is directly propor- tional to the Total Diffeeenob of Po- tential between the far end of the source of supply, and inversely propor- tional to the Electrical Resistance which this pressure overcomes in flowing through the dynamo or battery and the copper wire or electric current conductor. 4. There is no electrical conductor so large or so good tliat it does not offer some electrical resistance to the flow of the elec- trical energy along it, and consequently reduce the total difference of potential or electro-motive force. The smaller and worse the conductor the greater tiie loss of electro-motive force. * Read each of the corresponding statements i and i, 2 and 2, &c., in turn. t Whether the mechanical energy be developed from an elevated tank of water, or a pump or any other means, does not much matter as far as out present comparison of the forces, &c. , is concerned. X AVhether this electrical energy be developed from mechanical energy by the moving of a conducting circuit in a magnetic field, as in the case of dynamo machines, or from chemical energy by the burning of chemical substances in a battery, or by any other means, does not matter much as far as our present comparison of the forces, &c., is concerned. I $6 LECTURE XVII. Faraday's Law (183 1). — If any conducting circuit he placed in the magnetic field of a permanent magnet or of an electric current, then, if by either a change of relative position or a change of strength of primary current, a change is made in the number of lines of force passing through the Secondary, an ELECTRO-MOTIVE FORCE is set up in the Secondary proportional to the RATE at which the number of included lines offm-ce is varying. Lena's Law (1834).— J» all cases of ELECTRO-MAGNETIC INDUCTION the induced currents have such a direction that their reaction tends to stop the motion which produces therfi. Electro-Motive Force, Resistance, and Current. — ^You observe in Faraday's Law the expression " an Electro -motive Force is set up in the Secondary proportional to the Rate at which the number of included lines of force is varying." We would like you to understand the meaning of the term " Electro-motive force," and if possible the whole expression, although of course we cannot fully discuss this important subject in such an Elementary Manual as the present. We shall, however, frequently return to it, more especially in our Advanced Text-Book. We shall not here attempt to carry further the comparison between the hydraulic and electrical systems. We however think it advisable to state Ohm's law, which expresses the above- mentioned relation between Current strength and the Electro- motive force * which keeps the current flowing through the Resist- ance which the force meets with in an electrical circuit. Ohm's Law,-Current ^ Electro-motive force. Total Resistance. Or if represent the Current, E the Electro-motive force, and R the total Resistance in circuit, then =5., or E = X R, or R =^. Unfortunately, we have left no time or space to refer to the induction coils, dynamos, or transformers, whose principles of action depend so directly upon the fundamental experiments of Oersted, Ampfere, and Faraday. We must now take up the con- struction and action of batteries. * The terms electro-tnotive force and total difference of potential are syno- nymous. When we speak, however, of potential difference, we merely mean the difference of electrical pressure or voltage between any two points in the circuit, such as the potential difference between the ends of the copper wire conductor shown in the last figure. The term electro-motive force is reserved for the total difference of potential, or the force urging the current throughout the whole circuit. LECTURE XVII. — QUESTIONS. I S 7 Lkcturk XVII. — Questions. I . . What is meant by the term Electro-Magnetic Induction ? 2. Sketch and describe an experiment to illustrate the induction of currents in a coil of wire joined up to a galvanometer when a magnet is moved towards, and away from, the axis of the coil. Suppose that you shoot the magnet completely through the centre of the coil at one stroke, explain what occurs, and why ? 3. You have a metal hoop. Describe, and give a figure of, some arrange- ment by which, without touching the hoop, you could make electric currents pass round it first one way and then the other. (S. and A. Exam., 1882.) 4. Sketch and describe an experiment to illustrate the induction of currents in a secondary coil when it is moved towards and away from the end of a primary current-carrying solenoid. Why is the direction of the current different on approaching and withdrawing the secondary coil i Mark the directions of the primary and the induced currents in each case. 5. Why is the number of swings of a compass-needle considerably reduced by placing it in a metal box 1 6. A piece of covered wire is passed a tew times round a wooden hoop ; its ends are joined up to a galvanometer. The ends of another piece of covered wire which is wrapped round a similar hoop are joined up to a battery. What will happen if the two hoops are (a) brought quickly near to one another, and (6) if they are quickly separated ? (S. and A. Exam., 1879.) 7. How could you temporarily stop or weaken a current in a wire without disconnecting it from the battery, by means of the motion of another wire through which a current is passing ? (S. and A. Exam., 1883.) 8. State Earaday's and Lenz's Laws. 9. Draw a comparison (in your own words) between the flow of water through a pipe from a reservoir and the flow of electricity from an electrical source through a wire. 10. State Ohm's Law, and give your ideas of the meaning of the terms potential, electro-motive force, potential difference, voltage, electrical resis- tance and current. ( 158 ) LECTURE XVIII. Contents. — Historical Note on the Discoveries of Galvani and Volta, &c. — Volta's Pile — Origin of the Terms Voltage and Volt, &c. — Simple Vol- taic Cell and its Chemical Action — Polarisation — Local Action — Amal- gamation of Zinc Plates — Daniell's Cell and its Chemical Action — Find- ing the Fall of Potential through a Cell, and Measuring its Internal Resistance — Different Forms of Daniell's Cell— Grove's and Bunsen's Cells and their Chemical Action, &c. — Questions, Eistorical Note on the Discoveries of Galvani and Volta, &c. — About the year 1780, Galvani, Professor of Anatomy at Bologna in Italy, whilst experimenting in his laboratory with a frictional electrical machine, observed that some recently skinned frogs, lying on a table near the machine, twitched convulsively when- ever the machine was worked. Not long afterwards he noticed the same effect produced on several dead frogs which had been, hung on an iron balcony by means of cupper hooks, whenever the wind brought their legs into contact with the iron. He repeated these experiments, and concluded that at the iunction of the nerves and muscles there is a separation of the two elec- tricities ( 4- and — ), the nerve being positively, and the muscle negatively electrified, and that the convulsive move- ments were due to the two electricities being connected by the metal hook and the iron balcony. About 1800, Volta, Professor of Physics in the University of Pavia in Italy, in- vestigated these effects more thoroughly, and showed by means of a delicate con- densing electroscope * that the seat of the electrical energy lay in the contact between the two dissimilar metals (copper and iron), and that the frog's muscles and nerves simply served the purpose of completing the circuit, and of * This instrument and its action will be described when we come to Electro- Statics or Frictional Electricity, but, in the meantime, see the figure on the next page. Volta's Pile. DISCOVEEIES OF GALVANI AND VOLTA. 159 Voltaic Pile as now Made with Terminals. rendering the presence of the force visible by their contraction. He carried out a number of experiments, and finally produced what is known as the Voltaic Pile. This, as may be seen from the last and the next figure, consisted of discs of copper, C, and zinc, Z, with wet cloth or blotting-paper, W, moistened with brine ; the object of the wet cloth, in his idea, being merely that it might act as conductor, and prevent contact between each two pairs of copper and zinc plates and the next pair. With such a pile, composed of a large number of pairs of discs of dissimilar metals, and the con- densing electroscope, he excited considerable interest in the scien- tific circles of his day. He further discovered that by merely solder- ing together two bars of different metals, according as the one or the other was brought into con- tact with his condensing electroscope, the apparatus showed a free positive or a free negative charge. Volta's next discovery was that it was not necessary for the pairs of metals to be brought into actual metallic contact with each other, but that even better results were obtainable by plac- ing them side by side in a vessel containing some exciting liquid, such as dilute acid. Hence the simple voltaic cell illustrated by the next figure. For about forty years after Volta's dis- coveries, the cowtaci of dissimilar substances, either actually or through some conducting me- dium, was regarded as the seat of electro-motive force of the cell, and therefore the cause of the current. Afterwards, however, owing to the researches of Faraday and others, a new school sprang up, which advocated the L Hobizontal Voltaic Dry Pile with Movable Poles and Condensing Bleoteosoope. t6o LECTURE XVIII. chemical theory, i.e., that the chemical action which goes on in the cell not only produces the electro-motive force, but keeps it up, so as to supply a continuous current therefrom. Now-a-days it is generally believed that both contact and chemical action together play an important part in giving us an electric current. From this short historical note of the origin of the chemical method of producing electrical energy, you will understand why we adopted and still retain to this day the various terms Galvanic battery, Voltaic cell, Galvanism, and how the new term Voltage (or potential difference maintained by a battery or a dynamo between any two points on a conductor, as measured in Volts) has been derived. It is customary to commemorate famous scientific discoverers by naming after them some appa- ratus, effect, or unit of measurement, with the inception of which, or through whose investigations, we are indebted for any important addition to our knowledge of appliances, phenomena, or laws. Hence the name Ampere was given to the practical unit of Cur- rent, Volt to the unit of Eledriomotive force, and Ohm to the unit of Resistance. Simple Voltaic Cell. — If, in a glass or glazed stoneware jar three-quarters filled with dilute sulphuric acid in the proportions of I part concentrated acid to lo parts water, j'ou place a plate of pure zinc (Zn) and a plate of pure copper (Ou), as shown below, no change seems to take place in either the acid or tlie plates. ^^ ^ ^ ^i_. You can, however, I I ' -N ^#=C3=^ f f II detect by the aid ^ "•" -L A (11 A T— of a, ygjy delicate electrometer* that the free end of the copper pole is charged with posi- tive ( + ), and the free end of the zinc pole with negative ( - ) electricity; and further, that there is a definite difference of potential between these points. Now, such a cell is capable of furnishing a continuous current of elec- tricity; for whenever you join the ends of the copper and zinc poles *_An Electrometer is any instrument fur measuring differences of electro- static potential between any two points. + Cells are said to be joined in series so as to form a battery when the positive terminal or pole of one cell is joined to the negative of the next and ?o on, ■ 9 ! Two Simple Vomaio Cells Joined in Seeies.+ SIMPLE VOLTAIC CELL. i6i together through a galvanometer, you observe a constant deflection of the needle to one side, proving that a current flows in the outside circuit. Since it has been customary to consider the direc- tion of a current as if it were from the positive to the negative, or from a higher to a lower potential (just as we regard heat as flowing along a conductor from a higher to a lower temperature), you natu- rally assume that the current is flowing from the copper to the zinc pole in the outside or galvanometer circuit. To ascertain which way the current flows through or inside the cell, place a very delicately poised magnetic needle immediately under or immedi- ately over the cell, and with the aid of your right-hand test (so frequently explained before) you find that the current is moving Galvanomider \ Gipper Ji pUut&~Sitiphuric^-i -Zinc ^JMJMJMMMiVAfJ^^^. Simple Voltaic Cbll, showing the Elements of which it is Composed AND THE Chemical Action when the Circuit is Closed. The complete ovals denote molecules of dilute sulphuric acid. from the zinc to the copper plate, or in the opposite direction to the flow outside the cell If you keep the circuit closed for a sufficiently long time, you will further observe that the current becomes weaker and weaker, and if you take out the zinc plate for inspection, you will find that it has been considerably wasted, or eaten into, or burnt away, due to its forming zinc sulphate, ZnSo^^, with part of the sulphuric acid of the liquid. Polarisation. — You will also see, shortly after the starting of the current, that bubbles of gas (which, if collected and tested, will be found to be pure hydrogen) collect at and rise up from the copper plate. In other words, the cell becomes jioZarMed* * This application of the term must not be confounded with the polarity of a magnet (see our definition of the latter, Part L, page 15). In the present case it simply means that the difference of potential between the poles of the two plates has been reduced by the deposition of the hydrogen on the copper plate, owing not only to a reduction of the area of action of the same, but also to the fact that hydrogen has an opposite electrical polarity, or is electro- positive to zinc, and therefore tends to send a current from itself against the natural zinc-to-copper direction within the cell. 1 62 LECTUEE XVIH. The cell, so long as it lasts, is a little chemical furnace, whereio zinc is burned to produce the current ; but since the difference of potential does not keep constant, this form of cell is not used in the practical applications of electricity, such as telegraphy, telephony, and electric-bell circuits. It is, however, very useful upon an emergency in the class-room or laboratory, since it can be put together in a few minutes, and it serves as a stepping- stone to the understanding of the more complicated chemical actions in other cells. We wish you particularly to note the chemical changes which take place in the cell, as fully marked upon the second figure, and to remember them. Local Action.* — There is another objection to the use of this cell (in fact, to all cells wherein zinc is used), viz., the difficulty of obtaining pure zinc. If ordinary commercial zinc be employed, it contains numerous impurities, such as particles of iron and carbon, to most of which zinc is electro-positive as it is to copper. Consequently, little local cells are set up between the zinc surface and these particles of foreign matter, thus causing what is termed local action and a necessary short-circuiting or closed-circuit between the zinc and iron or carbon,