i ^ i M i wi w li i MiO' i i' * .^^BHit. \ \ (UnMll W^mmxi^ Jibatg !\v\iD 2236 QC 521. G66 cornel, university Ubrary Cornell University Library The original of tiiis 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/cu31924012333369 A PHYSICAL TREATISE ON" ELEOTEIOITY AJ^D MAGNETISM. VOL. II. A PHYSICAL TEEATISE OSf ELECTRICITY AND MAGNETISM. , -' V BY J. E.'.' H. GOEDON, B. A. Cams., ASSISTANT 8ECKETAKT OF THE BRITISH ASSOCIATION. TTV TWO V0LV3IES. VOL. II. NEW YORK: D. APPLETOlSr AND COMPAlSrY, 1, 3, AND 5 BOND STEEET. 1880. /xcrWEllX Lk UN!^/EH3!TYi UBRARY^ CONTENTS OF VOL. II. ^art Hi. (continued). ELECTRO-KINETICS. CHAPTER XXX. ON STANDAED COILS. TACB The " Constants " of an Instrument ....... 1 Eespective JRequirements of a Standard and of a Sensitive Galvanometer 1 Comparison of Coils 2 The great Electro-Dynamometer of the British Association ... 3 Appendix to Chapter XXX. — Determinations of the Constants of a Helix 6 Determination of the Number of "Windings .... 6 Difference of Magnetic Potential at the Ends . . 6 Formula of Integration ...... 7 Experimental Comparison of Helix and Dynamometer . 8 Verification ......... 10 Determination of Sum of Areas ...... 10 Calculation of the Strength of the Current in the Helix in Terms of the Deflection of the Suspended Magnet . 11 CHAPTER XXXI. ELECTEO-MAGNETS — DIAMAGNETESM AND MAGNE-OEYSTALLIC ACTION. Large Electro-magnet .......... 12 Torsion Balance for Electro-magnet . . . . .14 Axial and Equatorial Line defined ....... 14 Diamagnetics and Paramagnetics . 14 Faraday's Discovery of Diamagnetism ....... 14 List of Diamagnetic Bodies 16 Diamagnetic and Paramagnetic Metals ....... 17 Comparative Magnetic Strengths of Iron and Bismu'h . . . .17 VI Contents. Diamagnetic Polarity .... Faraday's Experiments Verdet's Experiments . Tyndall's First Experiments Weber's Mathematical Discussion Weber's Experiments . Tyndall's Second Experiments Polarity of Diamagnetic Liquids Magne-Crystallio Action Effects of Compression Artificial Crystal Effects of the Surrounding Medium PAGE . 17 . 18 . 19 . 20 . 21 . 21 . 21 . 26 . 27 . 28 . 29 . 29 CHAPTER XXXII. PEOPESSOE ADAMS's EXPEEIMENTAL DBTEEMINATION OF EQUIPOTENTIAL LINES AND SUEFACES AND LINES OF PLOW. Method of Observation .... Tracing of Equipotential Lines in Tinfoil Case 1. Square Tinfoil, 2 Electrodes 2 5 2. Circular „ 3. Large Square „ 4. Octant of Case (3) . 5. Large Tinfoil, 3 Electrodes 6. Circular „ 2 „ . . Method of Tracing the Carves in Three Dimensions Case 7. Liquid, 2 Electrodes . Linear Electrodes ..... Case 10. Sulphate of Zinc, 2 Linear Electrodes )j li* j» )» ^ Lines of Flow .... 31 32 33 33 34 35 35 37 38 39 40 40 40 41 CHAPTER XXXIIL THE INDUCTION COIL. Theory of the Induction Coil . . 42 Construction ............ 43 Description of a 17-inch Coil ........ 43 Necessity of Sudden Break ......... 43 The Condenser ........... 44 Contact Breakers ........... 44 The Vibrator 44 Clock and Hand Breaks 47 Spottiswoode's Wheel Break ....... 47 Contents. vli PAGE Spottiswoode's Sapid Break 47 Gordon's High-Speed ,Break 49 Mr. Spottiswoode's Coil . 49 Construction 49 Sparlts obtained with ■ 50 CHAPTER XXXI 7. ON THE DISCHARGE OF THE INDTJCTION' COIL AND DISCHAEGE GENEEALLT. Discharges in Air between a Point and Disc ...... 51 Brush Discharge ........... 52 Short Spark and Flame ......... 52 Shock 52 Looking-glass Experiment .... .... 52 Perforation of Plate Glass ......... 52 Discharge with the Bapid Break . 53 Secondary Condenser 53 " Quantity " and " Cascade " .53 Induction Coil and Magneto Machine ....... 54 Discharge in Earefied Air 54 Gordon's Experiments 55 The Coil 55 The Air-pump 56 The Discharging Tubes 58 The Experiments 57 Results 58 Table of Results CO, 61 Sir Wm. Thomson's Determination of the Electro-motive Force required to produce a Spark ......... 59 Comparison of Thomson's and Gordon's Results . . . , .62 De La Rue and Miiller's Determination of the Electro-motive Force required to produce a Spark ........ 62 Discharge in Different Gases ....... 64 Vacuum Tubes ........... 64 Construction 65 Fluorescence in 65 Negative Bulb only illuminated 65 Effect of Magnets on Discharge in Rarefied Air ..... 66 CHAPTER XXXV. STEI^. Striae in Narrow Tubes -67 Gassiott's Experiments .......... 67 Induction Coil not necessary, use of Large Battery . . .67 VIU Cotitents. Carbonic Acid Vacua .... Effeut of Condensers .... lutermittence of the Discharge Effect of alteration of Resistance Gassiott's Conclusion .... Experiments of De La Eue, Miiller, and Spottiswoode Chloride of Silver Battery used Condensers used ..... Stratifications accompany Pulsations of Current Experimental Proof of this . Spottiswoode's Experiments ..... Eapid Break used ..... StriiE softened by it Delicacy of Break Make and Break Discharges Complementary Plow of Stria3 May be controlled by Resistance Revolving Mirror ..... Use explained ..... Phenomena observed in it Flow of StriiB in Conical Tube, influence of Diameter Summary ...... De La Eue and MUUer's Experiments . 8040 Cells used Discharge always Disrupture . Discharge does not obey Ohm's Law Method of Exhaustion .... General Arrangement of Apparatus . M, =: 1 Millionth Atmosphere, defined . " Luminous Entities " . Summary of the Experiments . on Motion PAGE . 68 . 68 . 69 . 69 . 69 . 69 . 69 . 69 . 69 . 70 . 71 . 71 . 71 . 72 . 73 . 73 . 73 . 75 . 75 . 76 77 79 81 81 81 82 82 83 84 85 85 CHAPTER XXXVI. ON THE SENSITIVE STATE OP DISCHAEGES THEOUGH BAEEFIED GA8. Experiments of Spottiswoode and Moulton ...... 88 I^ofinition and Description of the Sensitive State . . . . .88 Sensitive State due to Periodic lutermittence . . . . .89 No Sensitiveness without Intermittence . . . . . .91 Effect produced by Conductor is due to reZief of Electric Strain . . 92 Eedistribution at each Pulsation 94 Effects due to Electro-static Induction 95 Relief Effect independent of Potential of Relieving Conductor . . 95 " Repulsion " and " Discharge " Eflfects defined • • • • . 95 Special or Non-Relief Effects defined .95 Contents. ix PAGE Difference between Relief and ITon-Kelief Effects . . . . .97 Examination of Non-Belief Effect 97 jSTon-Relicf Rings can cut Dlscliarge into two or more Segments 98 On the Nature of Striaj and the Artificial production of Striation . . 99 Each Shell made by a Non-Relief Ring is a perfect Stria . 100 3![od,us operandi of ordinary Striated Discharge ..... 101 Unit of Striated Discharge . . 103 Physical Structure of Striae ... ..... 103 Strite have a Material Structure . .... 103 Pairs of Striae 103 Discharge lasts too short a time for Successive Pulses to interfere . . 103 All Effects can be got from a Single Jar Discharge .... 103 All Relief and Non-Relief Effects are completed in each Pulsation . 101 Discharge consists of the Passage of Free Electricity .... 104 On Unipolar Discharges 105 Method of producing . 106 Discharge enters Tube and returns by same route . . . 106 Unipolar Discharge Sensitive and may be driven back . . 106 Tube with intermediate Terminal 107 Mutual Repulsion of two Unipolar Columns .... 108 Discharge from an Air-spark Terminal depends only on that Terminal and not on the other ......... 108 General Conclusion 109 Discharges from each Terminal independent of other Terminal 109 The State of the Tube during the Occurrence of the Discharge . . 110 Concluding Remarks 110 All Discharges are of same General Nature .... 110 All Discharges are Discontinuous Ill CHAPTER XXXVII. PHENOMENA ]N VEET HIGH VACUA — EXPEEIMENTS OF CEOOKES. Radiant Matter 112 Tubes exhausted to 1 M 112 Mean Free Path of Molecules 113 The Negative Dark Space 113 Radiant Matter exerts powerful Phosphorogenic Action where it strikes 114 Phosphorescence of Glass 114 of various Minerals, Emerald and Diamond . 115 of Ruby 116 Phosphorescence depends on Exhaustion 117 Radiant Matter proceeds in Straight Lines from the Negative Pole . 117 Position of the Positive Pole unimportant .... 120 Radiant Matter, when intercepted by Solid Matter, casts a Shadow . 121 Radiant Matter exerts Strong Mechanical Action ..... 122 Electrical Radiometer 124 X Contents. PAGE Eadiant Matter is Deflected by a Magnet 125 Trajectory is like that of a Cannon-ball 126 Difference between Magnetic Deflection of Discbarge in Ordi- nary and in Higb Yacua 127 Lecture Experiment on Magnetic Deflection and Mecbanical Action 127 Discharges are not Carrents, but simple Electrified Molecules . . 129 Eadiant Matter produces Heat when its Motion is arrested . . . 129 Melting of Glass 129 „ of Iridio-Platinum ....... 130 CHAPTER XXXVIII. E1E0TEOLTSI8. Description of the Phenomenon ........ 131 Earaday's Nomenclature 131 Laws of Electrolysis . . . . . . . . . .132 Theory of Clausius .......... 133 Electro-Chemical Equivalence ........ 134 The Voltameter 135 Electrolytic Polarization ......... 136 Experiments of Ayrton and Perry ...... 136 Measurement of Deflections ...... 136 Method of Experimenting ...... 137 Resemblance between Curves of Strain for Leyden Jar, Voltameter, and Bent Beam ..... 138 CHAPTER XXXIX. SECONDAET BATTEEIES — EHEOSTATIC MACHINES. Secondary Batteries ..... Plante's Researches on Secondary Batteries . " Formation " of the Plates Connection in Series, or Side by Side Heating Effects of Secondary Currents Magnetic Effects .... Duration of the Secondary Currents Constancy of the Current Preservation of the Charge Electro-motive Force Transformation of the Current of a Voltaic Battery by Secondary Battery ...... Discharge in Vacuum Tubes ..... Plante's Rheostatip Machine ...... 140 140 141 142 142 143 143 143 143 143 means of a 143 145 145 Contents. XI Rheostatie Machine for " Quantity '' .... Discliarge of the Quantity Machine Heating Effects .... Mechanical Effects .... Nodes of Vibration in a Metallic Thread Noise ...... Fragility of the Wire Plante's Conclusion as to the Mode of Propagation of Electricity PAaE . 146 . 147 . 148 . 148 . 148 . 149 . 149 . 149' CHAPTBE XL. MA&NBTO-ELEOTEIC AND ELECTEO-MAGNETIC ENGINES, Magneto-Electric Machines .... The Siemens Armature . Alternate Currents are Induced The Hand Gramme Machine . The Steam-power Gramme Machine The De Meritens Machine Electro-Magnetic Engines .... Eeversibility of Gramme's Machine Electric Transmission of Power 150 150 150 151 153 153 154 155 155 CHAPTER XLI. THE ELECTEIC LIGHT. The Electric Arc 156 Clockwork Regulator 156 The Jablochkoff Candle 158 CHAPTER XLII. EELATIONS BETWEEN ELECTEIOITT AND HEAT. Heating Effect of the Electric Current . Thermo-Blectricity Thermo-electric Law Thermo-electric Scale Thermo-electric Pile Reversal of the Current . Electro-motive Force — Tait's Formula Peltier's Phenomenon Measurement of Pyro-Electricity .... 159 159 159 160 160 161 161 162 163 163 Xll Contents. CHAPTER XLIII. ELECTEICITT OF CONTACT. PA OB Definition . . . • 165 Contact produces Difference of Potential, but cannot alone maintain a Current 166 Contact Theory of the Voltaic Cell — Volta, Thomson, Maxwell . .166 Production of Current by supplying Mechanical Energy to Metals in Contact — ^Thomson. 166 „ „ „ ,, Miiller and De La Rue 167 Contact Electricit}' of Insulators — Joseph Thomson .... 168 Experiments of Ayrton and Perry 168 Statement of the Law ........ 169 Induction Method explained ..... . 169 Description of Apparatus ....... 170 Hole, Slot, and Plane 171 Experiments to show that Copper and Zinc in Water are not at the same Potential ....... 172 When the Copper and Zinc are placed in Water, three successive States occur ......... 174 The Sum of the Contact Differences of Potential of the Materials of a Cell equals Difference of Potential of its Poles . . 174 Tables of Contact Differences— Solids with Solids . . . 176 „ „ „ Solids with Liquids, and Liquids with Liquids . . 177, 178 Summary of Results . 179 CHAPTER XLIV. DIMBNSIOKS OF tTNlTS. Dimensions ..... Derived Units — Telocity Maxwell's Notation for Units Unit of Force .... Ratio of Units .... The two Sets of Electric Units Electro-static Unit of Quantity Electro-static Unit of Current Electro-magnetic Unit of Current . Intensity of Magnetic Field . Electro-magnetic Unit of Current . Electro-magnetic Unit of Quantitj' . 180 . 180 . 181 . 182 . 183 . 183 . 183 . 184 . 184 . 185 ■ . 185 • . 186 Contents. xlii PAGE Eatio of the two Units of Quantity 186 Electro-static Unit of Potential 186 Electro-static Unit of Capacity 187 Electro-static Unit of Resistance 187 Electro-magnetic Units of Electro-motive Eovce and Potential . . 187 Electro-magnetic Unit of Capacity ....... 188 Electro-magnetic Unit of Resistance ....... 188 Summary and Ratio of Dimensions in Electro-static and Electro-magnetic System 188 Ratio of the two sets of Electric Units of Quantity .... 188 Ratio is a Velocity 189 Ratios between the other Units 190 CHAPTER XLV. EXPEEIMENTAL COMPAEISON BETWEEN ELEOTEO-STATIC AND ELECTEO- MAGNETIC UNITS. The Ratio is the Velocity of Electro-magnetic Induction . . . 191 Physical Proof that the Ratio is a Velocity ...... 193 Theory of the Experiments 193 Experimental Methods 193 Experiments of Weber and Kohlrausch 193 Same Quantity measured in the two Units . . . 194 Sources of Error 194 Sir Wm. Thomson's Experiments 194 Same Electro-motive Eorce measured in the two Units . 195 McKichan's Experiments 196 Maxwell's Direct Comparison ...... 196 Electro-static and Electro-magnetic Actions of same Electro-motive Force compared .... 196 Ayrton and Perry's Experiments 198 Same Capacity measured in the two Units . . . 198 Hockin's Experiments 199 Physical Nature of w 200 Rowland's Experiments on Magnetic Effect of Static Charge in Rapid Motion 201 Calculation of Ratio of Units from Rowland's Experiments . 201 Summary 202 xiv Contents. fart m, ELECTRO-OPTICS. CHAPTER XLYI. MAGNETIC BOTATION OF POLARIZED LIGHT. PAGE Pi-eliminary . 205 Natural Eotatioa 206 Faraday's Discovery of Magnetic Eotation ... . . 206 Difference between Magnetic and Natural Eotatiou .... 207 Faraday's Paper 208 Title 208 Action of Magnets on Light 209 Arrangement of the Experiments ...... 209 Eeturn of the Light first observed ...... 210 Direction of the notation . . . . . . .211 For Diamagnetics Rotation is in the same direction as Mag- netizing Current ........ 213 Liquids 214 Gases 214 Bodies having Natural Rotation 214 Action of Electric Currents on Light ..... 214 Law of Action 215 General Conclusion ........ 215 Further Investigations 218 Verdet's Experiments 218 Apparatus — Electro-magnet 218 Measurement of Intensity of Magnetic Field .... 219 Relation between Rotation and Magnetic Strength . . . 221 Effect of Thickness of Medium 222 Effect of Angle between Directions of Light and Magnetic Force 222 General Law . 224 Maxwell's Summary 224 Verdet's Experiments on Rotation in different Media .... 225 Mixture of two Media 225 Magnetic Rotative Powers 226 Effect of the Colour of the Light 227 Gordon's Determination of Verdet's Constant ..... 228 Constants of the Helix 228 The Light ' . . .229 The Jellett and Circle 229 Earth's Horizontal Magnetic Force 230 Contents. xv PAGE Formula ... 231 Result 213 H. Becquerel's Experiments ... ...... 238 Effect of Terrestrial Magnetism on Light .... 233 Magnetic Rotative Powers 233 Tables of ' 234,235 Relation between Index of Refraction and Magnttio Rotative Power 233 Rotation of different Rays 236 Formula for Rotation of any Ray in Bisulphide of Carbon . 236 KUndt and Rontgen's Experiments ....... 236 Rotation in Vapour ........ 236 Becquerel on Rotation in Gases .... ... 237 Ktindt and Rontgen on Rotation in Gases ...... 238 Results of First Research 239 Results of Second Research ....... 240 Rotation in Air caused by Earth's Magnetism — KUndt and Rontgen . 240 „ „ „ Becquerel . . . 240 Maxwell's Theory of Magnetic Rotation 241 Results of the' Theory 244 CHAPTER XLVn. DE. KEKH's DI800VEEIES. Relation between Statical Electbicitz and Polaeized Light. Glass, when subjected to Electro-static Strain, becomes doubly reflecting 245 Arrangement of the Experiment 245 Various Liquids act in same way ........ 246 Lecture Experiment 246 Summary of Dr. Kerr's Experiments in Liquids 247 Professor Rontgen's Repetition of Dr. Kerr's Experiments on Bisulphide of Carbon 249 Effect of Strained Glass Compensation ..... 250 Other Dielectrics 251 Partially conducting Liquids 251 Exhausted Tube 251 Moving Liquid 251 Dr. Kerr's Electro-optic Law 252 Instruments— The Cell 252 Chromatic effects 253 Instruments continued— The Electrometer . . . 255 The Jamin Compensator 255 Arrangement of the Apparatus 256 Calculation ......... 256 Establishment of the Law, and Summary of Results . 257 XVI Contents. CHAPTER XLYIII. TE. keee's discoveeies {continued). Rotation of the Plane op Polaeization of Light eeflected feom THE Pole and Side of a Magnet. PAGE Light Reflected from the Pole . . ...... 259 Experimental Arrangements . Sub-Magnet ..... Perpendicular Incidence . Gordon's Repetition o£ the Experiments Light Beflected from the Side of a Magnet . Experimental Arrangements . Summary of Results 259 260 260 261 261 261 262 CHAPTER XLIX. SELENIUM. Experiments of Adams and Day Resistance varies as Square Root of Light Plan of the Investigation .... Preparation of the Selenium .... Current Causes a Permanent " set " Light produces an Electromotive Force on the Selenium Summary of Results ...... . 264 . 264 . 265 . 265 . 265 . 266 . 267 CHAPTER L. cleek maxwell s electeo-magnetic theoet of light. The Ether 270 Max-well's Theory 270 Waves of Light, and Electro-Magnetic Induction, both Perpendicular to Ray 270 Conductors Opaque to Light ........ 271 Comparison of Velocities of Light and Electro-Magnetic Induction in Air and Vacuum ......... 271 Velocities in other Media 272 Gordon's Experiments ....... 273 Eefractive Index for Rays of Infinite Wave Length 273 Gibson and Barclay's Experiments .... 274 Boltzmann's Experiments ...... 274 Crystalline Sulphur 274 Schiller's Experiments 275 Silow's Experiments 275 Boltzmann's Comparison for Gases .... 275 General Conclusion 276 LIST OF PLATES IN YOL. II. • Gassiott's Vacuum Tubes 68,69 PLiTE PAGE XXVII, Eltctro-dynamometpr ....... 4 XXVIII. Electro Diamagnet 21 XXIX. -J J- 33 XXX. >Equipotential Lines .^ ....... 35 XXXI.J I 36 XXXII. 17-inch Coil 43 XXXIII. High-speed Break 49 XXXIV. Mr. Spottiswoode's Great Coil 50 XXXV. Vacuum Tube (coloured) chrome ..... 65 XXXVI. ■ XXXVII. . XXXVIII. De La Eue's Air-pumps ... ... 83 XXXIX. De La Rue's Apparatus ... .... 83 XL. XLI. XLII. XLIII. XLIV. XLV. XLVI. XL VII. XL VIII. XLIX. L. LI. LIL >De. La Rue's Striae 84, 85 Crookes' Radiant Matter 129 Plante's Eheostatic Machine ...... 145 Electricity of Contact — Ayrton and Perry .... 170 Thomson's Ratio of Units ...... 195 Maxwell's Ratio of Units 197 Verdet's Constant 229 Rontgen's Repetition of Kerr's Electro-optic Experiments 249 24 PABT III. ELECTRO-KINETTGS (^continued). A PHYSICAL TREATISE ELECTRICITY AND MAGNETISM. ^3art m* (continued.) ELECTRO-KINETICS. CHAPTER XXX. ON STANDAKD COILS.* In all absolute measurements it is necessary to know what are called the " constants " of the instruments. In measuring currents by galvanometers it is necessary to know accurately the number of windings, and the size and position of each. In order, then, that the windings may be accurately counted, and measured directly, and that small errors of measure- ment may not introduce a large percentage error, it is necessary that the coil should be large, and should contain only a few layers of wire. In constructing a sensitive galvanometer it is necessary to arrange the coils so as to produce the greatest possible effect on the needle, and, therefore, the wires must all be as near to the needle as possible, and there must be a great number of them. In other words, the coil must be small, and with a great number of turns all crowded close together. Thus the conditions to be satisfied in making a standard gal- vanometer are quite different from those required in making a sensitive galvanometer. On this subject Professor Maxwel says,t— * Maxwell's " Electricity,'' vol. ii. ch. xv. p. 312. t Ibid., 707, vol. ii p. 312. 2 Electro-Kinetics. " In constructing' a sensitive galvanometer we aim at making tlie field of electro-magnetic force in which the needle is sus- pended as intense as possible. In designing a standard galva- nometer we wish to make the field of electro-magnetic force near the magnet as uniform as possible, and to know its exact intensity in terms of the strenj^th of the current." On account of this difference the constants of sensitive galva- nometers are not determined by direct measurement, but by elec- trical comparison with large standard coils. To determine the values of the detlections of a sensitive galva- nometer we proceed as follows : — We place it concentric with a standard coil, and with its coils parallel to the coils of the standard. The latter being large, the sensitive galvanometer will go inside it. The plane of the coils is ])laced in the magnetic meridian. We then send currents, whose ratio is known, in opposite directions through the galva- nometer and the standard coil. Suppose we make the direction such that the current in the galvanometer tends to deflect the needle, and that in the coil to bring it back to zero. Let S be the deflection, C the current iu the standard coil, c the current in the sensitive galvanometer, H the earth's horizontal force at that time and place, 2 /)» the magnetic moment of the needle. Also let r be the couple of the coil with a unit current on a needle of unit moment. Then, if we are experimenting with the large coil only, and the sensitive galvanometer be removed, r will be the number by which H tan 8 must be divided to obtain the true strength of the current; and as the ring is large, this number will be equal to times the number of a windings. Let 7 be the corresponding quantity for the sensitive galva- nometer. This will not in general be proportional to the number of windings, and a cannot be measured directly. Let the sensitive galvanometer be replaced. The couples acting on half the needle now are Tending to deflect it as long as the diflection is very small, c 7 cos 8 . ; te. Comparison of Galvanometer with Standard Coil. 3 Tending to bring it to zero, C r cos 8 . Z m and H sia d. I m. When the needle is in equilibrium we have C r cos S + H sin S — c 7 cos S = 0, or dividing by cos 8 C r — c 7 = H tan 3. n If we vary the ratio — till S = 0, that is, till the needle comes to zerOj we have 7--r. The ratio of C to c is found without knowing either C or c, by dividing the same current into two circuits, and interposing resis- tances R and r in C and c respectively. Then if G is the resistance of the standard coil and (/ that of the small galvano- meter, we have C_ r+ff c~E + G and so ms.,|^^2g.^^. „ 28-1. Mean values of «, with different values of r, — (3) 7: Mo- «,. «2. K3. ■Ui. «s. «6- r= 100 2-724 P* 4-973 P 5-667 P 5 777P 5-574 P 4-808 P 2-724 P r= 200 2 762 P 4-976 P 6 659P 5-725 P 5 572P 4-814 P 2 722 P )• = inoo 2 722P 4 962P 5-681 P 5-798 P 5-592 P 4-639 P 2 723P Mean .. 2 736 P 4-970 P 5-669 P 5-766 P 5-579 P 4-754 P 2-723 P Specimen observation. Hence -=100, w3 = p2§i±i? = 5 777P 100 + LI /6h v,dx = -^4,-m centims. {16-707 + 48-620 + 34-596] P = 131-732 P. Now P = 81-1620, .-. 131-732 P = 10751-96. Now the value of N for any helix with unit current taken with respect to the whole length of its axis produced to an infinite length in both directions is, by Art. 676 of Professor Clerk Maxwell's " Electricity,'' 4n'«, where n is the number of windings. When the length I is finite compared with the radius a, the value of N for that part of the axis which is included between the ends is ■NT A Vi^ + o^ — a, li (see Clerk Maxwell's " Electricity," Art. 676). * P is the " power " of the dynamometer. (S) lo Electro- Kinetics. "Sovi if we calculate n = — =^ , taking a the mean radius, this 47r a/^ + a" — a will give the nnmher of winding-s In the hplii. Xow as a ^ 4'84 centims., and N ^ 10752, we have _ 10752 26 34 . ^^ ' ?26-34' + f84'}* — 4-84' .■ . log n = log 10752 +log 26-34 — \os. 47r — l\ horse-power gas-engine. This machine gives alternate currents whose direction is reversed about 16 x 1300 = 20,800 times per minute, and therefore no contact breaker or primary condenser are required. This method of working gives secondary currents having great " quantity." With a 20-inch coil the spark is about 7 inches long, and has the " full thickness of an ordinary cedar pencil.'" The discharge is extremely regular, and can be used for spectroscopic purposes without a secondary condenser. Dischabge in Raeefied Aie. When the discharge either of a coil or electrical machine is passed through a tube, or other vessel connected to an air-pump, it is found that as the pressure diminishes the length of spark which can be obtained increases. A great many experiments have been made to determine the exact law according to which the spark length increases as the pressure diminishes. In 1834, Sir Wm. Snow Harris stated J that, other things being equal, the length of the spark which an electric machine or Leyden jar will give in air varies in the simple inverse ratio of the pressure. He however gave no tables or figures in support of his law. * Phil. Mag., 1879, p. 390. t See Chapter XL. t Phil. Trans., 1834. spark- Length — Masson — Knochenhauer — Gordon. 5 5 In the experiments miide by M. Masson,* the spark passed either between two balls in the air, or between two similar balls inside a globe in which a more or less complete vacuum could be produced. The distances between the balls could be varied, as well as the pressures. Within the limits of his experiments, M. Masson found that the length of spark was inversely propor- tional to the pressure. The greatest length of spark which he used was 11"1 millims. In 1 843 M. Knochenhauerf worked with a constant length of spark of about f iuch,f and measured the electric density required to produce a spark in air at various pressures. Within the limits of his experiments he found that the ratio of the electric density required to produce a spark, to the pressure of the air, increases sensibly as the pressure diminishes. Now the length of spark is proportional to the electric density; and therefore Knochenhauer's results show that the law given by Harris and Masson does not hold for all distances and pressures. Wiedemann and E,uhemann§ found a purely empirical formula for variations in the lengths of sparks where the longest spark was 9'95 millims. . Goudon's Expeeiments. At the Dublin Meeting of the British Association the present writer gave an account|| of some recent experiments which he has made on the subject. In them an attempt has been made to determine the ratio of the spark-length to the pressure for dis- tances ranging from 6 inches to 30 inches by means of one and the same apparatus. The experiments diller from any former experiments with which the author is acquainted, in the fact that an induction coil was used as the source of electricity instead of an electric machine. Apparatus used. The coil was the 17-inch coil already mentioned (Plate XXXII.). * "Annales de Chiraie," 3' seiie, t. xzx. ; or Mascart, "Electricity Statique," t. ii. p. 94. t fugg. " Ann." Iviii. p. 219 ; or Mascart, t. ii. p. 95. \ He does not state the length of spark he used, but gives the height of his whole apparatus, and a drawing which, if it is to scale, shows that the discharging balls were about \ inch apart. § Mascart, t. ii. p. 97. II Phil. Mag., Sept. 1878, p. 185. Abstract in B. A. Ecport, 1878, p. 433. 56 Electro- Kinetics. It was worked by 10 quart cells of Grove's battery arranged in series. It was provided with a vibrator and with a clock contact-breaker, either of which could be used. The Air-Fump was of the ordinary Tait's construction. The Discharging Tubes. — These consisted of two cylindrical glass tubes about 4 feet (1'33 metre) long and nearly 3 inches diameter. At one end of each was a tap, the brass pipe from which ended in a ball which formed one of the discharging terminals. Holes in the side of the brass pipe admitted the air from the tap to the tube At the other end of each tube was a stuffing-box, in which a brass rod slid ; at the end of the brass rod was a point which could either be placed in contact with the ball or withdrawn some 3 feet from it. The end of the rod was kept always in the axis of the tube bj' means of three little glass arms, which were inserted into an ebonite collar fixed on the discharging rod a little behind the point. The two tubes were supported in a horizontal position, parallel to each other and about 18 inches apart, on four ebonite legs about 18 inches high. The tubes were joined to the air-pump by means of the pipes and taps shown in fig. 169, which were so arranged that the / 'Wires to euH gpcondffry\ f.r- i t . f' i f.i Fig. 169. tubes could be quickly connected to each other, to the external air, to a gas-bolder, or to the pump. Between the tubes and the pump the metal pipe was cut, and a piece of glass tubing about 18 inches long, well varnished with shellac, was inserted, Spark-Length — Gordon. 57 so that the electricity might not pass to earth through the pump* The Experiments. When the tubes were shut off from the pump, air could always b'; let into the glass pipe to prevent the discharge passing to earth inside it, as it would do at low pressures. The distance between the point and ball in each tube was measured as follows : — They were placed in contact, and an ink-mark was made on the discharging rod just outside the collar of the stuffing-box. When the rod was slid out, the distance of this mark from the collar was equal to the distance between the point and ball. The pressure was given by a U-gauge, about 4 feet high, attached to the air-pump at one end, open to the air at the other. The pressure P was given by the formula — P ^ {height of barometer} — {difference of level of mercury in the two arms of the IT}. Before being admitted into the tubes, the air was dried by being drawn through sulphuric acid. When it was desired that the pressure of the air in the tube should equal that of the ex- ternal atmosphere, air bubbled through the acid as loug as the difference of pressure inside and outside the tube exceeded that of the inch of acid which had to be displaced, and then the tap was opened direct to the outside air. The external diameters of the tubes were about 2"n4. and 2'76 inches respectively, and the diameters of the balls '94 and ■'J2 inch. In the experiments, one of the tubes (A) was left open to the atmosphere, and its discharging point placed at a standard distance either 6, 8, or 10 inches from the ball; and the other tube (B) being nearly exhausted, experiments were commenced at the low pressure, and then a little air was let in between each observation. The tubes were so connected to the coil that the discharge would pass in whichever tube offered least resistance. The discharging distance in B was then varied and adjusted to the sIwHest distance, which caused the whole discharge to pass in A. This distance having been noted, the points in B were brought nearer together till they reached the longed distance at which * Mr. Apps informs me that it is injurious to the coil to connect eithe secondary terminal to earth when using long sparks. 5 8 Electro- Kinetics. the whole discharge passed in B* The mean of these two dis- tances was taken as the distance which^ at the pressure then being wovlicd with, interposed in B a "resistance " equal to that of the standard length in A of air at the pressure of the atmosphere. Let us call this mean, " mean B spark." Now, if the law that the spark-length is inverseh'- proportional to the pressure holds, we should have for the same series of experiments — imean B spaik| Jpressure in B} ^ const. ; and to compare different sets made with different distances in A and with the barometer at different heights, we should have — { mean B sp ark} [pressure in B} _ {distance in A} {height of barometer} If the two tubes and the discharging points were precisely alike, this constant would be unity. Any slight difference in the shape of the points and balls would cause it to differ from unity, but would not affect its constancy. Eesults. The table (pp. 60, 61), which explains itself, gives the results of several sets of experiments arranged in ascending order of pressures. The results which I deduce from it are : — (1) From a pressure of about 11 inches up to that of the atmosphere Harris's law approximately holds good. No variation from it indicating any other law is observed. (2) No law can be said to be more than approximately true ; for when the density has almost reached the discharging limit, any slight accidental circumstance, such as the presence of a grain of dust, a little burning of the point by the last discharge, &c., will cause the discharge to take place. Professor Clerk Maxwell has compared this experiment to the splitting of a piece of wood by a wedge. It is possible to determine the average pressure on the wedge which will split the wood ; but in any particular experiment it is impossible to say that the wood will split exactly at that pressure. (3) When the pressure is diminished below 11 inches, the * The fact that the discharge only divided itself between the two tubes, « hen the " resistances" were almost equal, confirms Jlr. De La Rue's discovery (vol. ii. page 82) that disruptive discharges do riot obey Ohm's law. Spark-Length — Gordon — Thomson. 59 product ill column VII. vapidly dimiuishes. This shows that at low pressures the spark produced by a given electro-motivo force is much shorter than is required by Harrises law, or that the electro-motive force required to produce a spark of given length is at low pressures greater than that required by Harris''s law. This affrees with what Mr. De La Rue has shown (vol. ii. page 82), namely, that at all pressures, however low, the discharge is disruptive, and none of it passes by conduction. If any portion could at low pressures pass by conduction, we might expect that a smaller and not a greater electro-motive force would be required than that calculated by Harris's law from experi- ments at high pressures. SiE Wm. Thomson's Ekperiments. It is interesting to compare these results with Sir William Thomson's historical experiments " On the Electro-motive Force required to produce a Spark." * In these experiments the potential or electro-motive force required to produce a spark was measured by an absolute electro- meter.f The following table of results was obtained : — Electromotiveforce Length of spark in inches. Electromotive force (in arbitrary units). per inch of air (in arbitrary units), that i^ S. E.M.F. E.M.P. S. Inches. •007 2^4195 349-9 •0105 3 0000 285 7 •0115 3 1622 2750 014 3 6055 257-5 •017 4^0000 235-3 •018 4-3589 242 2 •024. 5-4772 228-2 ■0295 6-3245 214^4 •031 7-0710 208-0 •0385 7^7459 201-2 •Oil 83666 2114-1 •0445 8-9442 201-0 ■048 9-4868 197-6 •052 100000 192-3 •055 10-4880 190-7 •058 10-9544 188-9 •060 11-4017 190-0 * " Papers ou Electro-statics and Magnetism," p. 247 ; Proc. Roy. Soo., 1860, vol. X. p. 326; Phil. Mag., 1860, 2nd half-year, t Vol. i. p. 65. 6o E lectro-Kinetics. M w3'^*-0-*000-*iOOOO-*u500 ® (n t;- '-C !>. ::; CD CO -^ o -^ ^^ LT :p '^ t> in -X o o t* u3 a oq (N cj (M ^1 (M (M c-i 01 M w c-4 c-T c-t G 03 rt c '■/■( & Q o 2cDOt0OC0MtI«t0t£O00!i00O00;0CD00OX ■- .2 f-i " S 61-^- ! ■? 9 3 p. g e Tc, : ^ £ .2 o ^ -^ ^■^ I — I rr, . c c: « o « ;^ o o of > a. ■^ 3 =* ■ u': CO I T? t> jH --J ^^ -^ s -^ « - -^ ociiocp'^cpT-'MXpcp'T'Cicoooxcscqcoooco 'I;c■^lbdDr^iDc;i;-i(bf^35?cmlbx«A^(^^TflCDOT = TlC<)rHC<)i— I.— ti— 'rH(— li—li-l^r-li— )i— 'i-lrHi-Hf-ti-Hr-l h E 3 6 o"!. ®Cpf^t-qpO'7't^p^ll>QOe-l'>1X?1cq'05C5U5^l'>l-?»H.-(p 'oJ1W^t*I>630'-<.-''-H'HCO--r'T^T?u5u30COcbcb C iHrHrHr-(!-(r-lrHrHrHi-lfHrHT-tiH Spark-Length — Gordon. 6 1 COi-I'MffOOQOiMOO (M Oi Gvi Oi sa o;dco«qoccoxcdxcdoocoooo^xcdocoqoocdgoocoocoo7HOOOTOJ>CpcpTONCOCO{^ipCpCprHGON-^rH(»ip!pM 63fHifiT?cQAtbMom65dD»bcccbibaiMt^ico6'W'^j> _^_^r^_^^^>— I^^F— 4f— ll— I I— (l— I t— ll— I T— I I— I I— ll— ( T— tl— 1 •— ll— I .lU _^ ^p^^r^r^lHIHlHc^^oql^^^^lo:^(N(^lN(N^^lWMo:^Mo^o^NCN(Nl^^N(^^(^^(N(^^(^^ 28 62 Electro-Kinetics. The first column (on pag-e 59) gives the length of spark in inches, the second gives the corresponding electro- motive force, and the third is the ratio of the latter to the former, and gives the electro- motive force per inch of air at the different distances. If the electro-motive force required had varied directly as the air space, the third column would have been constant. It is seen, however, that the last column is by no means constant ; but the numbers show a very curious and unexpected result — namely, that greater electro-motive force per unit length of air is required to produce a spark at short distances than at long ones; or, if we adopt Faraday's view that the tension exists in every part of the air, they show that air in a thin stratum has greater strength than when it is in a thick one. It will be seen that my results (pages 61, 62) agree very well with Sir William Thomson's ; ^or he writes, " Greater electro- motive force per unit length of air is required to produce a spark at short distances than at long." For the words in italics I substitute " at low pressures than at high." We may then both write " with a low air ' resistance ' than with a high one," or " with few air particles between the points than with many." Sir William Thomson says of his result, " It is difficult even to conjecture an explanation;" I can only say the same of mine. De La Rce and Mullee's Experiments. On August 23rd, 1877, Messrs. De La Rue and Miiller com- municated to the Royal Society * a paper, in the early part of which they describe a series of experiments on the " striking distance ■" — that is, the length of spark obtainable, from batteries of from 1080 to 8040 chloride of silver cells, in various gases, and with terminals of various materials and shapes. The spark-lengths were measured by means of the discharger A B (fig. 170), which could be placed inside the receiver G G' of an air-pump, when it was desired to surround it with an at- mosphere of any gas. The discharges took place between the point P and the disc D. Teiminals of other shapes could be substituted for this point and disc. To measure the distance between P and D, the micrometer * Proc. Boy. Soc, xxyi., 1877, p. 519; Phil. Trans., 1879, vol. cbdx. p. 155. Spark-Length — De La Rue and Muller. 63 head A was read when a spark was just able to pass, aud then the screw-head was turned till P and D just came into contact. The micrometer A being again read, the difTerenoe of the two readings gave the " striking distance." The position of contact between P and D was determined by arranging 2 cells, so that their current, which was shown by a detector galvanometer, passed when contact was established. The rod R, which works in a Fig. 170. stuffing-box, enabled the screw-head A to be turned without opening the receiver. The following table gives a summary of the results ob- tained by Messrs. De La Rue and Miiller, for discharges taken between two spherical surfaces. For purposes of comparison, the numbers given in Sir Wm. Thomson's paper, in another table to that on page 59, have been reduced into volts, and are given in the black figures. 64 Electro- Kinetics. It will be seen that Mr. De La E.ue's results a^ree in the main with Sir Wm. Thomson's, but that the diminution is less rapid. Plain Numbers, Chloride of Silver Battery. Blaelc Numbers, Sir Wm. Thomjion's Results. Spark length in iuclieB. E.M.F. in silver cells. Difference of potential in volts per centimetre. 00497 000575 0-01434 001738 0-02524 003000 003703 0-04388 004925 0-05650 0-06287 0-07025 0-07550 008275 000340 000500 0-00600 000750 001110 001610 02220 002300 002710 '1-03560 004160 00522 1080 1200 2160 2400 i 3240 3600 4320 4800 5400 5880 6440 6960 7560 8040 88,060 84,590 61,090 56,010 52,050 48.660 47,320 43,210 44,460 42,210 41,780 40,180 40,160 39,420 80,230 77,000 78,660 67,260 60,220 45,450 43,210 41,870 42,250 40,490 39,630 39,310 Discharge in Different Gases. On May 17, 1877/ Messrs. De La Hue and Miiller stated that the length of spark given by a battery at ordinary atmo- spheric pressures in the following gases is the longest in the order in which they are enumerated — hydrogen, nitrogen, air, oxygen, carbonic acid — it being nearly twice as long in hydrogen as in air. The spark does not appear to be dependent on the specific gravity of the gas, but may have some relation to its viscosity. Vacuum Tubes. "When the pressure of the air is less than about 15 inches of mercury, the appearance of the discharge changes considerably. The whole gas within the tube glows, and if the light be examined with a spectroscope, it will be seen to give the cha- racteristic spectrum of the gas. * Proe. Roy. Soc, xxv!., 1877, p. 227. w '5 ■3 Gf o p; s D ^ ■00 a Wi 3 0=1 s =1 o > > X X Xi P-i Vacuum Tubes. 65 When the exhaustion is continued by a mercury pump till the pressure is only a very small fraction of a millimetre, the whole tube is filled with a bright light, of which the colour varies with the nature of the residual gas in the tube. If any fluorescent substances are placed in the tube or surround- ing it — if, for instance, a portion of the tube passes through a solution of sulphate of quinine, or part of the glass be coloured with uranium, they will glow with their characteristic colours when illuminated by the electric discharge. In these " vacuum tubes," as they are called, the electrodes usually consist of platinum or aluminium wires, passed through the glass, which is then fused round them. Platinum is particularly suitable for this purpose, because its expansion rate is about the same as that of glass, and, therefore, it does not crack out of the glass on cooling. A little opening being left at one side of the tube, the glass is drawn off into a capillary tube and attached to a Sprengel air- pump. When the exhaustion has been carried as far as required, the capillary tube is heated in a blowpipe flame till it softens, when it is drawn ofii" and so closed, a process which is assisted by the pressure of the external air. Plate XXXV. represents a tube in the possession of the author. The spiral portion near each end passes through a solution of sulphate of quinine contained in a wider external tube. The green portions are coloured with uranium. The red shows the natural colour of the discharge in rarefied air. The sulphate of quinine is quite colourless by ordinary daylight, and the uranium very nearly so. The illumination of these portions of the tube by the discharge shows that the latter is peculiarly rich in the ultra violet rays of the spectrum, for it is these which produce fluorescence.* It is observed that only one of the bulbs at the ends is strongly illuminated. It is the one connected with the negative electrode. On reversing the current, the other is illuminated. * See Lommel, " Optics and Light," oh. xiii. International Scientific Series (Keg&n Paul). 66 Electro-Kinetics. Effect op Magnets. It is found that the discharge in rarefied gas is attracted and repelled by a magnet in the same way as a wire carrying a current subject only to the differences caused by the fact that the wire is rigid and the discharge flexible. CHAPTER XXXV. STRI^. It is observed tliat when the vacuum tube is made somewhat narrow, as, for instance, when it is of the form shown in fig. 171 Kg. m. that in the narrow part the stream of light is not continuous, but is separated into a number of discs of light. Under certain circumstances these discs are also observed in larger tubes. They are called " strife ■" or stratifications. Their cause is not yet fully understood. Mr. Spottiswoode, Mr. De La Rue, and Mr. Miiller have been for some years investigating the subject, and most of what is now known about it is due to their labours, and to those of Mr. Gassiot. Mr. Spottiswoode's great coil, already described (vol. ii. page 49), was constructed especially for investigations of the strisB, but it has only lately been completed, and we shall have therefore to wait a little longer for the important discoveries which no doubt will be made by means of it. The following is a summary of the present state of our know- ledge on the subject : — EXPEEIMENTS OF GaSSIOT. On May 24, 1859, Mr. Gassiot communicated to the Royal Society* the fact that an induction coil is not necessary for the * Proo. Ko3'. Soc, vol. x. p. 36. 68 Electro -Kinetics. production of strite. He found that a "water battery" of 3520 insulated cells would produce a constant succession of sparks between two copper discs \ inch apart. When its poles were connected to the ends of a " carbonic acid vacuum" tube, whose electrodes were some % inches apart, a stratified discharge was obtained. The strise were also observed wlien 400 cells of Grove's battery were used. , Caebonio Acid Vacua. These carbonic acid vacua* were obtained as follows : — A tube, open at both ends, had one end connected to a Sprengel pump and the other to a receiver, containing carbonic acid. Some caustic potash was placed in the tube. When all the air had been replaced by carbonic acid, the end of the tube next the receiver was sealed. The tube was then exhausted as com- pletely as possible, and the second end sealed. On being heated, the caustic potash absorbed nearly the whole of the residual gas, and an extremely perfect vacuum was the result. Experiments Continued. On Feb. 6, 1860, Mr. Gassiot made another communication to the E-oyal Society,t in which he described some experiments made with 613 cells of Daniell's battery, with which he also obtained stratifications. 500 cells of this battery seems about the minimum number which will show this phenomenon, as with 480 Mr. Gassiot was unable to observe it. When condensers, consisting of from 110 yards to 16 miles of submarine cable, were connected to the tubes, the light in the vacuum tube lasted, after disconnection from the battery, for from a time too short to be measured with the 110 yards, to 1^ seconds with the 16 miles. Mr. Gassiot finally concludes that the stratified discharge of the induction coil arises from a force acting on highly at- tenuated but resisting media ; and that the same explanation is also applicable to discharges of the voltaic battery in vacua, and that the fact of this discharge having been ascertained to be also * Carbonic Acid Vacua ; Phil. Trans., 1859, page 137, and Bakerian Lec- ture, Phil. Trans., 1858, p. 1. t Pioc. Eoy. Soc, vol. x. p. 393. PiAiE XXXVI. — gassiott's vacuum tubes. Plate XXXVII. — gassiott's tacdctm tubes. Stritz — Gassiot — De La Rue, Milller, Spottiswoode. 69 stratified leads to the eonclusioa that the ordinary discharge of the voltaic hattery is not continuous, but intermittent : that it consists of a series of pulsations of greater or less intensity according to the resistance in the chemical or metallic elements of the battery, or the conducting media through which the dis- charge passes.* On December 11th, 1862, Mr. Gassiot announced f that the number and position of the striae is altered by altering the re- sistance in circuit. Plates XXXVI., XXXVII. represent some of the appearances observed in three different tubes, as the potash was heated or cooled, and the resistance in circuit varied. Mr. Gassiot found that when the pressure in the tube is ex- tremely low, the discharge prefers to pass through even a greater length of air at atmospheric pressure. At the end of his paper Mr. Gassiot says, " May not the dark bands be the nodes of undulations arising from similar impulses proceeding from positive and negative discharges; or, can the luminous stratifications which we obtain in the closed circuit of the secondary coil' of an induction apparatus, and in the circuit of the voltaic battery, be the representation of pulsations which pass along the wire of the former, and through the battery of the latter, — impulses probably generated by the action of the discharge along the wires ? " EXPEEIMENTS OF De La RuE, MuLLEK, AND SPOTTISTOODE. On April 8, 1875, a paper was read J by Messrs. De La Rue, Miiller, and Spottiswoode, describing experiments with 1080 cells of the chloride of silver battei'y (vol. i. page 216). With it several condensers were used : one of them consisted of 350 yards of wire, others of sheets of tinfoil. It was found that tubes which gave no stratifications with the battery, gave them at once when a condenser was added. It then occurred to the investigators that possibly stratifica- tions accompanied variations in the battery current, and this was found to be the case. The means adopted to ascertain this were as follows : — * Proc. Eoy. Soc, vol. x. p. 404. t Ibid., vol. xii. p. 329. X Ibid., vol.xxiii. p. 356. 70 Electro-Kinetics. The primary wire of a small induction coil (figs. 172 and 173) (without a contact-breaker) with or without an iron corSj was included in the yaouum tube circuit. Fiff. 172. Another small vacuum tube^ Vj, was attached to the secondary wire. Now, we know that as long as a steady current flows through the primary, there would be no effect on the secondary ; but that at every fluctuation a current would be induced in the latter. In the experiments it was found that whenever the discharge in Vj the first vacuum tube was stratified, the second tube Vj was lighted up. Fig. 173 shows the arrange- ments. It will be understood that the secondary tube was merely used as the most convenient method available, for detecting and esti- mating the currents of the secondary circuit. I do not know if the experiment has been tried of omitting the primary vacuum tube from the circuit ; but if this were done, I imagine that there would be no illumination of the secondary tube. If this should prove to be the case, it would show, not only that the stratifications are produced by varia- tions of the primary current, but that these variations are themselves produced in the primary tube, probably by some elastic yielding of the attenuated gas, analogous to the verti- cal oscillations of a weight while being raised by an elastic oord. Strice — Spottiswoode. 71 rig. 173. SZ ig the tattery. T the primary vacuum tute. pp' the primary ooU. AA' BB' the condenser. sg' the secondary coil. V the secondary vacuum tube. Experiments of Spottiswoode. On June lOth, 1875^ Mr. Spottiswoode* gave an account of some experiments with the " high break/'' or, as he now prefers to call it, the " rapid break," described in vol. ii. page 47, figs. 167 and 168. This instrument enabled him, with an induction coil, to obtain effects equally steady and equally under control with those obtained by the batteries. In this paper Mr. Spottiswoode says : — " With a contact-breaker of this kind in good action, several phenomena were noticeable ; but first and foremost was the fact that, in a large number of tubes (especially hydro-carbons), the strife, instead of being sharp and flaky in form, irregular in dis- tribution, and fluttering in position, were soft and rounded in outline, equidistant in their intervals, and steady in proportion to the regularity of the contact-breaker. These results are, I think, attributable more to the regularity than to the rapidity of the vibrations. And this view is supported by the fact that, although the contact-breaker may change its note (as occasionally * Proo. Roy. Soc, vol. xxiii. p. 455, 72 Electro- Kinetics. happens), and in so doing may cause a temporary disturbance in the stratification, yet the new note may produce as steady a set of strise as the first : and not only so, but frequently there is heard, simultaneously with a pure note from the vibrator, a strident sound, indicating that contacts of two separate periods are being made ; and yet, when the strident sound is regular, the strise are steady. On the other hand, to any sudden alteration in the action of the break (generally implied by an alteration in the sound) there always corresponds an alteration in the strise. "It is difficult to describe the extreme delicacy in action of this kind of contact-breaker, or ' high break,' as it may be called. The turning through 2° or 3° of a screw, whose complete revo- ution raises or lowers the platinum pin through -025 of an inch, is sufficient to produce or to annihilate the entire phenomenon. A similar turn in a screw forming one foot of the pedestal of the break is enough to adjust or regulate the strias ; and a slight pressure of the finger on the centre of the mahogany stand, ap- parently rigid, or even on the table on which the contact-breaker stands, will often control their movements. "The discharges described above are usually (although not always) those produced by breaking contact; but it often happens, and that most frequently when the strident noise is heard, that the current produced by making contact is strong enough to cause a visible discharge. This happens with the ordinary as with the high break ; but in the latter ease the double current presents the very remarkable peculiarity that the striae of one current are so arranged as to fit exactly into the intervals of the other ; and, further, that any disturbance affecting the column of striae due to one current affects similarly, with reference to abso- lute space, that due to the other, so that the double column moves, if at all, as a solid or elastic mass. And this fact is the more remarkable if we consider, as is easily observed in a revolving mirror,* that these currents are alternate, not only in direction, but also in time, and that no one of them is produced until after the complete extinction of its predecessor. And it is also worthy of note that this association of striae is not destroyed even when the two currents are separated more or less towards opposite sides of the tube by the presence of a magnetic pole. There seems, however, to be a tendency in that case for the striae of one current * Vol. ii. page 75. S trice — Spottiswoode. 'J2> to advance upon the positions occupied by those of the reverse current, giving the whole column a twisted appearance. But as there is no trace, so far as my observations go, of this association of alternate discharges when produced by the ordinary break, we seem led to the conclusion that a stratified discharge, on ceasing, leaves the gas so distributed as to favour, during a very short interval of time, a similar stratification on the occurrence of another discharge, whether in the same or in the opposite direc- tion. An explanation of the fact that the striae of alternate dis- charges occupy alternate and not similar positions is not obvious, and probably demands a better knowledge of the nature of the strise than we possess at present. " The column of strise which usually occupy a large part of the tube frora the positive towards the negative terminal have hitherto been described as stationary, except as disturbed by irregularities of the break. The column is, however, frequently susceptible of a general motion or ' flow,' either from or towards the positive pole, say a forward or backward flow, A similar phenomenon was observed by Mr. Gassiot in some tubes with his large battery ; but I am not acquainted with the exact circumstances under which it was produced. This flow may be controlled, both in velocity and in direction, by resistance introduced into the circuit, or by placing the tube in a magnetic field. The resist- ance may be introduced in either the primary or the secondary circuit. For the former arrangement I have successfully em- ployed a set of resistance-coils supplemented by a rheostat. For the secondary current, as well as for the Holtz machine, I have used an instrument devised and constructed by my assistant, Mr. P. "Ward, to whose intelligence and skill I am much indebted throughout this investigation, intended for fine adjustment Wherever the resistance be introduced, the following law appears to be established by a great number and variety of experiments, namely, that, the strise being previously stationary, an increase ot resistance produces a forward flow, a decrease of resistance a backward flow. I have generally found that a variation of 3 or 4 ohms, or, under favourable conditions, of 1 or 2 ohms, in the primary current is suflicient to produce this effect. But as an alteration in the current not only affects the discharge directly, but also reacts upon the break, the effect is liable to be masked by these indirect causes. The latter, so far as they are dependent 74 Electro-Kinetics. upon a sudden alteration of the resistance, may be diminished by the use of the rheostat ; but when the striae are sufficiently sen- sitive to admit the use of this delicate adjustment, some precautions are necessary to insure perfect uniformity of current, so as to avoid disturbances due to uneven contact in the rheostat itself. " When the striae are flowing, they preserve their mutual dis- tances, and do not undergo increase or decrease in their numbers. Usually, one or two remain permanently attached to the positive electrode; and as the moving column advances or recedes, the foremost stria diminishes in brilliancy until, after travelling over a distance less than the intervals between the two striae, it is lost in darkness. The reverse takes place at the rear of the column. As the last stria leaves its position, a new one, at first faint and shadowy, makes its appearance behind, at a distance equal to the common interval of all the others : this new one in- creases in brilliancy until, when it has reached the position originally occupied by the last stria when the column was at rest, it becomes as bright as the others. The flow may vary very much in velocity ; it may be so slow that the appearances and disap- pearances of the terminal striae may be watched in all their phases, or it may be so rapid that the separate striae are no longer dis- tinguishable, and the tube appears as if illuminated with a continuous discharge. In most cases the true character of the discharge and the direction of the flow may bo readily dis- tinguished by the aid of a revolving mirror. In some tubes, especially in those whose length is great compared with their diameter, the whole column does not present the same phase of flow; one portion may be at rest while another is flowing, or even two conterminous portions may flow in opposite directions. This is seen also in very wide tubes, in which the striae appear generally more mobile than in narrow ones. But in all eases these nodes or junction-points of the flow retain their positions under similar conditions of pressure and current; and it there- fore seems that, under similar conditions, the column in a given tube always breaks up into similar flow segments. "These nodes will often disappear under the action of a magnetic pole. Thus if the first segment, measured from the positive terminal, be stationary, and the second be flowing back- wards (/. e. from — to -f ) , a magnetic pole of suitable strength, placed at the distant end of the latter, will stop its flow, and the Stria — Spottiswoode. 7 5 whole column will become stationary throughout. An increase in the strength of the magnet, or a nearer approach of it to the tube, will produce a general forward flow of the column. The phenomena of the flow, as well as others of not less interest, are capable of being produced with the Holtz machine. Revolving Mieror. On May 18, 1876, Mr. Spottiswoode* gave an account of some experiments on the striee made with a revolving mirror. The break consisted of a plunger working in a mercury and platinum amalgam, and moved by a cam on the axis of the mirror, which insured contact being broken when the spark was in the centre of the field of view. The axis of rotation of the mirror was vertical, and the light of the tube, which was also vertical, passed through a vertical slit. Thus, if the mirror were at rest, a continuous discharge would appear in it as a vertical line of light, whose breadth was equal to the breadth of the slit — a striated discharge would appear as a broken vertical line whose bright portions corresponded to the positions of the striiB. Now let the mirror revolve : a continuous unbroken discharge would present the appearance of a sheet of light ; a continuous striated discharge, that of a series of horizontal bars, whose thicknesses were respectively equal to the length of the strise. An intermittent unbroken discharge would show a series of unbroken vertical lines; and an intermittent striated discharge, a series of broken vertical lines forming horizontal bands, whose thicknesses equalled the length of the striso. The ratio of the thicknesses of the vertical lines to those of the vertical spaces is of course the ratio of the duration of the discharges to the intervals between successive discharges. When the distance of the tube from the mirror, and the velocity of rotation is known, the absolute duration of each discharge can be calculated. Let us now fix our attention on a point of light, and suppose it to move vertically downward. We should see a diagonal line, whose slope depends on * Proc. Eoy. Soc, vol. sxv. p. 73. 76 Electro- Kinetics. the ratio of the velocity of the point to the velocity of the mirror* If the luminosity of the point were continuous, this diagonal line would be continuous ; if intermittent, it would be broken. Thus we see that with the mirror we can measure the dura- tion of the discharges ; the interval between each ; and the velocity of motion of the striae — also, if, ,as often happens, several discontinuous discharges unite to form one apparently con- tinuous, the mirror will resolve it into its constituent elements. Top of Tube (-)pole Middle of Tube. Fig. 174. Fig. 174 represents the appearance in the mirror of a carbonic .neid tube. The commencement of the discbarge is at the right * If a be the angle which this line makes with the horizontal, and v the velocity of the point of light, 6 the angular velocity of the mirror, r the distance from axis of mirror to tube, we have : tan a = -3. S(riis — Spottiswoode. J J hand (that is, the mirror was turning in the direction opposite to that of the hands of a watch), and the negative terminal is at the top. The positive terminal is a good deal below the picture, so the drawing represents the upper part of the tube during one complete coil discharge. It will be seen that the discharge commences with a perfectly regular set of strise having a steady downward motion. After a short interval of darkness, a second set are produced^ less regular, but somewhat longer lived than the first. It will be seen that in the upper part of the tube all luminosity soon ceases, but nearer the middle the discharges are repeated again and again. At the commencement of the region of longer duration we see that each stria moves downward for a short space, when it is extinguished, and its place is taken by another, starting .from nearly the same point as the first. Lower down still, the motion of each stria continues longer, and it is no longer formed and destroyed ;it the same fixed point in the tube. When this tube is viewed by the eye, it shows " flake-like fluttering strise with a slight tendency to flocculence near the head of the column.'^ Each of these strise is, we see, composed of the elements shown in the approximately horizontal bands, each of which is a group of elementary stria;. The curvature of these bands shows the proper motion of the compound strise : when it is downwards, they are moving from negative to positive; when upwards, in the reverse direction. Fig. 175, which represents another similar tube, shows the proper motion of the compound striae more clearly. Here it is first downward and then upward, but we see from the slope of the lines representing the elementary strise, that all the proper motion is really from — to + , and that the apparent reverse motion of the compound strise is due to each new elemen- tary stria being formed a little further up than its predecessor. Fig. 176 represents the discharge of a hydrogen tube of conical form, the diameter of which varied from capillary size to half- inch ; the capillary end being at the bottom. The positive ter- minal is at the top ; that is, the current is in the opposite direc- tion to that in figs. 174 and 175. The principal interest of this tube consists in showing the influence of diameter on the velocity of proper motion. The wider the tube, the freer, it seems, are the 29 78 Electro- Kinetics. slriijo to move. We see that this discharge is, though striated, practically continuous. Top of Tnhe (-) pole. /// '/mm///////"^" / / / / / *%w»'/////'^^ Pig. 176. Middle of Tube. Wide Top of Tube ( + )Pole. Capillary bottom of Tube C-) Pole. Fig. 176. Strics — Spottiswoode. 79 The following are the conclusions to which Mr. Spottiswoode thinks " the foregoing experiments seem to lead ;•" * — " (1.) The thin flake-like strisBj when sharp and distinct in their appearance, either are short-lived or have very slow proper motion, or both. " (2.) The apparent irregularity in the distribution of such striae, during even a single discharge of the coil, is due, not to any actual irregularity in their arrangement, but to their unequal duration, and to the various periods at which they are renewed. These striae are, in fact, arranged at regular intervals throughout the entire column. The fluttering appearance usually noticeable is occasioned by slight variations in position of the elementary striae at successive discharges of the coil. " (3.) The proper motion of the elementary striae is that which appertains to them during a single discharge of the coil. This appears to be generally directed from the positive towards the negative terminal. Its velocity varies generally within very narrow limits. It is greater the greater the number of coils employed, or the greater the electro-motive force of the current. In some tubes it may be seen to diminish towards the close of the discharge ; and even in rare instances alternately to increase and to diminish during a single discharge. " (4.) Flocculent striae, such as are usually seen in carbonic-acid tubes, are a compound phenomenon. They are due to a succession of short-lived elementary striae, which are regularly renewed. The positions at which they are renewed determine the apparent proper motion of the elementary stria. If they are constantly renewed at the same positions in the tube, the flocculent striae will appear to have no proper motion and to remain steady. If they are renewed at positions nearer and nearer to the positive terminal, the proper motion will be the same as that of the ele- mentary striae; if they are renewed at positions further and further from the positive terminal, the proper motion will be reversed. " (5.) The velocity of proper motion varies, other circumstances being the same, with the diameter of the tube. This was notably exemplified in the conical tube. In tubes constructed for spectrum analysis, the capillary part shows very slight, while the more open parts often show considerable, proper motion. * Proc. Roy. Soc, vol. xxv. p. 81. 8o Electro- Kinetics. " (6.) Speaking generally, the discharge lasts longer in narrow than in wide tubes. In spectrum tubes the capillary part gives in the mirror an image extending far beyond that due to the wider parts. " (7.) The coil discharge appears, in the earlier part of its development at least, to be subject to great fluctuations in extent. In all eases there is a strong outburst at first. This, although sometimes appearing as a bright line, is always, I believe, really stratified. Immediately after this there follows a very rapid shortening of the column. The extent of this shortening varies with circumstances ; but when, as is often the case, it reaches far down towards the positive terminal, a corresponding diminution of intensity is perceptible in the negative glow. The column of striae, after rising again, is often subject to similar fluctuations. These, which are sometimes four or five in number, are successively of less and less extent, and reach only a short distance down the column of striae. The rifts due to these fluctuations then dis- appear, and the striae either continue without interruption, or follow, broken at irregular intervals, until the close of the dis- charge. " (8.) The effect of the proper motion, taken by itself, is to shorten the column of striae. But, as we have seen, the striae are in many cases renewed from time to time. In regard to this point, the head of the column presents the most instructive features. After the cessation of these rifts, the general appear- ance of the field is that of a series of diagonal lines commencing .it successive points which, form the bounding limit of the column at successive instants of time. If the points are situated in a horizontal line, the striae are renewed at regular intervals at the same place ; and the length of the column is maintained by a periodic renewal of striae, a new one appearing at the head of the column as soon as its predecessor has passed over one dark interval. If the boundary of the illuminated field rises, the length of the column increases ; if it descends, the column shortens. In every ease, however, the growth of the column takes place by regular and successive steps, and not irregularly. The intervals of the new striae from one another and from the old ones are the same as those of the old ones from one another. " (9.) The principal influence of a change in the electro-motive force appears to consist in altering the velocity of proper motion. StricB — De La Rue and Milller. 8 1 A change iu the amount of battery-surface exposed produces a corresponding change in the duration of the entire discharge^ as well as apparently in the development of some of the minor details of the striaj. " (10.) When the proper motion of the elementary striae exceeds a certain amount, the striie appear to the eye to be blended into one solid column of light, and all trace of stratification is lost. When this is the case, the mirror will often disentangle the individual striae. But there are, as might well be expected, cases in which even the mirror is of no avail, but in which we may still suppose that stratification exists. A variety of experiments have led me to think that the separation of the discharge into two parts, viz., the column of light extending from the positive terminal, and the glow around the negative, with a dark space intervening, may be a test of stratified discharge ; but I cannot affirm anything certainly on this point." EXPEEIMENTS OF De La Eue AND MULLEE. On Aug. 23rd, 1877, Messrs. Warren De La Rue and Hugo W, Miiller communicated a paper to the Royal Society " On the Electric Discharge of the Chloride of Silver Battery ,•"' part I.* Part II. t was communicated April 10, 1878. In the first part of this paper they give an account of the construction of the great chloride of silver battery,J which now (1879) consists of 8040 cells. The first experiments with it were devoted to seeing whether the discharge, in highly-exhausted tubes, is of the nature of a true current, or whether it was disruptive like that through air at ordinary pressures. For this purpose they arranged that the discharge of 2100 cells should pass through a circuit con- sisting of a vacuum tube, and a large resistance. The resistance was then varied, so that the strength of the current varied in the proportion of from 1 to 135, but it was found that the difference of potential at the ends of the tube remained almost absolutely constant. Now, by Ohm's law, the potential along a con- ductor falls regularly along the resistance, and, therefore, if the vacuum tube had been an ordinary conductor, there would have been a uniform fall of potential along the whole circuit, * Phil. Trans., part i. vol. clxix. p. 55. t Ibid., part i. vol. clxix. p. 155. X See vol. i. p. 210. 82 Electro-Kinetics. consisting of tube A B and resistance B C (fig. 177) ; and the line L M C would have been straight from L to C ; as it was, how- ever, it was found that however much the slope of the part M C I. Fig- 177. varied, that of L M representing the fall of potential along the tube remained constant. This shows that the discharge is not a case of true conduction, but that even at the lowest pressure it is disruptive. Method of Exhaustion. We next come to the method of exhausting the tubes, so as to reproduce diflPerent phases of the phenomena at will. When tubes were exhausted and sealed once for all, it was found that, after a few discharges had passed, their character changed, and it was impossible to restore them to their original state. The tubes were therefore arranged so that the discharges could be passed while the exhaustion was in progress. When any particular phenomenon was observed, the pressure was noted, and on again adjusting to that pressure, the phenomenon could be reproduced. Plate XXXVIII. is a picture of Mr. De La Rue's air-pumps, &c. The arrangement comprises three means of exhaustion, which are successively employed as the vacuum becomes more perfect. The first is an Alvergniat high-pressure water trompe in con- nection with the high-pressure water-main of the West Middlesex Water Company, the head of water being 106 feet ; it produces a vacuum to within half an inch (0'47 in. = 12 millims.) of the height of the barometer. The pipe leading to it is so marked in the drawing; it is attached, through a cock to a four-way junction piece F, provided with three more cocks, communicating one to one end > X X X 04 S tries — De La Rue and Miiller. 83 of the tube T, one to the last drying bottle of the gas generator G G, and one to a mercurial gauge. The other end of the vacuum tube T communicates by means of a Y piece to both an Alvergniat mercurial pump on the right of the figure, and a Sprengel pump on the left. After the trompe has done its work, the Alvergniat is used for rapid exhaustion, and then shut off by means of the glass cock C, leaving the exhaustion to be completed by the Sprengel ; the authors have obtained by thejywMjf* alone in tubes 33 inches long, and % inches in diameter, vacua of only 0002 millim. pressure, equal to 36 millionths of an atmosphere — a vacuum so perfect, that the current of 8040 cells would not pass. The apparatus is in connection with a M'Leod gauge,* by means of which pressures to O'OOOOS millim. can be determined. Besides this gauge, the Sprengel and Alvergniat pumps have their own gauges, which read to a millimetre. AllHANGEMENT OP THE APPARATUS. Plate XXXIX. shows the general arrangement of the apparatus. AC is a rapid commutator for sending currents alternately in the two directions. K is a reversing key specially constructed to give good insulation with the immense electro-motive forces employed. When the handle is vertical, as shown, no current passes. Moving the handle to right or left sends the current in the two directions respectively through the tube. BB is a wheel-break for producing rapid intermittence. Z and Ag. are the zinc and silver terminals of the battery respectively. M (Plates XXX VI II., XXXIX.) is a rotating mirror, consisting of a four-sided prism, mounted on a horizontal axis, and provided with a multiplying wheel ; on each face of the prism is fastened a piece of looking-glass. The reflection of the tube in the mirror enables one to examine whether an apparently nebulous discharge consists really of strise, also whether and in what direction there is a flow of strise which may appear quite steady to the eye. The observations are facilitated by covering the tube with a half-cylinder of cardboard, having a slit in the direction of its axis about jSj- inch wide. R (Plate XXXVIII.) is a radiometer attached to the Sprengel; d r?a drying tube containing sticks of potash, used when gas is introduced from a reservoir through the Alvergniat. * (Phil. Mag., Aug. 1874.) When the mercury cistern is raised, a portion of gas at the same pressure as that in the tube is shut off at i, and com- 84 Electro-Kinetics. In order to save writing decimals, the pressures are usually ex- pressed in millionths of an atmosphere written |y|. 1 millim. of mercury = 1315-789 M- . ext-^ssa, 1 M = -00076 millims. In the present experiments the pressures varied from about 20,000 M to 3 M ; with less than about 3 IVI the current of 11,000 cells would not pass in tube 129. The exhaustion of an ordinary vacuum tube, arranged to give the best luminous effeets, is from 2 to 4 millims., say from 2,000 M to 5,000 M- The battery power used varied from 3,000 to 11,000 cells. Tubes whose length varied from 6 or 7 inches to 3 feet were used, and by means of the pumps they could be filled with different gases as de- scribed. The most marvellous and beautiful striae were observed in all manner of strange shapes. It would be impossible in the space at our disposal to give detailed accounts of the innumerable separate phenomena described in the paper. The student is recommended to study them in the " Philosophical Transactions." We have, how- ever, reproduced some of the pictures of the more striking forms. Tube 129 (fig. 178) was 32 inches long, and 1'6 inch in diameter. Its terminals were a ring and a wire of aluminium. It was charged with hydrogen. Plates XL., XLI., XLII. are sketches of some of the phenomena observed with this tube in different stages of exhaustion, and with different battery powers. pressed in the small graduated chamber, a, at the top of the bulb to different degrees in one gauge, from -^l^ to ts?^;, according as the gas is less or more rarefied ; the mercury at the same time rises in the pressure column p, and its height affords the means of determining the pressure of the gas in the tube. Tables have been prepared to give the value of the reading by inspection. ? >4 ij X s V. StricB — De La Rue and Milller. 85 These curious-shaped figures are all masses of light of various colours ; " luminous entities " the authors of the paper call them. Sometimes they are in motion, sometimes at rest. Plates XLIIL, XLIV., XLV. are engravings from photo- graphs of the discharges, taken actually from the tubes. Mr. De La Rue gives histories of all his tubes, and describes all the phases as they occur. He and Mr. Miiller thus sum up the results of all their experiments : — " 1. The discharge in a vacuum tube does not differ essentially from that in air and other gases at ordinary atmospheric pres- sures ; it cannot be considered as a current in the ordinary acceptation of the term, but must be of the nature of a disruptive discharge, the molecules of the gas acting as carriers of electri- fication. The gases in all probability receive impulses in two directions at right angles to each other, that from the negative being the more continuous of the two.* Metal is frequently carried from the terminals, and is deposited on the inside of the tube, so as to leave a permanent record of the spaces between the strata. " 2. As the exhaustion proceeds, the potential necessary to cause a current to pass, diminishes up to a certain point, whence it again increases, and the strata thicken and diminish in number, until a point is reached at which, notwithstanding the high electro -motive force available, no discharge through the residual gas can be detected. f Thus when one pole of a battery of 8040 cells was led to one of the terminals of tube 143, which has a radiometer attached to it, the other terminal of the tube, distant only 0"1 inch, being connected through a sensitive Thomson galvanometer to the other pole of the battery (earth), the current observed was not greater than that which was found to be due to conduction over and through the glass. Although no current passed, the leading wires, acting inductively, stopped the motion of the radiometer, as has been observed by Mr. Justice Grove. "3. All strata have their origin at the positive pole. Thus in * De La Eue and Miiller, PLil. Trans., 1878, vol. clxix. pp. 90 and 118. t Prom observations with pressure, varying from 6"4 to 145"! millims. Wiedemann and Euhemann conclude that the accumulation requisite to produce discharge increases with the pressure at first quickly, then more slowly ; towards the upper limit of their experiments it becomes nearly pro- portional to the pressures. See also vol. ii. pp. 55 to 64 above. 86 Electro- Kinetics. a given tube with a certain gas, there is produced at a certain pressure, in the first instance, only one luminosity, which forms on the positive terminal, then as the exhaustion is gradually carried further it detaches itself, moving towards the negative, and being followed by other luminosities, which gradually increase in number up to a certain point. " 4. With the same potential the phenomena vary irregularly with the amount of current. Sometimes, as the current is increased, the number of strata in certain tubes increases, and as it is diminished, their number decreases ; but with other tubes, the number of strata frequently increases with diminution of current. If the source of the current is a charged condenser, the flow being from one of its plates, througli resistances and the tube, to the other, then, as the potential of the condenser falls, and the current diminishes, the number of strata alters ; if the strata diminish in number with the fall of potential, then the stratum nearest the positive wire disappears on it, the next then follows and disappears, and so on with the others ; if, on the other hand, the charge of the condenser is very gradually increased, the strata pour in, one after the other, in the most steady and beautiful manner from the positive. " 5. A change of current frequently produces an entire change in the colour of the strata. " For example, in a hydrogen tube from a cobalt blue to a pink. " It also changes the spectrum of the strata ; moreover, the spectra of the illuminated terminals and the strata differ. " 6. If the discharge is irregular and the strata indistinct, an alteration of the amount of current makes the strata distinct and steady. Most frequently a point of steadiness is produced by the careful introduction of external resistance ; subsequently the introduction of more resistance produces a new phase of unsteadi- ness, and still more resistance another phase of steady and dis- tinct stratification. "7. Thegreatestheat is in the vicinity of the strata. This can be best observed when the tube contains either onlj' one stratum, or a small number separated by a broad interval. There is reason to believe that even in the dark discharge there may be strata ; for we have found a development of heat in the middle of a tube in which there was no illumination except on the terminals. Sirics — De La Rue and Miiller. 87 " 8. Even when strata are to all appearance perfectly steady, a pulsation can be detected in the current ; but it is not proved that the strata depend upon intermittence. " 9. There is no current from a battery through a tube divided by a glass division into two chambers, and the tube can only be illuminated by alternating charges. " 10. In the same tube, and with thg same gas, a very great variety of phenomena can be produced by varying the pressure and the current. The luminosities and strata, in their various forms, can be reproduced in the same tube, or in others having similar dimensions. " 11. At the same pressure and with the same current, the diameter of the tube affects the character and closeness of strati- fication. " We defer for the present the suggestion of any theory to account for stratification, in the hope of being able to confirm ex- perimentally certain views which we entertain as to the cause of this phenomenon. 88 Electro Kinetics, CHAPTER XXXVI. ON THE SENSITIVE STATE OF DISCHAKGES THEOUGH EAREFIED GASES. On April 2, 1879, Mr. William Spottiswoode and Mr. J. F. Moulton communicated to the Royal Society a paper " On the Sensitive State of Electrical Discharge through Rarefied Gases."* DEt'lNITION AND DeSCEIPTION. It has frequently been remarked that the luminous column produced by electric discharges in vacuum tubes occasionally displays great sensitiveness on the approach of a finger or other conductor to the tube. The exact effect of such approach varies considerably with the circumstances of the discharge. In many instances the luminous column is repelled ; in others, and espe- cially when the finger is brought into actual contact, the column is severed ; and in the latter case, in addition to the luminosity previously present, there often appears proceeding from the in- terior of the tube, at the point where the finger rests, the blue haze which usually characterizes the negative end of a discharge. In some cases the discharge is so powerfully aflijcted that tlie well-known green or blue fluorescence appears on the side of the tube opposite to the point touched. The degree of sensitiveness varies between wide limits. Dis- charges frequently occur in which close observation is necessary to detect any trace of it, while others may be produced so sensi- tive that the magnetic action of a powerful electro-magnet is comparable to the action which is due to it as a conductor. The condition in question does not appear to be confined to any par- ticular gaseous medium or to any special form of tube ; and it is * Phil. Trans., 1879, page 165. The Sensitive State — Spottiswoode and Moulton. 89 in fact probable that in almost any tube sensitiveness may be produced by adopting suitable precautions. This state of sensi- tiveness may be exhibited by stratified discharges, but it is more commonly associated with those which show no clear traces of stratification. It is not, however, universally present in either kind of discharge. The present paper is devoted to an examination of the causes which produce this state, and the laws which regulate it. Due to Inteemittence. The first result which the authors arrived at was that The Sensitive state is due to a Periodic Intermiitence of consider- able rapidity and regularity, the quantity of Electricity in each individual Discharge being S2ijjlciently small to permit the Discharge to be instantaneous. The simplest way to produce intermittence is to illuminate the tube by means of a Holtz machine or other constant source of electi'icity, and to interpose a small air-spark in one of the wires leading from the machine to the tube (fig. 179). As soon as the air-spark is made to intervene, the discharge in the tube becomes sensitive; and this sensitiveness may in general be increased by increasing the length of the air-spark, until the discharge becomes visibly intermittent, so as no longer to appear to the eye as a steady continuous discharge. Although this is by no means the only way in which the sensitive discharge can be produced, it is the one which is the most generally convenient for the purposes of experiment ; and it may on that account be regarded as the typical mode of production. Indications of the sensitive state can also be produced by the use of an induction coil in connection with a laro-e con- denser. 90 Electro- Kinetics. If the discharge he allowed to pass through the tube while the coil is at workj a certain amount of sensitiveness will usually be visible. But if the coil be stopped and the current allowed to flow from the condenser through the tube without disturbance from the entrance of the coil discharges, the discharge will in general be found to have lost all its sensitiveness. Again, certain tubes appear to render the discharge from a continuous source sensitive without the necessity of artificially producing intermittenee in the current. Again, a sensitive discharge may be produced by connecting one terminal of the machine with one terminal of the exhausted tube, and the other terminal of the machine with the outside of the tube (fig. 180). If the current be then permitted to pass + vTinrrrrirrTvriririnnrBTrirz "KJ Fig. 180. between the terminals of the machine by- leaping a considerable distance (say half an inch) in air, so that the discharges in the tube are caused partly by conduction from one terminal of the machine and partly by induction due to the rapid alternations ot high and low tension in the wire from the other terminal of the machine to the outside of the tube, the resulting discharge will be found to be sensitive. Again, rapid intermittenee and sensitiveness in what would otherwise be a continuous discharge may be produced bv the use of a "wheel-break " (vol. ii. page 47) . If the wheel-break be inter- posed in the circuit of a Holtz machine when producing a lumi- nous discharge in a vacuum tube, and the break be worked at a considerable speed, so as to cause the current to be interrupted some 400 to 2000 times per second, the discharge becomes highly sensitive. The wheel-break is used as a .shunt, viz., so as to divert from the tube the current given by the machine durino- the time which the platinum spring rests upon the metallic por- tion of the wheel. In this way the current is never actually broken, and one great advantage of this arrangement is that it The Sensitive State — Spottiswoode and Moulton. 9 1 simply produces iutermittence in the discharge in the tube without interfering with it in any other way. Another method of producing the sensitive state is by the use of a "Rapid Break."* If such an instrument be used with an Fig. 181. induction coil, the discharge, though often beautifully stratified, is intensely sensitive. The lowest limit of rapidity with which the authors have produced stratified sensitive discharges in this way is 240 breaks per second. No Sensitiveness without Intekmittence. The authors examined a great many sensitive discharges by means of a telephone and a revolving mirror, and they found that in every case intermittence was necessary to sensitiveness. The revolving mirror was used in the manner described in vol. ii. page 75. If the body of the tube containing the discharge be hidden by an opaque screen which contains a narrow longitudinal slit, and the image of this slit be observed in a rapidly revolving plane mirror, a series of bright and black bands appear whenever the discharge is sensitive, showing that there are intervals be- tween the luminous discharges during which the tube is dark. If the ordinary non-sensitive discbarge be observed in a similar way, there are no such dark bands, and no available speed of the * Vol. ii. page 47. 92 Electro- Kinetics. mirror suffices to show any break in the uniformity of the himi- nous image of the slit. The occurrence of these dark bands shows conclusively that the discharge in which they appear is intermittent and discontinuous. When the telephone is inserted in circuit with a non-sensitive tube, there is sometimes a rushing sound, but in a very largo number of cases there is absolute silence. But as soon as the discharge becomes sensitive, the silence is broken by a shrill sound ; or if the rushing sound of which we have spoken pre- viously existed, there is a sudden change in the characler of the sound, which usually becomes musical. The pitch of the note is always high, and naturally varies with the circumstances of the discharge, and it is therefore probable that, in cases where it is not heard, its pitch is too high, and the note itself is possibly too feeble, for audibility. When the air-spark is again abolished, the note ceases, or gives way to the rushing sound mentioned above. It is observed that all the methods of producing the sensitive state agree not only in the intermittent character of the discharge, but also in the shortness of duration of the individual discharges themselves, and this has induced the authors to regard brevity of duration as much an essential feature of the individual discharges that produce the sensitive state as rapidity or regularity of in- terval between these discharges. The latter characteristics are of more importance for main- taining the persistence of the sensitive state during a finite interval of time than for actually producing it, since careful experiments fail to show any inferior limit of rapidity of the periodicity necessary to produce such a discharge. In truth, there is no impossibility in producing by a single flash a dis- charge having the characteristics of sensibility. If a charged Leyden jar* be employed with a suitable tube, the instantaneous discharge that passes through the tube on the jar being con- nected with it will show all the symptoms of sensitiveness during its passage through the tube. The authors next state that The effect produced hy the approach of a conductor to a sensi- tive discharge is directly due to the relief given hy its presence to the instantaneous electric tension within and around the tube * Proc. Eoy. Soo., xxvi., 1877, p. 90. The Sensitive State — Spottiswoode and Moulton. 93 caused by the individual discharges in their passage through the tube. In the case of the sensitive or interrupted dischargej we have seen that there are separate pulses of electricity passing between the terminals. It is not improbable that each of these pulses leaves the posi- tive terminal in the form of free electricity. If ib does so, it will exercise induction in every direction, and cause a " state of strain" on the glass and in the space beyond. As the glass is a non-conductor, this state of strain continues ; but the instant a conductor is brought near, the state of strain is relieved, and a complete redistribution of the electricity outside the tube takes place. The state of the electric field in the neigh- bourhood of the conductor will then be different to that at any other part of the tube ; and this will in its turn react upon the discharge or upon the gaseous matter which exists within the tube. In order to show that the phenomena of sensitiveness are primarily due to the redistribution of the induced electricity and relief of the external strain, it is only necessary to observe that a non-conductor, however highly charged, does not affect the sen- sitive discharge. Nor will a conductor of small size, in contact with the tube (such as a piece of tinfoil), affect the discharge so long as it is insulated. But if the tinfoil be connected to earth, or to a distant conducting body, an effect on the sensitive dis- charge is at once seen. A consideration of these experimental facts leads the authors to the following conclusions : — (1) That the effect is due to a redistribution of electricity in the conductor ; and (2) That such effect is periodic. If a continuous electric state in the external body were the necessary condition, the observed effect could be produced by charged non-conductors. But as this is not the case, we must look to the facility of change in electric state afforded by con- ductors for an explanation of their effect on the sensitive dis- charge. This conclusion is supported by the fact previously stated, that a small piece of tinfoil placed upon the tube produces no effect so long as it is insulated. Such a piece of tinfoil would give but little scope for redistribution of electricity — at all events, 30 94 Electro-Kinetics. in such a way as to affect the space around it. But if it be con- nected metallically with a distant conducting body, so that positive or negative electricity can be driven from it to a sensible distance from the tube, the case is different ; and, if this be done, it is at once found to affect the sensitive discharge. If then the effect on the sensitive discharge is caused by the facility for a redistribution of electricity within the conductor, it follows that there must be a varying electric action upon it from the discharge in the tube. And that such is the case may be shown by connecting a ring, or segment of a ring, of tinfoil, placed on or near the glass (fig. 182), with the earth. ^K""'*'**» r ^f^tsj^f > Fig. 182. __ and interposing a telephone in the circuit between the tinfoil and the earth. As soon as the current becomes inter- rupted by an air-spark, a sound is heard in the telephone corresponding with the sound of the air-spark causing the inter- mittenee. This shows conclusively that at each pulsation there IS an electrical redistribution within the system composed of the earth, the wire, and the tinfoil. And as this continues indefi- nitely, without producing any charge upon the tinfoil, it is clear that there must, during the complete period of each pulsation, be a flow of one kind of electricity from the tinfoil, followed by its return, or by a similar flow of the opposite kind of electricity from the tinfoil. The authors go on to describe experiments to show that — (1) The effects are due to electro- static, and not to electro- magnetic, induction. The Sensitive State — Spottiswoode and Moidton. 95 (2) That the "relief effect" is independent of the potential of the conductor causing the relief. That is, that if the relief be caused by a ring of tinfoil attached to the tube, and connected by a wire to the inside of a rather large Leyden jar, it makes' no difference whether the inside is connected to earth, or insulated and charged to any potential, positive or negative. And, further, the relief effect is equally well obtained when the tinfoil is connected to a large conductor of varying potential; as, for instance, the wire of an induction coil, as long as its period of variation is not the same as that producing the intermittenee of the discharge. The authors next show experimentally that The relief effect {when the intermittenee is effected near the posi- tive terminal) assumes the form either of repulsion or of discharge from, the interior sit)f ace of the glass. These two effects are iden- tical in nature, and the form actually assumed depends in the same tube solely on the intensity of the action which calls it forth. They also find that they can pass continuously from the repul- sion to the discharge form of the relief effect in any one of three ways : — (1) By keeping the relieving system fixed at the same spot on the tube, and varying the completeness of relief,* the discharge remaining the same. (2) By keeping the relieving system constant, and varying its position on the tube, the discharge remaining the same. (3) By keeping the relieving system constant and fixed in position, and varying the interrupted ness of the discharge. It is found, however, that only certain tubes give the discharge effect, but that they all give the repulsion effect. On the Special or, Non-relief Effects peoduced on the Sensitive Luminous Discharge by Connecting it with the AlB-SPARK TeUMINAL. Let us first suppose the intermittenee to be caused by an inter- ruption in the circuit between the source of electricity and the positive terminal of the tube. For convenience we shall express this fact by calling the positive terminal the air-spark terminal. If we attach a wire to a piece of tinfol placed upon the * As, for instance, by making the contact between the tinfoil and the earth wire an imperfect one. 96 Electro- Kinetics. tubcj and connect the wire with any independent conducting system, we shall obtain, as we have seen, more or less complete forms of the relief effect. Both the wire and tinfoil will, in the majority of cases, repel the luminous column. But if the wire be connected with the positive terminal, a sudden change takes place in the phenomenon. Instead of the luminous column being repelled by the wire, the course of the latter along the tube (supposing it partly to rest upon the tube) will be marked by a bright line of luminosity on the inner surface of the glass as though it had attracted the luminous column instead of repelling it. And the effect of the tinfoil is changed in a no less remarkable manner. Instead of the former repulsion, a tongue of luminosity will be seen apparently starting from the actual inner surface of the glass under the tinfoil, and stretching toward the negative terminal of the tube, while the luminous column on the positive side of the tinfoil is usually depressed or repelled, and is often nearly severed in two. If the tinfoil be in the form of a ring round the tube, the appearance of the phenomena is very striking. In many cases the luminous column extending from the positive terminal is brought to an abrupt termination, and ends in a sharply-defined head, somewhat rounded at the extremity. Around this there is a well-marked hollow cone of luminositj^, springing from the side of the tube immediately beneath the tinfoil, bright and sharply defined on the outside, but hazy and blue on the inside, which is turned to, and in fact surrounds, the termination of the positive column above described. This hollow cone does not come to an apex on its external surface, but passes into a luminous column which stretches away towards the nega- tive terminal of the tube, and supplies the place of the former luminous column, which it resembles in all respects (tig. 183). Fig. 183. The Sensitive State — Spottiswoode and Moulton. 97 When the air-spark is considerably increased^ the truncated lumi- nous column is very much altered. It will be observed that these eflE'ects are totally dissimilar to the relief effects. EXPLAMATION OF THE DiFFEEENCE. The difference is accounted for as follows : — Each time that the discharge passes, there is a pulse of free positive electricity sent through the tube. In the case of the relief effect, this is accompanied each time by a pulse of positive electricity Ax\n%x\. from the tinfoil. In the case of the non-relief effect, it is accompanied by a pulse of positive electricity driven to the tinfoil, and so the electric strain, instead of being relieved, is increased by the action of the tinfoil. Examination of the Non-uelief Effect. The form of the non-relief effect obtained by making the posi- tive the air-spark terminal may be taken as typical of all other forms of it. If we place round the tube a narrow ring of tinfoil, and connect it with the positive terminal (where the air-spark is supposed to be) by a wire passing at a sufficient distance to prevent its directly affecting the luminous column, the following appearances will be noticed : — (1) The column which starts from the positive terminal will be found suddenly to terminate in a bright column of small dia- meter occupying the centre of the tube. This column is usually striated, and ends in a stria with rounded head. (2) On the side of the tinfoil towards the negative end a conical column of luminosity is seen to start from the inside of the tube immediately beneath the tinfoil, and to stretch towards the nega- tive terminal. This cone in fact forms the base of the new positive column. (3) On examination, this luminous cone is found to be hollow, the interior having an ill-defined and hazy surface in strong contrast witli the somewhat sharp and regular outline of the exterior. (See fig. 183.) The authors proceed to consider what is the explanation of these appearances. We know that strong pulsations of posi- gS Electro- Kinetics. tive electricity pass to the positive terminal of the tube and the tinfoil, keeping tirae with the passage of the air-sparks. These pulsations, when they arrive at the terminal, are of sufficient intensity to cause discharges to pass through the tube, and the pulsations that reach the tinfoil must be of exactly the same strength as those that go to the terminal, Such pulsations must drive off positive electricity in corre- sponding pulsations from the interior parts of the tube con- tiguous to the tinfoil. These latter pulsations are similar to the discharges that take place from the positive terminal, and they seek relief in the same manner, viz., by rushing towards the negative terminal of the tube ; in this process they form the hollow lumi- nous cone mentioned above. These discharges of positive elec- tricity from the inner surface of the tube leave behind them an excess of negative which would be held prisoned by the positive charge in the tinfoil if that were permanent ; but just as the latter was generated by the momentary charging-up due to the passage of the air-spark, so it is released by the relief given to such charging-up by the discharge through the tube. On such discharge taking place, the negative on the interior of the tube is set free, and in its turn satisfies the positive that meets it from the positive terminal. Thus we naturally get the termination of one positive column on the side of the tinfoil nearest to the posi- tive terminal, and a complete discontinuity between it and the second, which starts in a hollow cone from the edge of the tinfoil nearest to the negative terminal. To show more clearly that this is the true interpretation of the phenomena, and that the effect of the arrangement is thus to substitute for the original discharge two independent discharges occupying different parts of the tube, take two or more such rings separated from each other by spaces somewhat less than the diameter of the tube, and connect them as before with the positive terminal (fig. 184). Each of these will be found to be the base of a hollow cone similar to that above described ; and each such cone will form the base of a luminous column having all the features of a positive column, and terminating sharply behind the next tinfoil ring or at the borders of the usual negative dark place near to the negative terminal of the tube. If we bring the finger or other conductor to the side of the tube, these columns will all display sensibility, but each will The Sensitive State — Spottiswoode and Moulton. 99 move independently of the others and of the remainder of the positive column, and behave as if it started from the tinfoil ring at its base, still preserving, however, its position relative to the sectional columns on each side of it. Fig. 184. We now come to perhaps the most important portion of the paper, namely — that On the Nature of Striji, and the Artificial Production of Striation in the Luminous (Sensitive) Discharge. We have seen that the positive discharge due to a ring of tinfoil forms a hollow coqe with a sharply-defined luminous outer surface. This cone, if the nearest negative terminal is the negative terminal of the tube, passes into a column of diffused luminosity similar in all respects to the ordinary luminous column which starts from the positive terminal of a tube. But if there is another similar ring of tinfoil connected with the positive terminal (we are assuming that the effect which is being examined is the special effect when the air-spark is in the posi- tive circuit) between the former ring and the negative terminal, the luminous column that starts from ring No. 1 is stopped by ring No. 1, and from this latter ring there starts a second hollow luminous cone which stretches away in its turn towards the negative terminal in a different luminous column as before de- scribed. If these rings be placed at the proper distance from one another, and the size and exhaustion of the tube be suitable, the short luminous column between the rings will dwindle down to a hollow cone with blunt rounded head, this head being greatly superior in brightness to any other part of the cone, and stretching to a point close up or to even a little within the next ring, so that it is in the middle of the space enclosed by the hazy blue inside surface of the hollow cone that starts from that rins". 100 Electro- Kinetics. And, by using additional rings, this can be made to repeat itself until the whole luminous column is segmented into these hollow luminous cones or shells with bright rounded heads. The theory which the authors of the paper propose is that each of these luminous cones or shells is a perfect stria both in function and structure. The resemblance in appearance is most striking. There is the same convex bright outline pointing to- wards the negative terminal, the same hazy blue ill-defined hollow surface turned towards the shell immediately behind it, and there are the same dark intervals dividing consecutive shells as divide consecutive striae. Flat and annular striae can also be produced by proper adjustment of spark length, &c. Uut it is not only in structure that these luminous shells resemble strias. There is also an identity of function. We know that when the positive pulses arrive at the glass, they drive off similar positive pulses from the interior of the tube, and thus form the luminous shells. And knowing, therefore, that each luminous shell signifies a positive discharge, and also that no electricity passes through the glass, it is absolutely certain that a like amount of negative electricity must be collected at the surface of the glass within the tube, and must ultimately satisfy an equal amount of the original positive discbarge — i.e., of that which comes to it from the positive terminal, or from the shell immediately on the positive side of the one we are considering, as the case may be. We have then a negative discharge from the side of the tube, or from the gas immediately within it, satisfying a positive discharge advancing towards it along the tube; and we find that it causes the luminosity of this discharge to stop short and terminate in a bright, clearly-defined rounded head, which is separated by a dark space from the seat of the negative discharge. This, then, is the function of the shell : the bright part is to serve as the jilace of departure of the positive electricity that is about to pass across the dark space (or the place of arrival of the negative electricity after so doing), and the hazy interior of the cone is to form the place of its arrival (or the place of departure of the negative electricity) ; and, so far as we know, this is the sole function of these elements of the shell. Now let us take the case of the striated discharge. Here, also, we know that the positive electricity in the current must The Sensitive State — Spottiswoode and Moulton. loi leave the bright head of the stria, and, after passing the dark spaoe, arrive at the hazy inside of the next, and the negative electricity must take a reverse course. There is an absolute identity in the functions of the corresponding parts of the two structures, the only difference being that we know, from independent extrinsic evidence, that the electricity in the artificially segmented discharge does not flow continuously, but in intermittent discharges* Returning, then, to the case of the artificially produced conical shells, the modus operandi of the discharge is as follows : — When the pulse of positive electricity arrives at the terminal and causes the discharge into the tube, a positive discharge equal f to that which passes into the tube moves synchronously from the interior of the tube at each ring of tinfoil, forms the bright shell or stria, and passes on to the next shell or stria ; thus supplying the place of the positive pulse that the ring of tinfoil there has just sent on. The last shell passes its discharge to the negative terminal, and the first shell receives a discharge from the positive terminal. In this way a discharge passes through the tube identical in quantity and character to that which passes into it from the positive terminal. If, then, we are right in supposing that the series of artificially produced hollow shells are analogous in their structures and functions to strise, it is not difficult to deduce, from the explana- tion above given, the modus operandi of an Ordinary striated dis- charge. The passage of each of the intermittent pulses from the bright surface of a stria towards the hollow surface of the next may well be supposed by its inductive action to drive from the next stria a similar pulse, which in its turn drives one from the next stria, and so on. Thus the processes in the naturally' and artificially striated columns are precisely similar, save that, in the case of the latter, the pulses from the several strise are excited by * At the present stage of the inquiry the authors assume that all vacuum discharges are in reality intermittent. Any who do not wish to admit this must take the reasonings of this section as applicable only to those striated discharges which are known to be intermittent. ■f It may seem an unwarranted assumption to assume that each of these artificially produced discharges is equal to the whole original discharge, but the appearances (with suitable adjustments) seem to warrant it, and as the reasoning is simplified, and the validity of the theory is not affected by this assumption, it will, through the rest of this section, be supposed to be the case. I02 Electro- Kinetics. induction from without the tuhe, whiles in the case of the former, the induction is that of the discharge itself in its passage from stria to stria. The passage of the discharge is due in both cases to an action consisting of an independent discharge from one stria to the next; and the idea of this action can perhaps behest illustrated by that of a line of boys crossing a brook on stepping- stones, each boy stepping on to the stone which the boy in front of him has left. The unit of a striated vacuum discharge is therefore composed of the bright surface of a stria, the dark space in front of it, and the hazy interior surface of the stria on the further side of that dark space. In this unit we have a positive terminal, a space across which the discharge passes non-luminously, and a nega- tive terminal. And, in the opinion of the authors of this paper, all striated vacuum discharges are composed of reduplications of this unit, and that any phenomena connected with the negative terminal which seem to contradict this view, and to point to a special structure of the discharge near the negative terminal, un- like anything that exists in other portions of the discharge, are merely modifications due to the local circumstances of the terminal. The authors proceed to show that even the " negative glow " and " negative dark space " can be shown to be caused by modifications of a stria produced by the negative pole. The authors regard each segment as constituting a separate discharge. One phenomenon observed by them, of a diiferent kind to those of which we have been chiefly speaking, appears strongly to con- firm this. If a magnet be applied to a striated column, it will be found that the column is not simply thrown up or down as a whole, as would be the case if the discharge passed in direct lines from terminal to terminal, threading the striae in its passage. On the contrary, each stria is subjected to a rotation or deformation of exactly the same character as would be caused if the stria marked the termination of flexible currents radiating from the bright head of the stria behind it, and terminating in the hazy inner surface of the stria in question. An examination of several cases has led the authors of this paper to conclude that the cur- rents do thus radiate from the bright head of a stria to the inner surface of the next, and that there is no direct passage from one terminal of the tube to the other. The Sensitive State — Spottiswoode and Moulton. 103 Physical Strtjctueb op Strij3. It is natural to inquire what, in this theory, is the physical structure of striae. Are they merely luminous appearances (i. e., loci of maximum luminosity), or are they aggregations of matter having a material structure ? The authors consider that this is a question which it is be- yond the scope of this paper to discuss ; but the most probable view, in their opinion, is that they should be regarded as septa of complete electric porouiy, having a material structure. One of the most important facts favouring this conclusion is that when striae are formed by a coil working with a high- speed break, the striae produced by the two currents (the 11 HEffl Fig. 186. make and the break) adhere persistently together in pairs as though the alternate currents found ready to hand striae that only needed a little deformation to make them available for their purposes. The authors add that there are other facts tending to support this conclusion, but that a complete examination of the question would carry them beyond the limits of the present paper. Duration op the Discharge. The passage of the discharge through the tuhe occupies a time which is sufficiently small in comparison with the interval between the discharges to previent any interference betiveen successive elec- trical pulses. This was proved by an examination of the discharge by a revolving mirror. In order further to test the possibility of single pulses giving rise to the effects of which we have been speaking, the following experiment was tried : — The terminals of a tube were connected with the outside and the inside of a small Leyden jar. The poles of the secondary circuit of a coil were placed so that the discharge from the coil charged the jar by leaping over intervals of considerable size (about a quarter of an inch for the negative pole, and three- quarters of an inch for the positive pole), so that the make- current was excluded. When the coil was worked, there appeared a brilliant discharge caused by the jar periodically 104 Electro- Kinetics. discharging itself through the column. A slit was placed on the tube, and the luminous column was examined by the re- volving mirror, and it was found that the discharge was quite instantaneous, and that usually it was not followed by the next at any regular interval, but that occasionally it was multiple. The discharge was then tested for both relief and non-relief effects, and, notwithstanding the very large quantity that passed at each discharge, it gave them very markedly. The contact breaker was then worked by hand so as only to give single flashes. These were tested for sensitiveness and were found to be perfectly sensitive. Tims it appears from, experiment that the whole of the relief and non-relief effects are completed within each single pulsation, and that the effect of the rapid repetition of the discharges is merely to give the appearance of permanence to effects which in reality appear and disappear dibring each separate dis- charge. Natuee of the Disctiahge. The discharge is effected under ordinary circumsta:nces ly the passage through the tnbe from the air-sparh terminal of free elec- tricity of the same name as the electricity at that terminal. The authors prove this proposition by numerous experiments, of which perhaps the two most important are the following : — (1.) If a piece of tinfoil be placed at any part of the tube, even at the end farthest from the air-spark, the relief effect will be such as to show that the pulse of electricity is of the same kind at every portion of the inside of the tube. (2.) In order to prove the proposition in another way, a vacuum- tube was enclosed in a metal canister (the wires passing to its terminals through tubes of insulating material inserted in small holes in the canister), and a telephone was placed in circuit be- tween the canister and the earth. When a discharge with an air-spark in the external circuit was sent through the tube, a sound was heard in the telephone similar to that made by the air-spark. By a fundamental proposition in electricity* the free electricity on the surface of the canister (and which escaped through the telephone to the earth) was at any instant equal to the excess of one kind of electricity over the other in the space within the canister. Had the discharge been in the nature of conduction, as in a galvanic current, there would at no instant * See vol. i. page 16. The Sensitive State — Spottiswoode and Moulton. 105 have been an excess of either kind of electricity, and therefore there would not have been any sound in the telephone. The existence of a sound testified to variations in the algebraical sum of the free electricity in the tube. To show that this was not due to anything depending on the wires leading to the tube, the same experiment was repeated with a tube in which the middle por- tion was connected with the two end portions by very narrow passages. The middle portion was placed in the canister and the narrow parts passed through small holes made in its sides, so that only a portion of the complete tube was within the canister. The same results were obtained with this arransrement. On Unipolar Discharges. We have seen thatj inasmuch as the discharge from the air- spark terminal produces its special effect without any indistinct- ness or confusion close up to the opposite terminal, it would appear that the discharges from the two terminals are so far independent that the discharge may take place from one, and the free elec- tricity pass right through the tube to the immediate vicinity of the other, without evoking a specific response from the latter ter- minal. And if each such discharge does in any way call forth from the other terminal a specific response, it must be so slight that it does not affect materially the electrical condition of the interior of the tube, or the effect which that condition produces on conducting systems outside the tube. And we have also seen that this independence implies that the electricity leaves the terminal from which it starts in consequence of the electric tension within that terminal, and only in a very subordinate degree in consequence of the correlative action at the opposite terminal. Lest these should seem to be too hastily drawn con- clusions, the authors proceed to describe a class of phenomena which furnish very important evidence of their truth. If we take two exhausted tubes of the same general type, and connect one terminal of each with one terminal of a large Holtz machine, and connect their other terminals with the other ter- minal of the machine, intei-posingan air-spark (say in the positive circuit) so that the electricity has two alternative paths, the one through the one tube and the other through the other, the air- spark being common to both paths, a very remarkable phenomenon will be witnessed (fig. 186). If the air-spark be of a suitable lo6 Electro-Kinetics. magnitude, it will be found that one of the tubes is wholly traversed by the discharge, but that the other is occupied only by a luminoTis column extending from the positive terminal into + 1 Fig. 186. the tube for a considerable portion of its length, and gradually tapering to a point. If the air- spark do not exceed a certain limit, depending upon the " resistance " of both tubes, there will be no luminosity at the other end of the tube, and no discharge through it. No effect will be produced upon the luminous column, nor on any portion of the discharge, by breaking the connection with the distant terminal, showing, what the appearance of the column itself sufficiently indicates, that the discharge is unipolar or incomplete. Slight indications of blue haze are sometimes seen at the tip of this tapering column, due probably to some negative electricity gathered from the neighbourhood, but not directly discharged from the opposite terminal. The discharge is, in fact, one which jaasses into the tube, but not with sufficient force to pass through it, and which accordingly returns by the toay by which it entered. The cause of this recall will be examined later ; for the present it suffices to point out the fact that here we have a discharge from one pole, which is unable to approach near enough to the other pole to get relief there, and actually prefers to return by the way it came rather than to pass through the tube to the other terminal. Such discharges the authors propose to call unipolar discharges. < This unipolar discharge is of course intermittent, and therefore sensitive. If we take a glass rod (fig. 187) with a piece of tinfoil at the extremity electrically connected by a wire with the positive terminal of the tube, and hold it near to but a little beyond the end of the luminous column, we shall find the luminous column driven back; and by carefully advancing it towards the positive ter- minal we can often succeed in driving the luminous column wholly The Sensitive State — Spottiswoode and Moulton. 107 back and preventing' any visible discbarge taking- place into the tube. The explanation of this is obvious. At the moment that the charging-np, which causes the discharge^ takes place in the positive terminal, there is also a charging-up in the tinfoil, and this by its inductive effect tends to prevent the advance of any free positive electricity. Thus, however rapid the pulsa- tion, the force tending to oppose the discharge keeps exact time with it, and causes the heading back of the luminous column. If the tinfoil and wire be connected with earth, or otherwise made a relieving system, we find the usual to-earth effects produced on this unipolar discharge. A form of these experiments, which is in some respects even more striking, is obtained by taking a tube with an intermediate terminal (fig. 188), and connecting the inter- Fig. 188. mediate and one of the end terminals with the positive terminal of the machine, and the other terminal of the tube with the negative terminal of the machine. Let us interpose an air-spark in the positive circuit so that it forms part of the path to both of the terminals which were connected with io8 Electro-Kinetics. the positive terminal of the machine. "With an air-spark of proper dimensions it will be found that while the whole effective current passes from the positive intermediate terminal to the negative terminal, there is seen besides, first a tongue-shaped luminous column extending from the positive end terminal to- wards the intermediate terminal ; and, secondly, a similar tongue- shaped luminous column streiching out from the intermediate terminal to meet it (fig. 188). Or again, if we arrange two tubes as first described, and connect both terminals of the second tube with the positive terminal of the machine (fig. 189), we shall have two positive unipolar columns as before. These two do not join ; and it is clear that here we have naturally the same effect rig. 189. as that obtained by the use of the tinfoil in the former case. Each of these discharges acts repulsively on the other, and they drive each other back. If we use the tinfoil^ as in the former experiment (fig. 187), we can drive each in turn back towards, and sometimes into its terminal ; and within considerable limitswhen one column is driven back, the other advances, and vice versa. This experiment with the intermediate terminal shows very forcibly how the discharge from an air-spark terminal depends solely on the forces at work at the terminal itself, and has but little reference to the condition of the other terminal of the tuho. We see here that the positive electricity from the intermediate terminal actually issues copiously in the direction in which lies not only no negative terminal, but actually a positive terminal, which ultimately succeeds in repelling its advacen. In corroboration of the statement that these tongue-shaped The Sensitive State — Spottiswoode and Moulton. 109 luminous columns are parts of two distinct incomplete dis- charges, we may add that the magnet shows that they repre- sent discharges going in opposite directions, the positive elec- tricity in each proceeding from the base of each column towards the apex. Similar phenomena, save in respect of the shape of the lumi- nous columns, are seen when the two terminals are joined to the negative terminal of the machine. In the course of their experiments the authors showed that, if we take a discharge of small quantity from a coil of symmetrical make, the electricity passes into the tube simultaneously at both terminals, and that the two discharges meet and form a neutral zone near the middle of the cube. The result of connecting a small condenser with one of the ter- minals of the tube, when a coil discharge is used, is to depress the electrical tension produced at that terminal, and shift the position of the neutral zone. It is very instructive to compare these effects with the analogous effects in the case of unipolar dis- charges. If we join the effective terminal of the tube con- taining the unipolar discharge to a small condenser composed of, say, two pieces of tinfoil about three square inches in area, with a plate of mica between them, we shall see the luminous tongue slightly shorten. If, now, we connect the other side of this con- denser to earth, we see a further shortening of the column, which will often almost disappear. If we connect the terminal with a larger condenser or a Leyden jar, the discharge wholly disappears. Thus we see that the condenser or Leyden jar has, just as in the case of the coil discharges, the effect of muffling or toning down the intensity of the impulsive changes of electrical tension at the terminals, and thereby lessening the violence of the discharges into the tube. General Conclusion. The authors conclude from these experiments that the indepen- dence of the discharge from each terminal is so complete that we can at will cause the discharges from the two terminals to be equal in intensity but opposite in sign (as in the case of the coil) or of any required degree of inequality (as in the case of the coil with a small condenser). Or we can cause the discharge to be from one terminal only, the other terminal acting merely recep- 31 1 1 o Electro- Kinetics. tively (as in the case of the air-spark discharg'e) ; or we can cause the discharge to pass from one terminal only and return to it, the other terminal not taking any part in the discharge j or finally, we can make the two terminals pour forth independent discharges of the same sign, each of which passes back through the terminal from whence it came. The paper concludes with an examination of The State of the Tcbe during the Occuiirence op the Discharge. The authors find that the discharge does not take place simul- taneously all along the tube, but that it is progressive, and pro- ceeds from the air-spark terminal. If two pieces of tinfoil, connected by a thin wire, be placed on the tube — a small one. A, near the air-spark terminal, and a large one, B, near the opposite terminal — it is found that a relief is obtained at A as complete as if B (.still attached by a wire) were removed quite away from the tube. This shows that, at the time when the demand for relief is being made on A, no such demand is being made on B — that is, that the discharge has not reached B at the time when it is passing A. Concluding Remarks. The authors proceed to inquire whether the results which they have established from discontinuous discharges are also appli- cable to continuous ones, and they conclude that the essentials of the discharge of electricity through rarefied gases are the same, whether the discharge be interrupted, uninterrupted, or wholly discontinuous, and perhaps alternate. Now the simplest, and indeed the only obvious, explanation of this result is that the character which was found to be funda- mental in sensitive discharges, viz., disruptiveness, is common to both kinds of discharge ; and that the difference between the two kinds is to be sought in the scale on which that character is displayed.* In both discharges, each terminal pours forth its electricity to satisfy its own needs, and only in a very secondary degree to satisfy the needs of the opposite terminal. The one terminal does * It must be noted that in the ordinary discharge the discontinuom pulses in which the electricity leaves the terminals must be very minute. The Sensitive State — Spottiszvoode and Mon-iton. 1 1 1 not feel the electric state of the other directly^ as would be the ease were they metallically connected, but pours forth its electri- city in the shape of free electricity, and leaves it to wander at its own will in that shape. If these two matters could be demon- strated conclusively, a great step would be gained in our know- ledge of the nature of electric discharges; and, though the authors consider that this is not to be hoped for at present, they trust that the results recorded in this paper add con- siderably to the evidence in favour of them. The authors conclude with a number of arguments in favour of the view that a^^diseharges are discontinuous. In particular we may note that the probability that both kinds of discharge are really pul^atory is increased by the consideration that the strite ViX^ formed by the discharge. This increases the difficulty of sup- posing that a strictly continuous current could imitate effects which we have seen to be caused by discharges known to be in- stantaneous and disconnected. If the evidence given in the paper of the form of the discharge from one stria to another (see vol. ii. page 101) be considered sufficient, these remarks have still greater weight, for it is scarcely conceivable that a strictly continuous current should take so strange a course. The passage from stria to stria must then be taken as disruptive and discontinuous; and, if this be granted, then, as striae are only particular cases of ter- minals, it follows almost as a matter of course that all diseharo-es in rarefied air are equally disruptive and discontinuous. 1 1 2 Electro- Kinetics. CHAPTER XXXVIl. PHENOMENA IN VERY HIGH VACUA — EXPERIMENTS OF CROOKBS.* Mil. Crookes states that he has discovered that when the exhaustion of a vacuum tul)e is uamed considerably beyond that point which gives the best striae and himinous effects, a new set of phenomena not hitherto observed are produced, and that the residual gas developes so many new properties that he con- siders himself justified in saying that gus, when at these low pressures, may be regarded as matter in a fourth or ultra-gaseous state. To this he has given the name of " Radiant Matter/' According to Mr. Crookes, the' states of matter are four, as follow : — 1. Solid. 2. Liquid. 3. Gaseous. 4. Radiant. Although this view has not met with universal acceptance, it still appears to me to be legitimate, for the differences between the 3rd and 4th state seem to me to be at least as great as those between the 2nd and 3rd, and certainly to be greater than those between the 1st and 2nd. The pressure at which the new phenomena are best seen is about 1 M (one millionth of an atmosphere) . The difference between gas at 1 M and gas at say 3000 M appears chiefly to be caused by the fact that at the lower pressure the "Free Path" of each molecule — that is, the average distance which it moves without coming into collision with another molecule — is comparatively great, while at a higher pressure it is much smaller. * " On Radiant Matter," a Lecture delivered to the British Association ai Sheffield, August 22, 1879, by Wm. Crocdjes, F.R.S. Discharges in very high Vacua — Crookes. 113 At the hig'h pressure the molecules can hardly be said to move at all in straight lineSj because their direction of motion is constantly being changed by collision. Speaking roughly, we may say that, at a pressure of 1 M, the " moan free path," — that is, the average distance which each molecule wul move in a straight line before being deflected by collision — is about 3000 times as long as its mean free path in gas at 30U0 M, the ]iressure of an ordinary vacuum tube. In fact, as Mr. Crookes observes, "By great rarefaction the mean free path has become so long that the hits ill a given time, in comparison to the misses, may be disregarded, and the average molecule is now allowed to obey its own motions or laws without interference. The mean free path, in fact, is comparable to the dimensions of the vessel, and we have no longer to deal with a continuous portion of matter, as would be the case were the tubes lost, highly exhausted, but we must here contemplate the molecules indivkhtal/i/." The Negative Dark Space. In all wtH- exhausted vacuum tubes a small " dark sjJace " surrounds the negative pole. Mr. Crookes finds that, as the exlinustion improves, the dark space increases. He accuunte for the increase as follows : — Molecules of gas are driven off from the negative pole, and, as long as they do not come into collision with any ot'iur molecules, they do not produce any light. The space over which they travel without collision will be dark. When, by diminishing the pressure, the mean free path is lengthened, the dark space increases. The following experiment was devised for showing this "dark space " to an audience : — - The tube (fig. 190) has a pole at its centre in the form of a metal disk, and other poles at each end. The centre pole is made negative, and the two end poles connected togethei are made the positive terminal. The dark space will be in the centre. When the exhaustion is not very groat, the dark space extends only a little on each side of the negative pole in the centre. When the exhaustion is good, as in the tube which was shown in the lecture, and the coil is turned on. 114 Electro- Kinetics. the d.rk space is seen to extend for about an inch -on each bide or the pole. PlR. Inu. Radiant Matter exerts powerful Phosphokogenic Action WHERE IT Strikes. AVe have mentioned that the r.diant matter within the dark, space excites luminosity where its velocity is arrested bv residual gas outs.de the dark space. But if no residual gas is left the molecules w.ll have their velocity arrested by the sides of the glass; and here we come to the first and one of the most noteworthy properties of radiant matter discharged from the negative pole-its power of exciting phosphorescence when it strikes against solid matter. The number of bodies which respond luminously to this molecular bombardment is very great and the resulting colours are of every variety. Glass for instance, is highly phosphorescent when exposed 1o a stream of radiant matter. Fig. 19i represents three bulbs composed of Fig. IKI, * Here the mean free path of the molecules would be about an inch. Discharges in very high Vacua — Crookes. 1 1 5 diflPerent glass : one is uranium g-lass {a), which phosphoresces of a dark green colour; another is English glass [0), vyhich phos- phoresces of a blue colour ; and the third {c) is soft German glass — of which most of the apparatus used in the lecture was made — and phosphoresces of a bright apple-green. Many other substances are also phosphorescent under the influence of radiant matter. When luminous sulphide of calcium, prepared according to ]\Ir. Ed. Beequerel's description, is exposed even to candle-light, it phosphoresces for hours with a bluish white colour. It is, however, much more strongly phosphorescent under the influence of the luminous discharge in a good vacuum. The rare mineral Phenakite (aluminate of glucinum) phos- phoresces blue; the mineral Spodumene (a silicate of aluminium and lithium) phosphoresces a rich golden yellow; the emerald gives out a crimson light. But Mr. Crookes finds that, without exception, the diamond is the most sensitive substance he has yet met for ready and brilliant phosphorescence. A very curious fluorescent diamond was exhibited in the lecture, green by day- light, colourless by candle-light. It is mounted in the centre of an exhausted bulb (fig. 192), and the molecular discharge was Pig 192. directed on it from below upwards. On darkening the room, the 1 1 6 Electro-Kinetics. diamond was seen to shine with as much light as a candle phosphorescing of a bright green. Next to the diamond the ruby is one of the most remarkable stones for phosphorescing. A tube shown in fig. 193 was exhibited Fig. 193. containing a fine collection of ruby pebbles. As soon as the induction spark was turned on, the rabies were seen to shine with a brilliant rich red tone, as if they were glowing hot. It scarcely matters what colour the ruby is, to begin with. In the tube of natural rubies there were stones of all colours — the deep red and also the pale pink ruby. There were some so pale as to be almost colourless, and some of tlie highly-prized tint of pigeon's blood; but under the impact of radiant matter they all phosphoresced with about the same colour. Now the ruby is nothing but crystallized alumina with a little colouring matter. In a paper by Ed. Becquerel,* published twenty years ago, he describes the appearance of alumina as glowing with a rich red colour in the phosphoroscope. In another tube was shown some precipitated alumina prepared in the most careful manner. It had been heated to whiteness, and under the molecular discharge it glowed with the same rich red colour.t * Annates de C/iimic et de Physique, 3rd series, vol. Ivii., p. 50, 1859. t The spectrum of the red light emitted by these varieties of alumina is the snme as described by Becquerel twenty years ago. There is one intense ri-d line, a little below the fixed line B in the spectrum, having a wave- length of about 6895. There is a continuous spectrum beginning at about B, and a few fainter lines beyond it, but they are so faint in comparison with this red line that they may be neglected. This line is eafiily seen by examining with a suiall »>ocket spectroscope the light reflected from a good ruby. Discharges in very high Vacua — Crookes. 1 17 Fig. 194- represents a tube which was shown to illustrate the Fig. 194. dependence of the phosphorescence of the glass on the degree of exhaustion. The two poles are at a and b, and at the end (c) is a small supplementary tube connected with the other by a narrow aperture, and containing solid caustic potash. The tube had been exhausted to a very high point, and the potash heated so as to drive off moisture and injiire the vacuum. Exhaustion had then been recommenced, and the alternate heating and exhaustion repeated until the tube had been brought to the state in which it was exhibited. When the induction spark was first turned on, nothing was visible — the vacuum was so high that the tube was non-conducting. The potash was then warmed slightly, so as to liberate a trace of aqueous vapour. Instantly conduc- tion commenced, and the green phosphorescence flashed out along the length of the tube. The lieat was continued so as to drive off more gas from the potash. The green got fainter, and then a wave of cloudy luminosity swept over the tube, and stratifica- tions appeared, which rapidly got narrower, until the spark passed along the tube in the form of a narrow purple line. The lamp was taken away, and the potash allowed to cool; as it cooled, the aqueous vapour, which the heat had driven off, was reabsorbed. The purple line broadened out, and broke up into fine stratifications ; these got wider, and travelled towards the potash tube. Then a wave of green light appeared on the glass at the other end, sweeping on and driving the last pale stratifi- cation into the potash ; and, lastly, the tube glowed over its whole length with the green phosphorescence. If the tube is left to itself for some time, the green grows fainter and the vacuum becomes non-conducting. Radiant Mattee proceeds in Straight Lines. The radiant matter, whose impact on the glass causes an evolution of light, absolutely refuses to turn a corner. -Fig. 195 ii8 Electro-Kinetics. represents a V-shaped tube, a pole being at each extremity. The pole at the right side (a) being negative, the whole of the right Fig. i95. arm was flooded with green light, but at the bottom it stopped sharply and would not turn the corner to get into the left side. When the current was reversed, and the left pole made negative, the green changed to the left side, always following the negative pole, and leaving the positive side with scarcely any luminosity. To produce the ordinary phenomena exhibited by vacuum tubes, it is customary, in order to bring out the striking contrasts of colour, to bend the tubes into very elaborate designs. The Juminosity caused by the phosphorescence of the residual gas follows all the convolutions into which skil- ful glass-blowers can manage to twist the glass. The negative pole being at one end and the positive pole at the other, the luminous phenomena seem to depend more on the positive than on the negative at the ordinary exhaustion hitherto used to get the best phenomena of vacuum tubes. But at a very high exhaustion the phenomena noticed in ordinary vacuum tubes, when the induction spark passes through them — an ap- pearance of cloudy luminosity and of stratifications — disappear Discharges in very high Vacua — Crookes. 1 1 y entirely. No cloud or fog whatever is seen in the body of the tube, and with such a vacuum as is used in these experiments^ the only light observed is that from the phosphorescent surface of the glass. Fig. 196 represents two bulbs, alike in shape and riK. 198. position of poles, the only difference being that one is at an ex- haustion equal to a few millimetres of mercury — such a moderate exhaustion as will give the ordinary luminous phenomena — whilst the other is exhausted to about the millionth of an atmosphere. First the moderately exhausted bulb (A) was connected with the induction coil, and the pole at one side (a) being retained always negative, the positive wire was put successively to the other poles with which the bulb is furnished. It was seen that as the posi- tion of the positive pole was changed, the line of violet light joining the two poles changed, the electric current always choosing the shortest path between the two poles, and moving about the bulb as the position of the wires was altered. This, then, is the kind of phenomenon we get in ordinary I20 Electro- Kinetics. exhaustions. The same experiment was then tvied with a bulb (B) that was very highly exhausted, and, as before, the side pole [a) was made the negative, the top pole {ft) being positive. The appearance seen was very widely different from that shown by the last bulb. The negative pole was in the form of a shallow cup. The molecular rays from the cup crossed in the centre of the bulb, and, thence diverging, fell on the opposite side and pro- duced a circular patch of green phosphorescent light. The posi- tive wire was removed from the top and connected first to the side pole (c), then to the bottom pole (cV); but the green patch remained where it was at first, unchanged in position or intensity. We have here another property of radiant matter. In the low vacuum, the position of the positive pole is of every impor- tance, whilst in a high vacuum the position of the positive pole scarcely matters at all; the phenomena seem to depend entirely on the negative pole. If the negative pole points in the direction of the posi- tive, all very well ; but, if the nega- tive pole is entirely in the opposite direction, it is of little consequence : the radiant matter darts all the same in a straight line from the negative. Tf, instead of a flat disc, a hemi- cylinder is used for the negative pole, the matter still radiates normal to its surface. The tube represented in fig. 197 illustrates this property. It contains, as a negative pole, a hemi-cylinder [a) of polished alumi- nium. This is connected with a fine copper wire, I, ending at the pla- the upper end of the tube is another terminal, d. The induction-coil is connected so that the hemi- eylinder is negative and the upper pole positive, and, when exhausted to a sufficient extent, the projection of the molecular rays to a focus is very beautifully shown. The rays of matter, Fig. 197. tinum terminal, c. At Dischai'ges in very high Vacua — Crookes. 1 2 1 being driven fiom the liemi-cy Under in a direction norm;il to its surface, come to a focus and then diverge, tracing their path in brilliant green phosphorescence on the surface of the glass. Another tube was shown, in which the focus of molecular rays was received on a phosphorescent screen instead of on the glass. The effect produced was most brilliant, and lighted up the whole table. Radiant Matter, when intercepted by Solid Matter, casts A Shadow. Radiant matter comes from the pole in straight lines, and does not merely permeate all parts of the tube and fill it with light, as would be the case were the exhaustion less good. Whei-e there is nothing in the way, the rays strike the screen and pro- duce phosphoreseenue ; and where solid matter intervenes, they are obstructed by it, and a shadow is thrown on the screen. In the pear-shaped bulb (fig. 198) the negative pole («) is at the Fig. 198. pointed end. In the middle is a cross (i) cut out of sheet aluminium, so that- the rays from the negative pole projected along the tube will be partly intercepted by the aluminium cross, and will project an image of it on the hemispherical end of the tube which is phosphorescent. When the coil was turned on, the black shadow of the cross was clearly seen on the luminous end of the bulb (c, cT). Now, the radiant matter from the nega- tive pole has been passing by the side of the aluminium cross to 122 Eleclro-Kinetics. produce the shadow ; the glass has been hammered and bom- barded till it is appreciably warm, and at the same time another effect has been pi-oduced on the glass — its sensibilit}' has been deadened. The glass has got tired, if the expression may be used, by the enforced phosphorescence. A change has been pro- duced by this molecular bombardment which will prevent the glass from responding easily to additional excitement ; but the part that the shadow has fallen on is not tired — it has not been phosphorescing at all, and is perfectly fresh ; therefore, on the cross being thrown down * so as to allow the rays from the nega- tive pole to fall uninterruptedly on to the end of the bulb, the black cross (c. A, fig. 199) was seen suddenly to change to a Fig. 199. luminous one {e, /), because the back-ground was now only capable of faintly phosphorescing, whilst the part which had the black shadow on it retained its full phosphorescent power. The stencilled image of the luminous cross soon dies out. After a period of rest the glass partly recovers its power of phospho- rescing, but it is never so good as it was at first. Here, therefore, is another important property of radiant matter. It is projected with great velocity from the negative pole, and not only strikes the glass in such a way as to cause it to vibrate and become temporarily luminous while the discharge is CToing on, but the molecules hammer away with sufficient energy to produce a permanent impression upon the glass. Radiant Matter exerts Strong Mechanical Action where IT Strikes. We have seen, from the sharpness of the molecular shadows, that radiant matter is arrested by solid matter placed in its path. » This could be done by giving the apparatus a sliglit jerk, the cross having been ingeniously constructed with a hinge by Mr. Gimingham. Discharges in very high Vacica — Crnokes. 123 If this solid body is easily moved, the impact of the molecules will reveal itself in strong mechanical action. Fig. 200 represents Fig. 200. an ingenious piece of apparatus, constructed by Mr. Gimingham, which, when placed in the electric lantern, rendered this mecha- nical action very plainly visible. It consists of a highly- exhausted glass tube, having a little glass railway running along it from one end to the other. The axle of a small wheel revolves on the rails, the spokes of the wheel carrying wide mica paddles. At each end of the tube, and rather above the centre, is an alumi- nium pole, so that, whichever pole is made negative, the stream of radiant matter darts from it along the tube, and, striking the upper vanes of the little paddle-wheel, causes it to turn round and travel along the railway. By reversing the poles the wheel can be arrested and sent the reverse way ; and if the tube be gently inclined, the force of impact is observed to be sufficient even to drive the wheel up-hill. This experiment therefore shows that the molecular stream from the negative pole is able to move any light object in front of it. The molecules being driven violently from the pole, there should be a recoil of the pole from the molecules, and by arrang- ing an apparatus so as to have the negative pole movable and the body receiving the impact of the radiant matter fixed, this recoil can be rendered sensible. Fig. 20 i represents an apparatus whose appearance is not unlike an ordinary radiometer, with aluminium discs for vanes, each disc coated on one side with a film of mica. The fly is supported by a hard steel instead of glass cup, and the needle point on which it works is connected by means of a wire with a platinum terminal sealed into the glass. At the top of the radiometer bulb a second terminal is sealed in. The radio- 124 Electro-Kinetics. meter therefore can be connected with an induction-coil, the movable fly being made the negative pole. For these mechanical effects the exhaustion need not be so high as when phosphorescence is produced. The best pressure lor this electrical radiometer is a little beyond that at which the dark space round the negative pole extends to the sides of the glass bulb. When the pressure is only a few miliims. of mer- cury, on passing the induction current a halo of velvety violet light forms on the metallic side of the vanes, the mica side remaining dark. As the pressure diminishes, a dark space is seen to separate the violet halo from the metal. At a pressure Fig. 201. Fig. 202. of half a millim. this dark space extends to the glass, and rota- tion commences. On continuing the exhaustion the dark space further widens out and appears to flatten itself against the glass, when the rotation becomes very rapid. Fig. 2t)2 represents another piece of apparatus which illustrates the mechanical force of the radiant matter from the negative pole. A stem (a) carries a needle-point in which revolves a light mica fly {b I). The fly consists of four square vanes of thin clear mica, supported on light aluminium arms, and in the centre is a small glass cap which rests on the needle-point. The Discharges in very high Vacua — Crookes. 125 vanes are inclined at an angle of 45^ to the horizontal plane. Below the fly is a ring of fine platinum wire (c c), the ends of which pass through the glass at d, cl. An aluminium terminal (e) is sealed in at the top of the tube, and the whole is exhausted to a very high point. By means of the electric lantern an image of the vanes was projected on the screen. Wires from the induction-coil were attached, so that the platinum ring was made the negative pole, the aluminium wire (e). being positive. Instantly, owing to the projection of radiant matter from the platinum ring, the vane s rotated with extreme velocity. Thus far the apparatus had shown nothing more than the previous experiments had prepared us to expect ; but another phenomenon was then exhibited. The induc- tion-coil was disconnected altogether, and the two ends of the platinum wire connected with a small galvanic battery ; this made the ring c c red-hot, and under this influence it was seen that the vanes spun as fast as they did when the induction-coil was at work. Here, then, is another most important fact. Radiant matter in these high vacua is not only excited by the negative pole of an induction-coil, but a hot wire will set it in motion with force sufficient to drive round the sloping vanes. Radiant Maiter is deflected by a Magnet. We now pass to another property of radiant matter. The long glass tube, shown in fig. 203, is very highly exhausted; it h rig. 203. has a negative pole at one end (a) and a long phosphorescent screen (5, c) down the centre of the tube. In front of the necra- tive pole is a plate of mica {Ij, d) with a hole {ej in it ; and when the current was turned on, a line of phosphorescent lio-ht {e,f) was projected along the whole length of the tube. A powerful horse-shoe magnet was now placed beneath the tube and 32 126 Electro-Kinetics. the line of light (e, g) became curved under the magnetic influ- encCj waving about like a flexible wand as the magnet was moved to and fro. This action of the magnet is very curious, and, if carefully- followed up, will elucidate other properties of radiant matter. Fig. 204 represents a tube exactly similar, but having at one Pig. 204. end a small potash tube, which, if heated, will slightly injure the vacuum. When the induction current is turned on, the ray of radiant matter is seen tracing its trajectory in a curved line along the screen, under the influence of the horse-shoe magnet beneath. Let us observe the shape of the curve. The molecules shot from the negative pole may be likened to a discharge of iron bullets from a mitrailleuse, and the magnet beneath will represent the earth curving the trajectory of the shot by gravitation. The curved trajectory of the shot is accurately traced on the luminous screen. Now suppose the deflecting force to remain constant, the curve traced by the projectile varies with the velocity. If more powder be put in the gun, the velocity will be greater and the trajectory flatter ; and if a denser resisting medium be interposed between the gun and the target, the velocity of the shot will be diminished, and it will move in a greater curve and come to the ground sooner. The velocity of this stream of radiant molecules cannot well be increased by strengthening the battery, but they can be made to suffer greater resistance in their flio-ht from one end of the tube to the other. In the experiment shown, the caustic potash was heated with a spirit-lamp, and so a trace more gas was thrown in. Instantly the stream of radiant matter responded. Its velocity was impeded, the magnetism, Discharges in very high Vaciia — Crookes. 127 had longer time on which to act on the individual molecules, the trajectory became more and more curved until, instead of shooting nearly to the end of the tube, the " molecular bullets" fell to the bottom before they had got more than half-way. It is of great interest to ascertain whether the law governing the magnetic deflection of the trajectory of radiant matter is the same as has been found to hold good at a lower vacuum. The experiments just described were made with a very high vacuum. Fig. 203 represents a tube with a low vacuum. \Then Fig. 203. the induction spark is turned on, it passes as a narrow line of violet light joining the two poles. Underneath is a powerful electro- magnet. On making contact with the magnet, the line of light dips in the centre towards the magnet. On reversing the poles, the line is driven up to the top of the tube. We notice the difference between the two phenomena. Here the action is tem- porary. The dip takes place under the magnetic influence; the line of discharge then rises and pursues its path to the positive pole. In the high exhaustion, however, after the stream of radiant matter had dipped to the magnet, it did not recover itself, but continued its path in the altered direction. By means of the little wheel (fig. 206) skilfully constructed by Mr. Gimingham, Mr. Crookes was able to show the magnetic deflection in the electric lantern. The negative pole {a, h) is in the form of a very shallow cup. In front of the cup is a mica screen {c,d), wide enough to intercept the radiant matter coming from the negative pole. Behind this screen is a mica wheel {e,f) with a series of vanes, making a sort of paddle-wheel. So arranged, the molecular rays from the pole a b will be cut off from the wheel, and will not produce any movement. A magnet g was now put over the tube, so as to deflect the stream over or under the obstacle c d, and the result was rapid motion in one or 128 Electro- Kinetics. the other direction^ according to the way the magnet was turned. The image of the apparatvis was thrown on the screen. The Fig. 206, spiral lines painted on the wheel showed which way it turned. The magnet was arranged to draw the molecular stream so as to beat against the upper vanes, and the wheel revolved rapidly as if it were an over-shot water-wheel. On turning the magnet so as to drive the radiant mntter underneath, the wheel slackened speedj stopped, and then began to rotate the other way, like an under-shot water-wheel. This reversal can be repeated as often as the position of the magnet is reversed. We have mentioned that the molecules of the radiant matter discharged from the negative pole are negatively electrified. It is probable that their velocity is owing to the mutual repulsion between the similarly electrified pole and the mole- cules. In less high vacua, such as that shown in fig. 205, the PiR. 207. discharge passes from one pole to another, carrying an electric current as if it were a flexible wire. Now it is of great interest a < s !> Discharges in very high Vacita — Crookes. 129 to ascertain if the stream of radiant matter from the negative pole also carries a current. Fig. 207 is an apparatus which de- cides the question at once. The tube contains two negative terminals {a, h) close together at one end, and one positive ter- minal (c) at the other. This enables two streams of radiant matter to be sent side by side along the phosphorescent screen ; or, by disconnecting one negative pole, only one stream. If the streams of radiant matter carry an electric current, they will act like two parallel conducting wires and attract one another; but if they are simply built up of negatively electrified molecules, they will repel each other. The upper negative pole {a) was first connected with the coil, and the ray was seen shooting along the line d, f. The lower negative pole (i) was then brought into play, and another line (e h) darted along the screen. Instantly the first line sprung up from its first position, d f, to d g, showing that it was repelled, and the lower ray was also deflected downwards : therefore ihe Itco parallel streams of radiant matter exerted mutual repxdsion, acting not nice enrrent carriers, but merely as similarly electrified bodies. Radiant Matter produces Heat when its Motion is aerested. Another property of radiant matter is that the glass gets very warm where the green phosphorescence is strongest. The mole- cular focus on the tube (fig. 197) is intensely hot. An apparatus was exhibited by which this heat at the focus was made visible to the audience. A small tube (fig. 208) was prepared with a cup- shaped negative pole. This cup projects the rays to a focus in the middle of the tube. At the side of the tube is-a small electro- magnet, which can be set in action by touching a key, and the focus is then drawn to the side of the glass tube (fig. 209). To show the first action of the heat the tube was coated with wax. The apparatus was put in front of the electric lantern, and a mag- nified image of the tube was thrown on the screen. (Plate XLVI.) The coil was set to work, and the focus of molecular rays was pro- jected along the tube. The magnetism was turned on, and the focus drawn to the side of the glass. The first thing seen was a small circular patch melted in the coating of wax. The glass soon began to disintegrate, cracks shooting starwise from the centre of heat. The glass softened, next the atmospheric pressure I30 Electro-Kinetics. forced it in, and then it melted. A hole (e) was perforated in the middle, the air rushed in, and the experiment was at an end. We can render this focal heat more evident if we allow it to play on a piece of metal. The bulb (fig. 210) is furnished with a negative pole in the form of a cup (a). The rays will therefore be projected to a focus on a piece of iridio-platinum (5) supported in the centre of the bulb. Fig. 208. Fig. 209. Fig. 210. The induction-coil was first slightly turned on so as not to bring out its full power. The focus played on the metal, raising it to a white heat. By bringing a small magnet near, the focus of heat was deflected just as was the luminous focus in the other tube. By shifting the magnet the focus can be driven up and down, or drawn completely away from the metal, so as to leave it non- luminous. On withdrawing the magnet so as to let the molecules have full play again, the metal became white-hot.' On increas- ing the intensity of the spark, the ividio-platinum glowed with almost insupportable brilliancy, and at last melted.* * The highest vacuum Mr. Crookes has j'et succeeded in obtaining has been the l-20.000,000th of an atmosphere, a degree which may be better under- stood if we say that it corresponds to about the hundredth of an inch in a barometric column three miles high. CHAPTER XXXVIII. electrolysis. Description of the Phenomenon. Ip a binaiy compound body in a liquid state has a current of electricity passed through it, it is in general decomposed into its constituent elements, one of which appears at each of the points where the current enters and leaves the liquid. If two platinum wires be immersed in acidulated water, aud connected to a battery, the water will be decomposed ; and hydro- gen will appear at the negative pole, oxygen at the positive, and the volume of the hydrogen produced will always be double that of the oxygen. If a solution, say of sulphate of copper, is substituted for the acidulated water, copper is deposited on the negative pole, while sulphuric acid is liberated at the positive. Faraday's Nomenclature.* The process of resolving compound bodies into their consti- tuents is called Electrolysis. The bodies acted on are called Mec- irolytes. The poles at which the decomposition takes place are called Electrodes. The electrode attached to the zinc of the battery is called the catJiode ; and the other, the anode. The products of decomposition are called ions ; those which go to the anode are called anions and those which go to the cathode cations. Thus chloride of lead is an electrolyte, and when electrolysed, by having the electrodes of a battery immersed in it, evolves the two ions, chlorine and lead, the former being an anion, the latter a cation. * " Exp. Ees.," 665, vol. i. p. 197. 132 Electro-Kinetics. Laws op Electeolysis.* No elementary sulstance can he an electrolyte. For by definition, an elementary substance is that wbich can- not be separated into two constituents. Electrolysis only occurs while the hody is in the liquid state. The free mobility of the particles is a necessary condition of electrolysis, for the process can only take place in one of two ways. The molecule next one of the electrodes is decomposed. One constituent of it goes to the near electrode, and the other either travels to the other electrode or combines with a constituent of the molecule next to it, setting free a portion similar and equal to itself; which in its turn combines with the corresponding portion of the molecule next to it, and so on. In either case the free mobility of the particles is an essential condition. Nevertheless, electrolysis sometimes occurs in viscous solids ; but only in proportion to their fluidity. Fused nitre is an excellent conductor in the liquid state. If, however, a cold platinum wire connected to a battery be dipped into it, electrolysis does not commence till the crust of solid nitre, which is formed round the cold wire, has had time to re-melt. On this point Professor Maxwellf says, — " Clausius,t who has bestowed much study on the theory of the molecular agitation of bodies, supposes that the molecules of all bodies are in a state of constant agitation, but that in solid bodies each molecule never passes beyond a certain distance from its original position, whereas, in fluids, a molecule, after moving a certain distance from its original position, is just as likely to move still farther from it as to move back again. Hence the molecules of a fluid apparently at rest are continually changing their positions, and passing irregularly from one part of the fluid to another. In a compound fluid he supposes that not only the compound molecules travel about in this way, but that, in the collisions which occur between the compound molecules, the molecules of which they are composed are often separated and change partners, so that the same individual atom is at one time * See Miller's " Chemistry," 4tli ed. vol. i. p. 516. t Maxwell's " Electricity," 256, vol. i. p. 309. t Pogg. Ann. Bd. ci. S. 338 (1857). Electrolysis — Theory of Clausius. 133 associated with one atom of the opposite kind, and at another time with another. '' This process Clausius supposes to go on in the liquid at all times ; but when an electro-motive force acts on the liquid, the motiona of the molecules, which before were indifferently in all directions, are now influenced by the electro-motive force, so that the positively charged molecules have a greater tendency towards the cathode than towards the anode, and the negatively charged molecules have a greater tendency to move in the opposite direc- tion. Hence the molecules of the cation will, during their in- tervals of freedom, struggle towards the cathode; but will con- tinually be checked in their course by pairing for a time with molecules of the anion, which are also struggling through the crowd, but in the opposite direction." The direction of the molecules is always the same with regard to the direction of the battery current. The following very instructive experiment for showing the definite direction of electrolytic force is due to the late Dr. W. A. Miller. He says, — " Let* four glasses be placed side by side as represented in fig. 211, each divided into two compartments by a partition of card Fig. 211 or three or four folds of blotting paper, and let the glasses be in electrical communication with each other by means of platinum wires which terminate in strips of platinum foil. Place in the glass No. 1 a solution of potassic iodide mixed with starch ; in No. 2, a strong solution of common salt, coloured blue with sul- phate of indigo ; in 3, a solution of ammonium sulphate, coloured blue with a neutral infusion of the red cabbage ; and in 4, a solution of cupric sulphate. Let the plate H be connected with * Miller's " Elem. Chem.," vol. i. p. 517. 134 Electro- Kinetics. the positive wire, and let A complete the circuit through the negative wire. Under these circumstances iodine will speedily be set free in B, and will form the blue iodide of starch ; chlorine will show itself in D, and will bleach the blue liquid ; sulphuric acid will be seen in P, and will redden the infusion of cabbage ; sulphuric acid will also be liberated in H, as may be seen by intro- ducing a piece of blue litmus paper, which will immediately be reddened ; whilst a piece of turmeric paper will be turned brown in A, from liberated potash ; in C, it will also be turned brown by the soda set free ; in E, the blue infusion of cabbage will become green from the ammonia which is disengaged ; and in G, metallic copper will be deposited on the platinum foil." For a constant quantity of electricity, whatever the decomposing conductor may he, whether water, saline solutions, acids, fused bodies, Sfc, the amount of electro-chemical action is a constant quantity. That is, the same quantity of electricity will always produce the same amount of chemical effect.* The same current electrol^'ses different quantities of different substances, but the proportion of one to the other depends only on their chemical equivalents. Thus, if a current from a battery be sent through a series o troughs containing respectively, — Water (H^ 0), Fused plumbic iodide .... (Pb I2), Fused stannous chloride . . . (Sn CI2) then for each 65 milligrammes of zinc dissolved in any one cell of the battery, there will be produced, — (2 X 1) = 2 milligrammes of hyiJrogen, 16 „ of oxj-gen, 207 „ of lead, (2x127) = 254 „ of iodine, 118 „ of tin, and (2x35-5) = 71 „ of chlorine; and these numbers, — 65, 1, 16, 207, 127, 118, 35.5 correspond to the chemical equivalents of the elements respec- tively. If three similar vessels A, B, C, with platinum plateSj and con- * Faraday's " Exp. Ee-.," 505, vol. i. p. 145. Electrolytic Decomposition — The Voltameter. 135 taining acidulated water, be avraiiged as in fig. 212, and a battery Pig. 212. current passed through them, the sum of the quantities of gas produced in B and C will be exactly equal to that produced in A. The Voltametek. This fact enabled Faraday to invent the Fb^/aMefer, which con- sists of a trough containing acidulated water, and having elec- trodes inserted in it. Eeceivers over the electrodes collect the gas produced. The qiiantily of gas j^fodnced per minute is an absolute measure of the mean strength of the current during that time ; and the total quantity of gas is a measure of the total strength of the current.* It is necessary to collect the gases separately, as chemically clean platinum has the power of inducing their recombina- tion. t Fig. 213 shows a com- mon form of the instru- ment. The tubes are previously filled with water and in- verted over the electrodes. As the gas rises it displaces the water. The amount of gas formed is known by graduations on the tubes. Electrolysis is of great practical importance, for nearly all the * For instance, if C were the strength of the cui-rent, measured by a tangent galvanometer, and A, the quantity of gas produced between the times Fig. 213. = 'fc^ <, and t , we may write K=. hj f. C dt, where ^ is a constant, t Faraday, "Exp. Ees.," Series vl. vol. i. pp. 165—194. 136 Electro- Kinetics. operations of plating, whether with copper, silver, or gold, are performed by making the substance to be plated, the negative electrode in a solution of a salt of the metal with which it is desired to plate it.* Electeolttic Polakization. We have hitherto supposed the currents to be strong enough to decompose the liquids employed. When, however, only one Daniell's cell is used, decomposition does not take place ; but a state of " polarization " or strain is set up which very closely resembles that set up in a charged Leyden jar. In fact, electrolytic polarization may be compared to the ordinary charging of a Leyden jar, and decomposition to the case where the charge of the jar is strong enough to perforate the glass. Messrs. Ayrton and Perry t have measured the rate of charg- ing of a voltameter, and the rate of the return of the charge, and they have found a very close resemblance between the electrolytic curves and those obtained for the charging and discharging of a Leyden jar. They have also foundj that both the electrolysis and the Leyden jar curves are precisely similar to those expressing the deflection of a beam by weights, and its return when the weights are removed, and that the same form of mathematical (differential) equation will express all three phenomena. Measurement op Deflections. The rapidly changing currents and potentials were measured by means of a reflecting galvanometer and electrometer whose light-spots were thrown on to a large rapidly revolving barrel covered with white paper. The limits of swing were noted by making rapid dots with a pencil at the extreme positions of the light-spot. By this means two curves were obtained (fig. 214, 1, 3, 5, 7, and 2, 4, 6.) It is clear that the curve A, B, . . . E, expressing the mean value of the current or potential, must lie somewhere between these curves. The authors show mathematically that each point on the * Miller's " Chemistry," vol. i. pp. 541—548. t Journal of Soc. of Telegraph Engineers, 1877, vol. v., Nos. xv., xvi. p. 391. X See vol. i. p. 66. Electrolytic Polarization — Ayrton and Perry, 137 mean curve may be obtained by bisecting that portion of the ordinate which lies between the extreme curves. Fig. 214. We fee that the zigzag line 1, 2, 8, 4, h, 6^ 1, is the path of the spot of light. Method of Experimenting. Two platinum plates, each ten by eight centiras., were placed 21"3 centims. apart in a mixture of pure water and sulphuric acid (sp. gr. of mixture 1'016 at 50° Fahr.). These plates could be connected by means of a kej' to one Daniell's cell, the current flowing in at any time being measured by a galvanometer. The plates were kept permanently connected to a quadrant electrometer by which their potentials could be measured. When the circuit was broken, the plates were left insulated, and subsequently connected by resistance coils, and a very delicate reflecting galvanometer, by means of which the "return current'" or " residual charge " was measured. The time of any observation was noted by means of a " break- circuit chronograph.^' This consists of a rapidly-running " Morse Ink-writer," in which a pen is held down by an electro-magnet upon a rapidly-moving ribbon of paper and produces a continuous line as long as the current flows in the electro-magnet. At the commencement of each second, a clock breaks the circuit for a moment, producing a gap in the line. 138 Electro-Kinetics. At the iHstant of any event, which it is desired to record^ the circuit is broken by hand or otherwise, and a gap made in the line whose position between the two nearest time-gaps gives the time of the event. Results. The first result obtained was that, at the instant of making contact, there is an enormously large current into the voltameter, far greater than the experimenters were able to measure. In, fig. 215, time is measured along OX, starting from 0, which represents the moment of closing the circuit. Galvanometer and electrometer deflections are measured along OY. CUKVE AB. The curve expressing the fall of battery current is something of the shape of that of AB (fig. 215), only that during the first quarter of a minute the ordinate of the curve would be much greater than OA. This is analogous to the fact that a rapid stream of sparks can be sent into a Leyden jar for a few seconds until it is charged, after which it will not receive any more. Curve A'B'CC. The ordinates of this curve represent electrometer deflections. The line A'B' represents the rise of potential during forty- six minutes, during which the battery was kept connected to the electrometer. At the end of that time the battery was disconnected, and the plates left insulated, except that they were connected through the acidu- lated water. The fall of the potential is re- presented by the curve B'C. After sixty-six minutes more, Fl(?. 215. -1 they were connected through 12,000 Ohms resistance, and the curve CC shows the fall of potential to the end of the experiment. Electrolytic Polarization — Ayrton and Perry. 139 Curves DE, AEPGG'. When the voltameter was connected to the battery^ the current diminished rapidly. The fall during eleven minutes is represented by the curve DE. During the same time the potential increased from A' to E. At E the plates were insulated, and EF shows the fall of potential. At F, twenty-two minutes from the commencement of the experiraentj the plates were connected through 12,594 Ohms, and in 3| minutes the potential fell to G. The plates were than again insulated, and the potential rose toG'. The shape of the curve GG' is exactly similar to a curve obtained by the " soaking out'' of the residual charge in a Leyden jar which has been discharged for a short time, and iheii insulated. ^40 Electro-Kinetia. CHAPTER XXXIX. SECONDAllY BATTERIES — RHEOSTATIC MACIIINDS. When a current is sent through a voltameter for a consideratle time, the plates acquire some sort of electrical polarization, such that, if the battery be removed and the plates connected bj' a wire, a current will be observed in the wire in the reverse direction to that of the battery current. When the plates of the voltameter are made very large, it takes a longer time to polarize the plates, but the reversal ov "secondary" current is extremely powerful. The secondary current only lasts a short time, but its total energy is equal to the total energy which it has received from the battery in a long time, and, therefore, during the time which it lasts, the secondary current will be much stronger than the '^ primary " or battery current. Advantage has been taken of this fact in the construction of " secondary batteries," which enable us with two or three cells of Grove or Bunsen to obtain, for a short time, effects equal to those which could only be obtained directly by the use of many hundred cells. Plante's Researches. The most important researches which have been made on secondary batteries are those of M. Gaston Plante,* now (Feb. 1880) in course of publication. The form of " Secondary Element " which he uses is shown in * " Recherches sur I'lilectricite," par Gaston Plants. (Paris : A. Fourneau.) Tom. I., 14 Fev., 1879. Tom. II., fixsc. i., 30 Sept., 1879. Tom. II., fasc ii., 16 Oct., 1879. Plantd's Secondary Batteries. 141 fig. 2] 6. It consists of two large sheets of lead, of rather more than one square metre area, kept apart by narrow bands of gutta-percha, and imrqersed in diluted sulphuric acid of the strength 10 to 1.* Any number of these secondary ele- ments can be connected so as to form a secondary battery. M. Plante has also constructed batteries consisting of a series of flat lead plates, immersed in acid, arranged alternately like the plates of a condenser.f "When a secondary battery, consisting of six plates, each 020 metre by 0"22, and having an available surface of about J a square metre, was excited by two Bunsen cells^ the secondary current produced was found to be strong enough to heat to redness wires of iron, steel, and platinum one millim. in diameter. Pig. 216. " Formation " of the Plates. It was found that, when the plates had been in use some time, they gave better effects than when they were new. An investi- gation of the conditions under which their action improved led M. Plante J to the discovery of a method of "forming" the plates — that is, of causing them to assume the best condition for the production of the desired effects. It is found that two or three Bunsen or Grove cells will produce a better "formation" than any number of DanielFs. The process is as follows : — • On the first day the secondary element is charged alternately in the two directions some five or six times, and discharged between each reversal of the primary. It will be found that the secondary current gets stronger after each reversal. * Plante, Tom. I., p. 35. t See vol. i. p. 67. X Plante, Tom. I., pp. 49—55. 33 142 Electro-Kinetics. The six chargings are as follows, where " positive " means one direction of the currentj " negative " the other direction. 1 Current + for \ hour. 2 » — ,. i „ 3 I> + !> 5 5. * jj >) a ») 5 i> + „ 1 „ 6 „ — „ 1 „ After the last charging, the secondary element is not discharged, but is left charged (— ) until the next day. The next day it is charged alternately + and — several times, each charging lasting 2 hours, after wliich it is found that the secondary current does not increase any more. The element is then left at rest, charged (— ) for 8 days, when it is again charged + but not reversed again that day. Then it is allowed to rest for 15 days, then for one month, two months, &c., and it is found that its power still goes on increasing, the increase being only limited by the thickness of the lead plates. The improvement appears to be caused by the penetration of the electrolytic action into the interior of the plates, and the intervals of rest are necessary in order that the crystalline layers which are found to be formed in the metal may have time to harden before the reversal of the current. Connection in Semes on Side by Side. A number of secondary elements can be connected either in " series " or " side by side'' in precisely the same way as a num- ber of ordinary battery cells (Vol. I., page 267), and with exactly similar results. Heating Effects of the Secondary Cuerents. If four or five elements be connected " side by side," and then discharged through a short thick iron wire, it will be fused into a ball as in fig. 217. The surface of the incandescent ball will appear to " boil,'" and will be covered with spots, as bubbles of gas burst through from the interior. The bulbs develope themselves P'g-21''- very rapidly, and generally end Effects of the Secondary Battery. 143 by bursting the envelope of liquid iron which surrounds thena, and flying to pieces. Sometimes, however, the wire fuses first, and the ball cools and is preserved. Magnetic Effects. The secondary currents are able to magnetize electro-magnets much more powerfully than the primary currents from which they are derived. Duration of the Secondary Currents.* The secondary currents last longer when the plates have been well "■ formed'" than when they are new. Their duration also depends on the resistance through which they are being discharged, being of course longer when the resistance is high. The discharge of one element will keep a platinum wire of one millim. diameter red hot for from one to ten minutes accordino' to the degree of its "formation." An element which will only keep a thick wire red hot for a few minutes will keep a platinum wire \ millim. in diameter in a state of incandescence for a full hour. Constancy of the Current. It is found, when the resistance is considerable, that the cur- rent remains sensibly constant during the time which it lasts. Preservation op the Charge. It is found that a well-" formed" element will give a good current two or three weeks after it has been charsred. Electro-motive Force. It is found that the electro-motive force given by one element is about 1'45 to 150 times that of a Bunsen cell, i.e. about 2"5 volts. Transformation of the Current op a Voltaic Battery by means of a Secondary BATTERY.f In order to obtain currents of high potential, a number of secondary elements are arranged "side by side" and charged, and * Plante, Tom. I., p. 65. t Ibid., p. 93. 144 Electro- Kinetics. then are connected in series. While the elements are being charged, they are arranged as in fig. 218. Kg. 218. The connections are then altered to the arrangement of lig. 219, when the difiFerenees of potential given to each element separately Fig. 219. are all added together and produce a great difference of potential at the ends of the battery. Fig. 220 represents an ingenious mechanical contrivance for Fig. 220. making this change of connections rapidly. A wooden roller C C, can be turned by means of a handle, B. Broad strips of > 1-1 Secondary Batteries — Plantes Rheostatic Machine. 145 copper (one of which is seen in front) are fixed along each side, and short copper rods (seen vertical) are fixed through the roller. For charging, the roller is turned as shown in figs. 220, 221, so that c 11 ii i- 1 T'iS- 221. Ms. 232. springs irom all the negative poles press on one strip, and springs from all the positive poles press on the other. To connect in series, the handle B is turned through 90°, so that each spring is connected by one of the copper rods to the one opposite to it, as seen in fig. 222. TT' are the discharging poles for heating long wires, QQ' those for heating shorter ones. With a large secondary battery, consisting of 800 elements, some very fine luminous effects were obtained. It was charged by two Bunsen cells for several hours, and then discharged in the course of a few minutes. With a secondary battery of 200 ele- ments a platinum wire -^ to -j*„- of a millim. diameter and 10 metres long was heated to redness. Discharge in Vacuum Tubes. A secondary battery of 800 elements will illuminate a vacuum tube of high "resistance" for 3 J hours or more without re- charging. The discharge was found to be beautifully stra- tified.* Plante's Rheostatic Machinb. — Plate XLVII. The success of his experiments with secondary batteries led M. Plante to construct a Bheostatic machuie'\ for converting voltaic electricity into electricity of high potential. It consists of a number of mica and tinfoil condensers, and an arrangement exactly similar to that of the secondary battery for connecting the conductors in "side by side" for charging, and " in series" for discharge. As it is possible to charge and discharge these condensers very rapidly, the handle is rotated continuously, and a continuous stream of sparks is obtained. Plate XLVII. represents a large rheostatic machine containing 80 condensers. The cylinder at the top is 1 metre long and 15 * Plante, Tom. i., p. 259. t Plante, Tom. i., p. 252; Tom. ii., p. 2. 1 46 Elect7'o- Kinetics. centims. ia diameter, and the machine gives sparks of 13 centims., or nearly 5 inches. By experiments where only portions of the machine were used, M. Plante found that the length of spark was proportional to the number of condensers. The length of spark increases as the number of elements in the charging battery is increased, and increases faster than the number of elements ; but M. Plante was unable to determine the exact law of increase. The rheostatic machine was charged by means of a battery of from 600 to 800 secondary elements, or by from 50 to 70 Bun- sen cells. M. Plante did not find it to be of much practical use ; but it is of considerable theoretical interest. Rheostatic Machixe fob Quantity.* M. Plante has also arranged a rheostatic machine for " quan- tity " efiects. It, like the machine just described, has its con- densers arranged " side by side " for charging by the secondary battery, but by a different arrangement of the commutator they are also discharged "side by side" instead of in series. The commutator (figs. 22-3, 231) is arranged to give the con- Fig. 223. Pig. 22t. denser discharge "side by side " without mixture with the direct discharge of the secondary battery. On an india-rubber cylinder are four strips of copper, each f the length of the cylinder, of which two, m n and p, are seen in the figures. Six springs, BCE, B'C'E', press on the cylinder. The pair BB' are connected with the secondary battery, CC with the charging poles of a rheostatic machine (Plate XLVII.) whose commutator has been previously set in the position which connects its condensers " side by side." The discharge is taken from the springs EE'. * Plants, ii. p. 23. Plants' s " Quantity " Rheostatic Machine. 147 When the commutator is in the position shown in fig. 223, the battery is connected with the rheostatic machine and charges the condensers. When it is in the position shown in fig. 224, the battery poles are insulated, and the condensers are connected to the discharging poles EE'. When the cylinder is revolved rapidly, it gives an almost continuous series of discharges. This commutator, instead of being revolved separately, is usually adapted to the machine itself («' h' fig. 225). Fig. 223. When it is desired to use the machine for " quantity," the pin K is raised, which disconnects the two cylinders and allows a'3' to revolve while a b remains at rest. When it is desired to use the machine for " series " efEects, K is pressed down, which connects the two cylinders and causes a b to revolve, and the machine to act in all respects like the machine previously described (page 145). Although the short cylinder db' also revolves, it does not then produce any eflPect. Discharge of the "Quantity " Machine. A series of brilliant sparks are obtained, but only of a length of from T^ to -j^j. millim., much shorter than the direct discharge of the secondary battery. The spark, however, is much brighter and more violent than that of the direct discharge. The difference between the discharges of the secondary battery with and without the quantity machine is closely analogous to 148 Electro- Kinetics. that between the discharge of au induction coil with and without a Leyden jar* Heating Efj?ects. The heating eifect of the rheostatic machine is much greater when it is arranged for quantity than when it is arranged for series effects. Mechanical Effects. The mechanical effects of the quantity machine are very remarkable. If the machine is connected to a voltameter, the passage of each spark through the conducting liquid is accompanied by a loud noise resembling a small explosion. Nodes of Vibuation in a ^Metallic Thread. If the current of the "quantity " machine be sent through a fine platinum wire (a h fig. ^26) about -^ millim. in diameter and Fig. 22«. 40 centims. (16 inches) long, it will be seen that a series of acute angles are formed at tolerabl}' regular intervals all along the wire as in dV . If the poles are brought nearer together so as to slacken the wire, fresh angles are formed, and the wire takes the shape a'W. If the length of the wire be reduced to about 10 centims. (■i inches), the current makes it white-hot, and it is twisted into the sharp angles d"h"^, presenting the appearar.ce of a continuous electric spark. *■ Vol. ii. p. 53. Summary of Plantes Researches, 149 Noise. The discharge through the wire is aeeompanied by a continuous crackling sound, very much like that of an electric spark, but produced in the wire itself. Fragility of the Wire. The wire becomes very brittle during the passage of the discharge. If the experiment lasts more than about two minutes, it breaks spontaneously. Conclusion. The following is the conclusion to which M. Plante considers these and other experiments (described in his book) to lead : — ■ " The phenomena which we have just described are of a nature to throw some light upon the mode of propagation of electri- cit}'. The molecular vibrations revealed by the nodes formed in the metal wire, by the noise observed in it, and by the notable change in its cohesion under the influence of the " dynamo- static " current, which we have just studied, ought to be produced in a less degree in conducting bodies traversed by electric currents of less tension. These vibrations would be too small to be perceptible, but they are none the less real. " We may then conclude that the " electric movement " ought to propagate itself in bodies in the same manner as " mechanical movement," properly so called, by a series of very rapid vibrations of the more or less elastic matter which it traverses." * * I do not myself express any opinion on this conclusion. 1 50 Electro-Kinetics. CHAPTER XL. magneto-electeic aa'd electro-magnetic engines. Magneto-Electric Machines. The fact that electric c-urrents are prodiieed in a wire when it moves in the neighbourhood of a magnet has been utilized in the construction of magneto- electric or dynamo-etectrie machines in which very powerful electric currents are produced by revolving coils of wire between the poles of large horse-shoe magnets. The motion is given either by hand or steam power. The Siemens Armatuee. — Fig. 2i27. 1 n order to obtain the maximum effect, it is necessary that the Fig. 227. moving wires should cross the lines of magnetic force at right angles, and that the poles of the magnet between which the coil revolves should be as close together as possible. To satisfy these two conditions, ]Mr. Siemens has invented the " armature " shown in tig. 227. It consists of an axis which can be rapidly revolved, and on which a coil of wire is moved longitudinally. We see that this takes up but little room between the poles, and that the wires move to a great extent at right angles to the lines of force. Alternate Currents. Let A (fig. 228) represent a cross-section of one of the wires of the revolving coil, and suppose it to be moving round the centre O in the direction of the arrow between the poles SN of a horse- shoe magnet. Dynamo-electric Machines. 151 We see that, as lono^ as it is to tbe right of the line EE', it will be crossing the lines of force in one direc- ^^^^ tion; and as long as it is to the left, it will \ \ ^ / / be crossing them in the other direction.* \ \ ..-l-A / / The current in the wire will therefore be \ l+:}:x~^^ j reversed every half- revolution. ^— ^-^S^ii^'^— ' For some purposes these alternate currents i' are preferable to a continuous current. '^' ^^^' When a continuous current is required, a " revolving com- mutator" is attached to the axis of revolution of the coil and collects the alternate currents, and sends them all in the same direction through the wire. PiiACTiCAL Forms of the Machine. An immense number of different forms of magneto-electric machines have been constructed. We shall now only describe one or two typical forms, referring the student for further details to treatises on Electric Lighting.f The Hand Gramme Machine. — Fig. 2:ij», which can either be lowered through the slot ss, or by turning the rod rr caused to rest across it. The upper plates are supported by clean glass rods G, which are kept dry by sulphuric acid in the lead cups 17. The whole apparatus, including the short circuit key and elec- trometer, was, to avoid induction from outside, enclosed in a large zinc case connected with the earth, and was not opened at all during one complete experiment, consisting of some ten short circuitings of the upper plates, reversals of the table AB, and cor- responding readings to the right and left of the electrometer needle. * Phil. Tran?., 1880, p. 15. PLdeJmn CoTiiact/ ^iectricvfy 75 of fuJJy ,.Jlyrton and' Ferry Contact Electricity — Ayrton and Perry. 1 7 1 The following is a complete operation to obtain the contact difference of potentials^ between a metal and liquid^ for example. Suppose the permanent adjustments to have been made, and the gilt plates 3 and 4 are quite bright. The plate P is cleaned with emery paper that has touched no other metal, and all traces of the emery removed by means of a clean dry cloth ; it is then placed on the three levelling screws I, and fixed in position by hole, slot, and plane.* The porcelain dish containing the liquid is laid in a metal one just fitting it, and on the base of which is a hole, slot, and plane ; this is now laid on the other levelling screws I. The rod r r is then lowered until the disk d d rests on a brass plate let into the top of the wooden framework at the top of the instrument — that is, until the induction plates 3 and 4 are in their lowest position. The levelling screws 1 1 are now raised until a small metal ball, of a diameter of eight millims., is in contact at three fixed points with the plate 4 and the plate P, or until, when in contact with the plate 3, it and its reflection in the liquid L appear to meet. To avoid any harm arising from possible contact of the liquid with this gauge ball, it was made of a material not acted on by the particular liquid under experiment. Before proceeding further, each pair of quadrants is in succes- sion put to earth, the other pair remaining insulated in order to test for any possible leakage from the needles to the quadrants. Each pair of quadrants is now charged with a battery, the other pair being connected with the earth, in order to test for any leakage along the glass rods G, the small glass rods supporting the quadrants in the electrometer, or along the paraiEned ebonite pillars of the short circuiting key. It having thus been ascer- tained that there is no leakage, the strip of metal which has been cut from the same sheet of metal as P itself, and temporarily attached to it by a binding screw soldered to P, is made quite bright with emery paper and a cloth, and its end is dipped into the liquid L, as shown in Plate XLVIII., fig. 4. The zinc case is then closed up, plates 3 and 4 connected together, and with the earth, by means of a key (the handle of which was a long * " Hole, slot, and plane.'' This is an arrangement invented by Sir Wm. Thomson to allow any apparatus supported on three feet to be removed from a table and replaced exactly in the same position. Let 1, 2, 3 be the feet; 1 is placed in a small hole made in the table ; 2 in a short slot whose direction if produced would pass through the hole ; 3 rests on the plane surface of the table. 1 72 Electro-Kinetics. thin ebonite rod projesting through, the zinc case), and the electrometer reading taken. 3 and 4 are then insulated from one another, and from the earth, and raised by means of the rod r r projecting above through the zinc case ; the table A B is turned from below by means of a handle passing through the base of the instrument ; 3 and 4 are then lowered into exactly their former position, this being ensured by the parallel motion of the sup- porting beam and by the limiting stop, d d. The reading of the electrometer is now taken. Then the processes of " short circuit, insulate, raise, reverse, and lower, and take a new electrometer reading, &c.," are repeated. Some ten readings having thus been obtained, a fresh set of experiments is alwaj s made with the same two substances in the following way in order to compensate for the error introduced by defects in parallelism of the apparatus affecting the result obtained from two rigid surfaces (as those of copper and zinc), differently from the result found with one or with two liquid surfaces under examination. Instead of commencing, as before, with the liquid L under 3 and the plate P under 4, the experimenters start with the plate under 3 and the liquid under 4, and readjust, by means of the levelling screws I, the heights of the surfaces, until their distance from the plates 3 and 4 is, as before, 8 millims. They then short circuit, insulate, raise, reverse, and lower, and take exactly as many readings as before ; and the mean of the two sets of readings, obtained with the two modes of levelling, is regarded as the result of the particular experiment. " To test the accuracv of the statement, quoted on page 169 from Professor Jenkin's "^Electricity,^ that when copper and zinc are both plunged into water they are all at the same potential, the following sets of experiments were made. The plates 1 and i (fig. 238) were respectively zinc and copper, and they were connected together by means of a liquid in a small beaker having no direct inductive action on the plates 3 and 4. First, however, the apparatus was calibrated thus: — " Table YIII.— 13th April, 1876. Plates 10 mm. apart. Latimer Clark's Standard Cell. Zero. KeadinK. Deflection. 9550 892-0 63-0 9545 1018-5 64-0 954-5 891-8 62-7 953-1 lOir-1 640 Contact Electricity — Ayrton and Perry. 1 7-3 Mean . . . 634 Assumed to be 1'457 volts. Direct reading is 355 Therefore ratio is „-.— r- 63'4 or 5'6. Experiments : — Zinc and copper connected by distilled water at 17° C. Zinc is negative to copper. Zero. Reading. Deflection. 953 9602 7-2 952 947-0 5-0 952 9600 8-0 952 946-5 5-5 951-9 961-0 9-1 953 945-0 7-0 952 961-0 9-0 952-9 946-2 6-7 An interval of 15 minutes. 953 961-0 8-0 952-8 945-1 7.7 Mean . . 7-32 oi ■ 0-168 volts. "Zinc and copper metallically connected. Zinc positive to copper. Zero. Beading. Deflection. 953-0 926.0 27-0 952-7 990-0 37-3 951-0 920-3 30-7 950-1 985- L 35-0 950-0 919-5 30-5 950-2 984-6 34-4 951-0 918-0 33-0 951-1 985-2 84-1 Mean . . 32-7 or 0-751 volts. " Zinc and copper connected by saturated pure zinc sulphate at 17° C. Zinc negative to copper. Zero. Reading. Deflection. 952-0 961-5 9-5 951-9 944-3 7-7 951-8 960-0 8-2 951-9 943-1 8-8 952-0 960-0 8-0 952-1 944-6 7-5 35 174 Electro-Kinetics. Interval of 10 minutes. 953-1 960-0 6-9 953-2 945-0 8 2 953-1 961-2 81 953-3 945-2 8-1 953-7 961-() 7-3 953-9 945 2 87 Mean of first six . . 83 or 0191 volts. Mean of last six . . 7-9 or 0-182 volts. " From these experiments it followed that the above statement made in text-books, and which was based on certain experiments of Sir "William Thomson, is only approximately correct." From Professor Ayrton and Perry's experiments, and from those previously made by Sir William Thomson, they were led to con- clude " that, when zinc and copper are immersed in water, there are three successive states to be noticed. At the instant of immersion the zinc and copper may possibly be reduced to the same potential, so that the electro-motive force of the voltaic cell E is equal to the difference of potential ZC between zinc and copper in contact ; the zinc now becomes negative to the copper, so that E reaches a limit which is greater than ZC ; lastly, if a current be allowed to pass by metallically' connecting the zinc and copper, polarization occurs and the zinc becomes gradu- ally less negative to the copper, E diminishing, therefore, from its maximum value. But when a saturated solution of zinc sulphate is employed instead of water, the first state, if it exists at all, exists for so short a time that practically zinc and copper in zinc sulphate are never at the same potential. Thus when care is taken to keep the zinc and copper in a water cell well insulated from one another, E is found to increase from a value very little greater than ZC, the electro-motive force of contact of zinc and copper, to a limit, but in a zinc sulphate cell no such great increase is observed." Subsequently the difference of potentials of a number of sino-le contacts of dissimilar substances were measured, as well as the electro-motive forces of complete and incomplete cells built up with the very same specimens of the materials immediately after the previous tests were made. The following are some of the results obtained : — Let C, Z, and L represent the copper, zinc and liquid respectively of a simple cell let L, and L. be the Contact Electricity — Ayr ton and Perry, 175 liquid in contact with the copper, and the liquid in contact with the zinc of a Daniell's cell ; let CL be the electro-motive force of contact of C and L, and let CL be identical with— LC. Then : — I. Daniell with pure saturated copper sulphate and nearly pure saturated zinc suphate. Observed J5MF of cell measured directly. 1-068 to 1-081, increasing slowly. 0-995. roio. 1-000. CL, + L,L, + L,Z + ZC = 0028 - 0-033 + 0-358 + 0-750 = 1-103 II. Daniell with distilled water and pure satu- rated copper sulphate. CL, + L'L, -I- L=Z + ZC = 0028 + 0-071 + 0126 + 0-750 = 0-975 III. Daniell with very dilute zinc sulphate and slightly impure saturated copper sulphate. CL, + L,L, + L,Z + ZC = + 0-063 + 0-177 + 750 = 0.990 IV. Simple cell, nearly pure saturated zinc sulphate. CL + LZ + ZC = — 0-113 + 0-358 + 0-750 = 995 V. Simple cell, distilled water. CL + LZ + ZC = 0-074 + 0-126 + 0-750 = 0-950 0-832 to 0-942 increasing slowly. " In every case the sum of the separate contact electro-motive forces is so nearly equal to the observed maximum electro- motive force of the cell, that we have good reason for con- cluding that the electro-motive force of contact of any two substances measured inductively is constant for exactly the same specimens of the materials under exaclli/ the same condition as re- gards temperature, the gaseous medium surrounding them, &c.,and is quite independent of any other substances that may be in the circuit." In the investigation made during 1877-78, and described in their third paper,* the authors have obtained the followino- results : — * Phil. Trans. Eoj. Soc, 1880, p. 15. 176 Electro-Kinetics. i/j. o > o 5 C z ^ -S « <» -tli r^ O -T r^ ■^ CI r^ 'ri c nri CO r^ ^^^ 1 - -fi c^ -r Cl CI I I 00 --f -^ I- ^" C^' -* O C: -t O r^l O -f' Ca 00 1^ CC .— I -r rH O <— > 1— 1 r-< -i 1— I on CV) — ^

CO CI '::^ <:o c.\ — r* I I 1 I I I I 00 CI -— ' -H C5 o I•- J^^ C-- — ^T O 1>- O Cl cc I I -f .— o ^ CO '-^ r^ ci 00 *-o o — -r c-' L- 10 c ^- Cl -f !>. CO O CO -f' * * * 00 CO O '^O X 10 T-H — cr- — 00 ^ I- O Ol 1 1 1 1 1 1 1 . . D ■ • • s: 1 , 1 1 -4-^ ^ 1— ' tD ^ Ti i-^r - -J " •Ji ^l F— ( S 10 -S ^ >-. rH • - 2 ^- S -?■! ^ -— ^ • """ ^^ -f ^ a -' ci g-? S^ '-Z^ 0"=" "'a ,__l ^ S CD (U ^ ^ c 4^ r^ JO ci c '^ c*-. ;s^ r^ =S 6c o '^ p ■ (U ^0 a) g tf a 3 §=2 1 g 0" n d %* "^ 1 -. >— ■ ':■( S n — 1 r-^ '>• 'Ti •^3 --, ',i ^ (7) ^j — ^ — • 1 t-. S 3 0^ ,^ ,_] CO ^ ^ "^^ ■i-H r^ r* C3 C4-r OJ '- u a <;+-. "t; -*^ C3 -. n & 2 --< d P. p ■-5 ^ 2 CU +3 S s '-* 7) c; ■^ - ^ 'ii p p X Vj 5 "5 ^ 1 -O T^ B-^ ^ ij c 7: ^ ■^ i-w ' _!, o j: -^ - ^ --' ^ c5 _2 ^ O C ^ - > -^ ^ ^^J :i — O c: ,;_i -4_ -— .4J t- x . t: c s; < = Contact Electricity — Ayrton and Perry. 177 a n o m P o ^ •piO'G ouqitiSuOJ^g ::!;::::: : : ■atj-Budins | ^ ouTZ iioTtTipa pa^BJ ; : S : ; : : : : : •0 6.o9T5'8 j .0 c^T.t '^qiABiS ogpada i i : i 3 : : : : : : : uoT^nxoa aq-Bqdins 0UT2 '0 e-oSt 3^ paa^Jm-BS -H . "-O . . . . . ■Oo9[ {j-B paj'BJni'Ba -uoi-} -ntoa a^CBqdLUs jaddoo ; ^ . . . . ; 05 : oi o ■ P ? ■ ° 7< 1 1 1 •0 oST ^t'B paiBi -nijBS 'uorjnps mtqy ■ja^-BAi. paiit'4Bia: ... g ... 3 . ■A'jnojaj^ :;•:;;:•; ; ; •Bscja; rH ■* in QO : S S : I 9 S : : : : 1 1 1 ■ouiz pa^'BuiBSi'Bra.y : g : : : : : S : ,-1 • ■ - . • (M • •omz .iO_CDCD . , irat^o fX) . "* .— dioeo . . CDcoco CO . ^ • ^ -t^ ^ «S • • ip CO Tjl W • Tfi '1 + 1 '1 '\ \ '1 '1 "tt!! 1 1 1 •aitiTn^B[j CD iS "2 CD CO I>- rH?>^lSCOM. -OOO- •p-Bai ^ C5 . . t> OS . 1 1 1 ■UOJI 1 c^J an CO , . >o N to rH C3 • . CO CD ■ 1 1 1 •jaddoo ■308 •269 to } ■100 -127 •070 •103 -■476 - 396 •uoqj'BO oi^oo.. ■ ... . ; : Mercury , Distilled water /"Alum, saturated at\ f i6°-5c. . . . ; Copper sulphate, satu-1 rated at 15° 0. . . i Copper sulphate solu- 1 tion ; specific gravity, 1087atl6°-6C. . 1 Sea salt; specific gravity l 1-I8at20°-6C. . ( Sal ammoniac, saturated i Zinc sulphate, saturated! at 16°-3 0. . . / Zinc sulphate solution ; \ specific gravity, 1'125 \ at 16° 9 0. . . I 1 distilled water, mixed ^ with 1 saturated solu- \ tion zinc sulphate . ; 1 distilled water, mixed) with 3 saturated solu- V tion zinc sulphate . ) i ■suoTquiog ! 1 178 Electro-Kinetics. I r- & i2 a o c 1 CO ... 'piD^ OTj^TTi aaoj^B 1 o : : ' : : : : : : 1 • O S p ■ajBiidius omz ■nonnios paiBJUiBS ill : \ : \ : ; : ill eST.I '^^lAUia oypads ; uopnios aj'BTxd[UB oaiz : : : = ! ^ = ^ ^ = ^ t ^ *0 e-oSI *'B paqumiBS ^ ■Oo9I q-B p3a'BJna'BS 'uot^ -nios e^'Bq.d[ns jaddoo rH III '0 o5T I'B peaBj: -niBS 'aopnxos Tnniy CD la, M usqBAi. psxmsia CO : ; : : 1 • : « i ! ■^Tiojejl : ; ■ = ; : i i : p li! ■SBMa : : I i : ■ | i : = 2 ^-^ ^22 ■ouTz p8i'BraB3['maiv ; 1 a -^ fe s" Idlf ■omz ■-(ON -f- ■* oc -I -^ , . . ... « « w T' : : : : : : III 1 o « S f= liil •mx •nnnrrj-Bia •pB3i S gc3 . . : : : : : : r »-«?>: : 1 TIOJI ; : : ; 1 : : ^: : •jaddoo ^ •uoqjBO : : : : |P = S 5 j= g 3 S : : -SI lEJl 1= ilSj 1- £ £ £ 3 -SiSj; S^ E_ j"_ t.'^ ■ . - fi^-sM =1 II II i-S fe-jo^S3 -r.S 1.S ,-..3 ^^.S ■= ■ - .5 = .S a .i 3 >. a, >, ~ h J3PCS C C ,Kg? 111! isii fill Mil •piOB ounqdins amna 'pariBjgaaouoo a ?- -s Contact Electricity — Ayrton and Perry. i 79 The authors point out that in all these experiments two air- contacts enterj and that up to the present time no direct experi- ment has enabled the difference of potential at each of these to be measured. They therefore show the importance of repeating their quantitative experiments on the electro-motive force of contact in other gases besides air, and especially in a very perfect vacuum ; and they mention that, although they made the working drawings of the apparatus necessary for this extended investigation at the beginning of 1877, it was not until now that they have been enabled to commence it. They further add : — " If the gas measurements such as we have indicated be extended to a good Crookes' vacuum, we may then possibly approximate to the real value of A B, the contact difference of potentials of A with B, the value in fact that -we should obtain by a measurement of the Peltier effect. Kesults. " The results which have been already obtained in this present investigation group themselves under three heads : — " 1st. The contact difference of potentials of metals and liquids at the same temperature. " 2nd. The contact difference of potentials of metals and liquids when one of the substances is at a different temperature from the other in contact with it ; for example, mercury at 20° C. in contact with mercury at 40° C. " 3rd. The contact difference of potentials of carbon and of platinum with water, and with weak and strong sulphuric acid. "But those contained under head No. 1. are alone contained in the present paper." * * The authors hope to have the honour of suhmitting the remainder of" their completed experiments on a subsequent occasion to the Royal Society. 1 8o Electro-Kinetics. CHAPTER XLIV. Dimensions of Units.* As a familiar illustration of dimensions let us consider a linear, square, and cubic yard. A linear yard is said to be of one dimension in length. A square yard of two dimensions because it is a (yard)'. A cubic yard of three dimensions because it is a (yard)'. Thus, any length is expressed by a number multiplied by the unit of length, and this unit of length is said to be of one dimension. The units of area and volume are said respectively to be of two and three dimensions. This much is obvious. Debited Units — Velocity. Xow let us consider derived units : of these, velocity is the simplest. The velocity with which anything moves, when moving uni- formly, is the distance traversed divided by the time occupied by the journey. Thus, a train which travelled 140 miles in four 140 hours would have a velocity of — - = 35, when the units of length and time were the mile and the hour. If, however, the units were the yard and hour, the same velocity would be ex- pressed by the number (35 X 1760), whereas, if the units were the mile and minute, the same velocity would be expressed by 35 the number — . Thus the numerical number of the same velo- 60 city is greater when the unit of length is small, and, further, it varies inversely as the unit of length. * On this subject the student is requested to read " Units and Physical Constants," by Dr. Everett (Macmillan), from which much of this chapter is taken. Dimensions of Units — Maxwell! s Notation. 1 8 1 But the magnitude of the unit of velocity is inversely as the number of units which make a given velocity. Hence the unit of velocity varies directly as the unit of length. Again, with regard to the unit of time. The numerical value of a given velocity is less when the unit of time is less, and it varies directly as the unit of time. But the magnitude of the unit of velocity is inversely as the number of such units which express a given velocity. Hence the unit of velocity varies inversely as the unit of time. That is, the unit of velocity varies directly as the unit of length and inversely as the unit of time. This is expressed by saying that velocity is of one dimension in length and minus one ( — 1) in time.* Kepler's law of planetary motion discusses the area swept out in a given time by the straight line (called the radius vector) joining the sun and a moving planet. The area swept out in a unit of time may be called the area-velocity of the radius vector By exactly similar reasoning to that on the last page we shall see that the unit ot area velocity is directly as the unit of area and inversely as the unit of time. Hence its dimensions are one in area and minus one in time. But the unit of area is the unit of length squared, that is, area is of two dimensions in length. Hence the unit of area-velocity is of two dimensions in length and minus one in time. MAXWiSLL's Notation. It is customary to write units in square brackets. Thus, a length L may be written L[L] where L is a number and [L] the unit of length. On this plan, then, we may write [Length] = [L] [Area] = [I/] [Volume] = [L^] [Velocity] ^ Tl T J sometimes written - [Area Velocity] = \jj t''] or F _ J * See Todhunter's " Algebra ;" Theory of Indices, p. 147. Art. 258. 1 82 Electro-Kinetics. UxiT OF Force. When a body is in uniform motion, Newton's second law* tells us that force is required to change the motion, and that the change of motion is proportional to the impressed force. Change of motion means change of velocity, and this includes change of direction ; for to change the direction of motion of a body a velocity must be given to it in a direction inclined to its first direction. Suppose a body to be moving northward with a given velocity, then, to cause it to move in a north-easterly direction with the same velocitj"-, we must add a velocity whose direction is between north-east and south. Thus we have a natural method of measuring forces when we assert that a unit force is that which, acting on a body of unit mass, can produce a change of velocity equal to unity in a unit of time. For instance, the velocity of a falling body continually in- creases because the force of gravitation is accelerating it. The number of centims. per second by which the velocity increases is the measure of the force with which gravitation acts on each gramme of the body. Xow, if the unit of velocity is great, a greater force will be equal to unity, as it will have to make a greater change of velo- city in a given time ; therefore the unit of force varies directly as the unit of velocity. If the imit of time is great, the unit of force will be small, as a less force will be required to produce a given change in a long time than in a short one. Hence the unit of force is inversely as the unit of time, and directly as the unit of velocity. Again, if the unit of mass is great, the unit of force will be great, for more force is required to produce a given change of velocity in a given time on a great mass than on a small one ; hence the unit of force varies directly as the unit of mass. We have then — unit of force varies directly as units of velocity and mass, and inversely as unit of time ; or if we write [F] [V] [il] for units respectively of force, velocity, and mass, we have * Lex II. — Mntationem motus proportionalem esse vi motrici impressa, et fieri secnndum lineam rectam qua vis ilia inipriiaitar. Dimensions of Electric Units. 183 [F] = [M V T J or |_ T J' But the unit of velocity also contains the unit of time once in- verselyj for [V] = L T '. Therefore, on substituting its value for [V] , we shall have [F] = [m. L t" T" '] = [m L T '] or Ratio of Units. It is obvious that the dimensions of different units bear certain ratios to each other ; sometimes these ratios have an obvious physical meaning, sometimes not. For instance, t!ie ratio of volume to area is the meaning of which is clear enough. The ratio of force to velocity is fMLl L V. -/ which has no obvious meaning. The Two Sets of Electric Units. Now we know that there are two systems of measuring electric effects, the electro-static and the electro-magnetic. If we in each case set to work to derive the units by which the effects are to be measured from the fundamental units of time, length, and mass, we shall arrive at two different systems. We propose to do this, and, having compared the two systems, see what physical meaning we can give to their ratio. Electuo-static Unit op Quantity. In the electro-static system the unit quantity of electricity is that quantity which, if collected at a point, will repel another equal quantity at a unit distance with a unit of force. In the C. G. S. system a unit of electricity is that quantity which would repel 184 Electro- Kinetics. an equal quantity at a distance of one centim. with a force of one dyne.* The force between two quantities of electricity, each equal to 5' at a distance / from one another, would be P' The unit of electrical force may then be written m But in order that this may agree with the mechanical units where unit of force is we must have or ra=P/] which gives us for the dimensions of unit electrical quantity in the electro-static system, Electeo-static Unit of Cueeent. The numerical value of a current in electro-static units is on the C.G-. S. system defined to be the quantity of electricity which passes in a unit of time. Hence the dimensions of a current are, in electro-static measure, Electeo-uagnetic Unit of Cueeext. On the electro-magnetic system, a current flowing along a circular arc is measured by the "intensity of magnetic field" which it produces at the centre of the arc. * A dyne is that force which, acting on a unit of mass, would change its velocity 1 centim. per second in one second. Gravity = about 981 dynes per gramme; weight of a gramme ^ 981 djTies; Ibrce of gravity on a gramme ^ 981 dynes. Dimensions of Electric Units. 185 Intensity op Magnetic Field. The intensity of magnetic field is equal to C^ the strength of the current^ multiplied by length of arc, divided by square of radius. Therefore, if [I] be the unit of intensity of magnetic field, we shall have -CL- y^=\m or [C] = [I L]. The unit magnetic pole is that which repels a similar unit pole at unit distance with a unit of force. In the C.G.S. system it is one which repels a similar pole distant I centim. with a force of 1 dyne. The force between two poles of strengths Pi and Pj is equal to their product divided by the square of their distance from one another. Hence we have for the equation of units or [P] = [m^- l^ t']. The intensity of a magnetic field is the force which a unit pole will experience when placed within it. Denoting this intensity by I, the force on any pole will be I P. Hence Dividing both sides of the equation by [P] we have "ML"! r i^ CUEEENT. Returning to our current equation [C] = [I L] ML r|i2 M2 T T we have [C] = [m* L L-* T '] = [m* L* t']- 1 8 6 Electro-Kinetics. Electeo-magnetic Unit of Quantity. Xow in the electro-magnetic system the quantity of electricity conveyed by a current is equal to the strength of the current multiplied by the time which it lasts. The unit of electrical quantity on the electro-magnetic system then is [Q] = [C T] = [m* L^ T-' tJ = [m* L*]. Ratio of Two Units of Quantity. Postponing for a while the discussion of the dimensions of other electrical quantities^ we will consider the ratio of the two units of electrical quantity. We have Dimensions of electrical quantity On the electro-static system r 1 3 -in Lm^l^t J- On the electro-magnetic svstem [mH4]. Ratio of Dim, in E. S. Dim. n. in E. S. _ r -i-, rL~| 11. in E. M. ~ LLT Joi-LtJ- Thus the ratio of the dimensions of the two units of quantity is a velocity. It will be shown in the next chapter,* that this velocity has a real existence, and the physical meaning of it will there be explained. "We will now briefly give the dimensions of other electrical quantities. Electeo-static Uxit of Potential. The dimensions of work are force multiplied by distance through which it acts Hence [W] = [m L T"' l] = [il L t']. The work done in raising the potential of a quantity of elec- tricity Q through a difference of potential V is * Yol. ii. pp. 192, 200. Dimensions of Electric Units. 187 Hence we have WJ xw" v tJ Electro- STATIC Unit of Capacity. The capacity of a conductor is the quotient of the quantity of electricity with which it is charged by the potential which this charge produces in it. Hence we have Electko- static Unit of Resistance. The resistance of a conductor is equal to the time required for the passage of unit quantity of electricity through it when unit difference of potential is maintained at its ends. It therefore varies directly as the time, also directly as the difference of potential ; for if we increase the difference of poten- tial, we must increase the resistance if we wish to keep the time the same. It varies inversely as the quantity, for if a greater quantity is to pass in a given time, the resistance must be less. Hence 1- <4 -J (^ M* L5 T J or , m viz. reciprocal of a velocity. Electeo-magnetic Units of Electro-motive Force and Potential. The work done in urging a quantity q of electricity through a circuit by an electro-motive force E is E jf. And the work done in urging a quantity q through a conductor by means of a difference of potential E is also E q. Hence the dimensions of electro-motive force and also the dimensions of potential are * Compare veil. i. p. 67. i88 Electro- Kinetics. Electeo-jiagnetic Uxit op Capacity. Capacity is the quotient of quantity of electricity by potential. Its dimensions therefore are I 13 el \mvr J [l-t] Electe-o-magnetic Uxit op . Resistance. Resistance equals E C therefore C I 3 -2^ = [lt-]- That is, as we saw in discussing the B A unit in vol. i,, page 28fi, electro-magnetic resistance is a velocity. SUMMAET. The following taLle summarizes the results we have just obtained : — DImens. in E. S. Dimens. in £. 'SI. g^jj^Dim.iTiF.S. Dim. ia E. JI. Quantitj' . m*l?t' 31*L^ lt"' : Current . M* L^ t" il* L* t' lt' Capacitj' . L L-T^ l=t' Potential aud Electro- motive Force 1 M^ l' t' l'' t Eesistance L-T ; L T ' i'^t' Ratio of the Two Sets op Electeic Units. The following explanation of the ratios between the two sets of electric units is due to Professor Everett :* — * " Units and Physical Constants," p. 132. Dimensions of Electric Units. 1 89 We know that in the C.G.S. sys- tem — The unit of length = 1 centiin. The unit of mass = 1 gramme. The unit of time = 1 second. Let us consider some other general system in which The unit of length =: L centims. The unit of mass := M grammes. The unit of time ^ T seconds. Then the new electro-static unit of quantity will equal M^ L^ T C.G.S. electro-static units of quantity. And the new electro-magnetic unit of quantity will equal M^ X? C.G.S. electro-magnetic units of quantity. Now it is possible so to select L and T that the now electro- static unit of quantity is equal to the new electro -magnetic unit. In order to determine what the values of L and T must be to satisfy this condition, we have, substituting the values of the new units in C.G.S. units, the equation f M* L^ t' C.(J.S. electro-static 7 _ f M^ L* C.G.S. electro-magnetic 7 ^ units of quantity. J (. units of quantity. ' ... 11 Dividing out by M'^ L^ we have L T C.G.S. electro-static units of quantity = 1 C.G.S. electro-magnetic unit of quantity. or the ratio of the C.G.S. electro-magnetic unit of quantity to the C.G.S. electro-static unit is — . We see that = is clearly the value in centims. per second of that velocity which would be denoted by unity in our " new " system. This is a definite concrete velocity, and its numerical value will always be equal to the ratio of the electro- magnetic to the electro-static unit of quantity, whatever units of length, mass, and time are employed. It will be observed that the ratio of the two units of quantity is the inverse ratio of their dimensions, and the same can be proved in the same way of the other four electrical elements. The last column of the table on page 188 shows that M does not enter into any of the ratios, and that L and T always enter with equal and opposite indices, that is that all the ratios depend only on the velocity — . 36 1 90 Electro-Kinetics. Thus if the concrete velocity jp be a velocity of v eentims. per second, there will be the following relations between the C.G.S. units. 1 electro-magnetic unit of quantity =» electro-static units. 1 „ „ current = d „ „ 1 >, „ capacity = «* „ „ V electro-magnetic units of potential ^ 1 electro-static unit. t!^ „ ,, resistance ^1 ,, „ iqi CHAPTEK, XLV. EXPERIMENTAL COMPARISON OP ELECTRO-STATIC AND ELECTRO- MAGNETIC UNITS. We have shown that electro-static and electro-magnetic pheno- mena are measured by two diflferent sets of units, or rather that two distinct systems of units have grown up — one based on an electro- static, and the other on an electro- magnetic unit of quantity; but either may be used in the measurements of any phenomena. We have also seen that the relation of the electro-magnetic unit of quantity to the electro- static is the relation of a length to a time — in other words, it is a velocity. By a purely mathematical process of reasoning,* which it is impossible to put into a non-mathematical form. Professor Maxwell has shown that this velocity is the velocity with which electro-magnetic disturbance is propagated through space — that is, if a sudden difference of magnetic potential be caused at any point, the disturbance due to it will be felt at any other point after an interval which, on being compared with the distance between the points, shows the disturbance to have been propa- gated with this velocity. This velocity has never been measured directly, as, even at a distance of 100 yards, the disturbance caused by any change of potential which we can produce would be quite insensible, and the velocity, which, as we shall presently show, is about the same as that of light, is so great that the time required for the disturbance to pass over a short interval is extremely small. It is, however, possible that, if necessarv, some method of direct measurement might be devised ; but the indirect method of comparison of units is as certainly a measure of the velocity of the disturbance, and is capable of far greater accuracy than is ever likely to be obtained by the direct method. • Clerk Maxwell, "Electricity," Articles 783—786, vol. ii., pp. 384—387. 192 Electro-Kinetics. The Ratio is a Velocity. The following physical proof that the ratio of the units is a velocity is given by Professor Maxwell : — Let there be two parallel currents — ^the attraction experienced by a length a of one of them is — r = 2CC'~, where C, C are the numerical values of the currents in electro- magnetic measure, and h is the distance between them. If we so choose the length a that we are considering that h zil'-I a we shall have — F = CC. Let us put n for the number of electro-static units in one electro-magnetic unit, we have to show that » is a velocity. The quantity of electricity transmitted by a current C in a time t is we know equal to C < in electro-magnetic measure, and therefore to nCt in electro-static measure, because n is the number of electro-static units in one electro- magnetic unit. We know that the repulsion F between two statically-charged bodies at a distance apart r and having charges ^ and (^ is F — g g' * Let two small conductors bo charged with quantities q q' equal to the quantities transmitted in time t by the currents C C re- spectively, then their charges in electro-static measure will be — and the electro-static repulsion F' between them will be — F' = " C f, « C i! _ CC'n'f Let the distance r be varied until this repulsion equals the electro-magnetic attraction F, we have F = F' or Dividing out by C C we have 1 — !L-? or * Vol. i. p. 19. Comparison of Units, Theory, Weber & Kohlrausch. 1 93 that is n t = r or r t — that is n, the number of electro-static units in one electro- magnetic unit, equals a length (r) divided by a time {t) — that is, equals a velocity. The absolute magnitude of this velocity is the same whatever units we adopt. Theory of the Experiments. We now come to the experimental methods of determining the ratio of the units. The general principle of them all is to measure the same thing both electro-statically and electro-magnetically. Different num- bers are obtained in the two cases. But when the same thing is measured by two different sets of units, the ratio of the numbers obtained in the two cases is the inverse ratio of the units used. For instance, suppose we did not know the ratio of a foot to a yard, but were able tq measure any distance both in feet and in yards. To determine the ratio of the units we should talce an arbitrary distance and measure it— first using the yard for the unit and then the foot. Suppose we found that the number obtained in the first ease was 60, and in the second 180, we then have, ratio of number obtained with yard unit to number obtained with foot unit equals 60 to 180, or 1 to 3. Therefore the ratio of the yard to the foot is the inverse of this ratio, or 3 to 1 . Similarly, in the electrical ease, we measure the same quantity of electricity first in electro-static units and then in electro- magnetic units. The ratio of the numbers obtained is the inverse ratio of the electro-static to the electro-magnetic unit of quantity. Experimental Methods of Determining the Ratio between Electro-static and Electro-magnetic Units. The first numerical determination of this velocity was made by Weber and Kohlrausch.* * Pogg. xcix., August 10, 1856, and Maxwell, 771, vol. ii. p. 370. 1 94 Electro-Kinetics. The following account of these experiments is given by Pro- fessor Maxwell : — " Their method was founded on the measurement of the same quantity of electricity — first in electro- static and then in electro- magnetic measure. " The quantity of electricity measured was the charge of a Leyden jar. It was measured in electro-static measure by the product of the capacity of the jar into the difference of potential of its coatings. " The capacity of a sphere is expressed in electro-static measure by its radius. Thus the capacity of the jar may be found and expressed as a certain length.* " The difference of the potentials of the coatings of the jar was measured by connecting the coatings with the electrodes of an electrometer whose constants had been carefully determined, so that the difference of the potentials was known in electro-static measure. By multiplying this by the capacity of the jar the charge of the jar was expressed in electro-static measure.^' To determine the value of the charge in electro-magnetic measure the jar was discharged through the coil of a galvano- meter. The total current could then be calculated from the limit of the first swing of the needle. This comparison gave for the ratio of the units^ which is commonly called v, V =: 3'1074 X 10'° centims. per second. Professor Maxwell has pointed out that the phenomenon known as "Electric Absorption," which^ at the date of these experi- ments, was not well understood^ makes it almost impossible to estimate correctly the charge of a jar unless the experiments are performed instantaneously. He shows that the effect of neglect- ing absorption would be to make the value of v deduced by this method too high. Sir Wm. Thojison's Compaeison. — Plate XLIX. Sir Wm. Thomson has determined v, the ratio of the units, by measuring the same electro-motive force in both sets of units. Electro-motive force is measured in electro-static units directly by means of an electrometer. * See vol. ii. p. 187. l-i '1 SUctmmetxr Se^ ^ ^ ^ .NX ^ ^ ^ ^ ^ £ltctrairvcttr K&y Experimental Comparison of Units — Thomson. 195 By Ohm's law we know that and C and r can be measured in electi-o-magnetic measure. For suppose that E equals A electro-static units, and that the current C which it can produce through a resistance r is such that Cr = B electro-magnetic units ; then, if we measure A and B, we shall have _ ^ «, the ratio required, for 1 electro-static unit of potential equals v electro-magnetic units. In Sir Wm. Thomson's experiments the difference of potential was measured (statically) by electrometers, and the current, through a known resistance, by the deflection of the suspended coil of an electro-dynamometer.* Plate XLIX. shows the arrangements. f But little explanation is required. The dynamometer has been already described, as have the various electrometers. The chief circuit is that of the resistance coils, battery, and dynamometer, with branches to the absolute electrometer. The gauge, Leyden battery, and replenisher, are the usual appendages of the electrometer. The quadrant electrometer was used solely as a convenient method of determining the resistance of the dynamometer coils while the experiments were going on ; and thus eliminating changes of resistance due to changes of temperature produced by the current. It was used by connecting its terminals alternately at each side of the resistance coil, whose resistance was known, and of the dynamometer coils. We can deduce, from vol. i., p. 306, that in any circuit the resistance between any two points is simply proportional to the difference of potential at those points. Hence the resistance of the dynamometer is to that of the coils as the difference of the potentials on each side of it to the corresponding difference at each side of the resistance coils. The battery used was sixty sawdust Daniell's in series. The value of the ratio given by Sir William Thomson in 1869 as determined by this method, is as follows : — " Eleven sets of * Yol. ii. p. 3. t " Reports on Electrical Standards,'' p. 186. 196 Electro-Kinetics. experiments made at various dates from March 10th to May 8th, 1868, have indicated values for the ratio which is called v, of which the greatest was 2-92 x 10'°, the smallest 2-7 54 x 10'° and the mean 2"825 x 10'" centimetres per second." Sir William Thomson, at the end of his paper, expressed his intention of carrying his experiments to a much greater degree of accuracy. McKichan's Experiments. This intention was carried out by Mr. Dugald McKichan, who, working in Sir William Thomson's laboratory, made, in 1870-72, a series of measurements of which the results were communicated to the Royal Society on April 15, 1873, and will be found in the Philosophical Transactions for that year.* The experiments only differed in detail from those already described. The final series of values of v determined on February 21, 1872, were as follows : — V in centims. yer second. 2934 X 10-° 2-935 2-931 2-923 2-935 2-935 The mean value adopted by Mr. McKichan is 2-93 X lO'" Clerk Maxwell's Direct Comparison. Professor Clerk Maxwell has compared the two units of electro-motive force by balancing the attraction between two oppositely-charged discs against the repulsion between two cur- rents carried in two flat spirals of known resistance, the ratio between the electro-motive forces used in charging the discs and in sending the currents through the spirals being known, and the electro-motive forces being measured electro-statically. Plate L. shows the arrangements. * Phil. Trans., 1873, p. 409. The mathematical theory of the experiments is very fully given in this paper. Plus L^ilucwell's batio op units. Comparison of Units — McKichan — Maxwell. 197 Explanation op Plate L. A Suspended disc and coil. A Counterpoise „ „ Fixed „ „ Bi Great Battery. Bj Small Battery. Gi Primary coil of GalTanometer. 63 Secondary „ „ II Gi'eat Eesistance. S Shunt. K Double key. C Electrode of fixed disc. X Curren through E. X jj „ (x|. a-— a^ „ „ S. y „ „ the 3 coils and Ga. M Mercury cup. T Torsion head and tangent screw. g Graduated glass scale. G and S 10 ft. from Electric balance. One disc and spiral were fixed, and the others were attached together to the same end of the arm of a torsion balance. To the other end of the arm was fixed an exactly similar coil, through which the same current travelled in the opposite direc- tion to that in which it passed through the first suspended coil. The eifect of the extra coil was to neutralize the action of terres- trial magnetism on the arm of the torsion balance. The small suspended disc was' surrounded by a guard-ring siniilar to that used by Sir William Thomson in his absolute electrometers. This insured that the electrical action on the small disc should be equal to that due to a uniform distribution over its front surface. The suspended disc was four inches diameter, and was kept at the same potential as the case of the instrument. The fixed disc was 6 inches diameter, and could be moved nearer to, or farther from, the guard-ring by means of a micrometer screw. It was insulated and maintained at a high potential during the experiments. When the suspended disc was in its position of equilibrium, its plane corresponded with that of the guard-ring. The coils were fixed at the backs of the fixed and suspended discs respectively ; special means were taken to insulate the one placed in contact with the disc which was intended to be charged to a high potential. The electro-static charge was given by connecting the discs to the terminals of a great battery of 2600 cells charged with corro- sive sublimate. The difference of potential at its ends was mea- sured by observing, by means of a galvanometer, the current which it could send through a known very large resistance. The currents through the coils were produced by a smaller bat- tery, and measured in the ordinary way by a galvanometer. 198 Electro-Kinetics. Thus we have two electro-motive forces whose ratio is known — one acting electro-statically, the other electro-magnetically. Their strength heing adjusted until the attraction of the discs equals the repulsion of the coils — ^that is, until the suspended arm of the torsion balance is in equilibrium ; and proper correc- tions having been made for the different distances apart of the discs and the coils, we have at once all the elements for a com- parison of the electro-static and electro-magnetic action of the same battery. That is, the results of Professor Maxwell's ex- periments give a direct value of the relation between electro- static and electro-magnetic units of electro-motive force. The following are a series of values of v given by Professor Maxwell : — 2-8591 X 10'° 2-8430 „ 2-8886 „ 2-8686 „ 2-8910 „ 2-8850 „ 2-8762 „ 2-8795 „ 2-8735 „ 2-8752 „ 2-8709 „ 2-9474 „ Mean value of » 28798 x 10'° or 288,000,000 metres per second, or 179,000 statute miles per second. The "probable error" is about one-sixth per cent. Ayuton and Pekey's Deteumination.* At the Dublin meeting of the British Association in 1878 Professor W. E. Ayrton gave an account of a determination of v, which had been recently made in Japan by Professor Perry and himself. The plan adopted by them was to measure the capacity of an air-condenser, each plate of which was 1323"14' square centimetres in area, both electro-statically and electro-magnetically. * Report, Brit. Assoc, Dublin, 1878, p. 487 ; or Phil. Mag., 1879, i. p. 277, and Jour. Soc. Tel. Engineers, May, 1879. Comparison of Units— Ayrton & Perry — Hockin. 199 The electro-static capacity was ascertained by linear measure- ment of the condenser, while the electro- magnetic capacity was determined by noting the first swing on discharging the con- denser through a special form of Balistic galvanometer devised by them for the purpose with forty magnets in two spherical masses so as to have great sensibility and exceedingly little air- damping (see vol. i. page 240).* The source of electricity was 383 new Daniell's cells. To determine the relation of battery and galvanometer constants, a known fraction of the current was sent through the galvanometer directly, by which means, from the limit of the swing obtained on discharging the condenser through the galvanometer, its capa- city in electro-magnetic measure could be determined indepen- dently of the absolute strength of the magnetic field in the laboratory. The required velocity v was the inverse ratio of the square roots of the two determinations of capacity,t and Messrs. Ayrton and Perry claim that their method has the advantage that the formula for reduction of the observations involves only the square root of a resistance, so that if any unknown error existed in the resistance coils, only the square root of that error would be introduced into the result, whereas, in the methods previously described, the error in v is directly proportional to that in the coils ; and also that only one accurate electrical measuring instrument — a balistic galvanometer — is employed, whereas the other methods required two, such as an absolute electrometer combined with a galvanometer, &c. The following results were obtained on three different days : — 1878. V June 18 2-974 x 10'° June 23 2995 y. 10'° June 25 2972 x 10'° Mean of 98 discharges of the air condenser . 2'980 x 10'" Hockin's Expeeiments. On August 26, 1-\ = [M'^ L"* T], for it is a number divided by a current, or, by what is of the same dimensions, a difference of magnetic potential. This is Verdet's constant in absolute measure. For light of a given wave-length passing through a given substance, it is a fixed and definite physical quantity, depending only on the units of length, mass, and time.f to, then, is defined to be Verdet-'s constant for the thallium ray in bisulphide of carbon, expressed in C.G.S. measure. I may also here insert the result of a former paper of mine on the same subject. It is that for distilled water with white light. m — 4-496 (10-«).§ I do not, however, attach much value to the result, as the different determinations are in the ratios of 7-563, 7-406, 8-295, 8-401, 6-916, showing variations from^ the mean of ± 7 per cent., or giving only about ^ of the accuracy of the present paper. The methods used for determining the constants were also susceptible of less accuracy. * 2 E is the difference of the circle readings when the cun-ent was reversed, and is, of course, double the rotation produced by it. t See vol. ii., p. 180. % The magnetic rotative power of bisulphide of carbon here comes in the same way as the specific heat of water comes into Joule's equivalent. § By an error in arithmetic this was printed lO'' in the abstract of the paper published in the Proc. Boy. Soc, June, 1875. Magnetic Rotative Powers — H. Becquerel. 233 Effect of Terresteial Magnetism on Light. In the "Comptes Rendus "for 1878/ M. Henri Becquerel pointed out that from these results we can calculate what would be the effect of the earth's magnetism on light in certain media. If a canal one mile long were dug from north to south near Kew, and filled with bisulphide of carbon, a ray of the green polarized light experimented on entering at one end would, by the action of the earth's magnetism, have its plane of polari- zation twisted just 50°. If the canal had been full of distilled water, the twist would have been about 7^°. H. Becquerel's Experiments. On July 10, 1876, M. Henri Becquerel presented to the Paris Academy of Sciences a paper entitled " Recherches Experi- mentales sur la Polarization Rotatoire Magnetique.'^ f The first portion of the paper contains an account of a comparison between the magnetic rotative powers of different substances, that of bisulphide of carbon being taken as unity. M. Becquerel's experiments were made on yellow and on red light, according to the colours of the substances used; but I think we may fairly assume that the ratio between different substances is approximately the same for all kinds of light. On this assumption we can calculate from M. Becquerel's table, and from my measurement of Verdet's constant, what the rotation in circular measure would be in each substance for the thallium ray between two points whose magnetic potentials differ by unity. See next page. Relation between the Index of Refraction and the Magnetic Rotative Power. In the same paper M. Becquerel has shown that for the same groups of substances the following relation approximately holds : — . ^ const. where R is the magnetic rotative power and /u the index of refraction. * T. kxxvi. p. 1077. t " Ann. de Chem. et de Phys., 1877, 5"= Seiie," torn. xii. p. 6. 234 Electro- Optics. Table of Magnetic Rotative Powees. SnbstaiLce. Index or BefractioD. RltiO of Botation to that in Bisulphide of Carbon. a = EotatJon in circalar mea- nireofthegreenl thallinm ray between two points whose magnetic potentials differ by nnity. Bisulphide of carbon . 1-263 Tellow light. 3^04763 X 10-> 1-6249 1 1-000 Nitric Acid— fmning „ „ ordinary .... ... yvno 1-3740 -206 -291 •6278 ■8868 1 Solphnric acid — monohydrate „ SO3 + 4 HO . 1-854 1-4284 1-4" 54 •247 •286 ■7527 •8716 Pare hydrochloric acid . . . - 11630 1-4071 •490 1^4933 1 Alcohol— methyl. . . . . propyl. . butyl. „ amyl. -836 -811 ■SIJ7 -815 1-3530 1-3S36 1-3934 1-4046 •253 -279 •294 •311 •7710 1 -S5'.2 ! -8960 •9178 Chloroform . . - 1 ... 1-4520 •380 ri58l Xylene . ... -866 1-4932 ■5-'5 1^6000 Toluene . . .... •871 1-4929 -576 1-7554 Benzine •883 1-4998 -636 1-9382 Silvine KCl (crystallized) . ... 1-4830 -672 2-0480 Diamond (octohedrai crystal) ... 2-4200 •301 -9173 Fluor Spar— 1st specimen . 2nd specimen . ... } 1-4332 \ -2-17 -63f)8 -7131 Rock salt 2-260 1-5430 -843 2-6691 Glass— Xo. 1. Heavy flint . 2. „ „ . . 6. Flint .... 7. „ . . . 8. Crown .... ♦Moltenborateonead(l) . (2) . , . Silicate of lead molten . , , . 4-380 4-660 3-168 3-540 2-559 1-7200 ] -76-50 1-6790 1-6140 1-5260 } 1-7800 { 1-8200 1-380 1-633 ■771 •987 •481 1-405 1-439 1^832 4-1447 4-6720 2-3497 30080 1-4659 4-2819 4-3855 1 5-6832 Solution of sub-acetate of lead in water ... 1-3670 •375 1-J428 Protochloride of carbon ... l-«80 -404 1-2312 Perchloride of carbon .... ... 1-5620 -761 2-3192 Cast borax ... 1-5010 ■405 1-2343 Concentrated solntion of nitrate of 7 silver 3 ... 1-4580 •424 1^2922 Chloride of silica 1-623 1-4090 •444 ! 1-3531 Concentrated solution of nitrate of 1 bismuth 1 ... 1-4S90 •452 1^377S ' Fondu " may mean either " molten " or " cast.' Magnetic Rotative Powers — H. Becquerel. 235 Table of Magnetic Rotative Powers {continued). Substance. 1 Index of Refraction. Ratio of Rotation to that in Bisulphide of Carbon. (0 = Rotation in circular mea- sure of the green thallium ray between two points whose magnetic potentials differ by unity. Concentrated aqueous solution of \ potash, . ... 1 ,,^ Yellow light. 1-4141 1-4230 ■464 Spinel coloured by chrome . 1-7150 -496 1-5116 Concentrated solution of chloride of 1 magnesium . . . , J 1-4300 •619 1-6817 Protochloride of phosphorus l-MO 1-608 •661 1-9840 Concentrated solution of perchloride > of antimony . . , . > 1-4600 •743 2-2643 Bichloride of sulphur, SCI ■ 1-6190 •932 2-8403 Protochloride of sulphur, SaCi . 1-687 1-618) ■ ^984 2-9988 Chloride of arsenic .... 2-172 1-6006 1-000 3 0476 Bichloride of tin 2-200 1-6060 1-036 3-1542 Perchloride of antimony 2-280 1-6910 1-656 6-0468 Bisulphide of hydrogen (impure) ... 1-8S50 l-7« 6-3120 Molten sulphur at 114° . 1-96 1-9290 1-9U4 5-8026 Sub-sulphide of phosphorus, PhgS 1-8007 2-0661 2-692 7-8991 Molten phosphorus at 33° . 1-77 2-0740 3-120 9-5086 Blende .... 4-095 2-3690 6-295 16-1372 Bichloride of titanium .... 1-6043 -•368 - 1-0910 Bromide of sulphur . . . . 2-696 Bed light li. 6-9184 1-7630 1-942 Brome 2-970 1-616 1-960 6-9735 Chloride of selenium . . . . 2-689 1-8070 2-403 7-3387 Selenium 4-300 2-665B 10-960 33-4020 Oxydide of copper .... 5-992 2-819 14-060 42-8496 Chlorochromic acid ... ... --080 --2438 Distilled water .... ... 1-3340 -308 -9386 X lO-s Distilled water, direct observation \ (Gordon) \ ... ... •4496 X 10-" * There is a very large discrepancy here between M. Becquerel's result and my own. I cannot account for it. 236 Electro- Optics. The constant, however, has very different values for different groups of substances. Rotation of Different Rays. M. Becquerel has found that for the same substance the relation between the magnetic rotation of different rays is ex- pressed very exactly by the formula — ^ const. M^C/'^-i) where X and (x are the wave lengths and refractive indices for the different rays respectively. Now, for bisulphide of carbon we have — Bay. Wave length.* Refractive Index. D . . . . 5-892 . l-6333t E . . . 5-269 l-6465t Green Thallium . 5-340 . 1-6448+ Hence we have that the rotation of any ray whatever in bisulphide of carbon, between two points whose magnetic potentials differ by unity, will be in circular measure. _ (1-6448)" 1(1-6448)' — 1} (3-04763 x IQ-') V " (5-349 X 10-')" ' fj.'{p.' - 1) = 5-9966 X 10* Af-' - 1) Rotation in Vapour. In the Philosophical Magazine for ]\Iarch, 1879, Professors A. Kiindt and "\V. C. Rontgen publish an account of some experiments in which they hava succeeded in obtaining a magnetic rotation of the plane of polarization of light in the saturated vapour of bisulphide of carbon at a temperature of 100° C, They first used a column of vapour rather more than 1 metre long, then one of 2-4 metres. A set of helices, contain- ing 2400 turns of wire, and a battery of sixty-five Bunsen cells * ^Vatt's " Index of Spectra." t Everett, " Units and Physical Constants," p. 72. X By interpolation. Rotation in Gases — Becquerel. 237 were employed. They obtained a distinct rotation, wliich tKey estimate at about 5°- At the time when this paper was written they had not as yet succeeded in obtaining any effect with air, but had seen a rotation in gaseous sulphurous acid at 100° C, and a pressure of twenty atmospheres, and in sulphuretted hydrogen gas at a pressure of twenty atmospheres and ordinary temperature. Rotation in Gases. On March 31, 1879, M. Henri Becquerel* announced that he had measured the amount of rotation obtained with various gases. The apparatus consisted of a copper tube, three metres long, containing the gas and surrounded by six helices each half a metre long, and each containing about 15 kilos, of copper wire three millims. in diameter. The light of a lime-light was used, and by means of reflectors it was caused to travel nine times along the tube,t so that it passed through 27 metres of gas. "With coal gas and yellow light, a double rotation of + 6 ''8 was observed. With the same magnetic force, under the same conditions, bisulphide of carbon gave a rotation of + 513° or 30,780'. The ratio of the rotation of coal gas to that of bisulphide is about + -00023 On multiplying my value of Verdet's constant by this ratio, we find that between two points in coal gas, whose magnetic potentials differ by unity in C.G.S. measure, the green light of thallium would have its plane of polarization twisted 6 X 10-9 of a unit of circular measure. J Some experiments on oxygen showed that its " magnetic rotative power " is negative, or opposite to that of bisulphide of carbon and of coal gas. * " Comptes Eendus,'' torn. Ixxxviii. p. 709. t See vol. ii. p. 207, fig. 239. X The calculation comes out 6-732 x 10'', but M. Becquerel informs me that he does not consider his result trustworthy to more than the first 238 Electro-Optics. Rotation in Gases— Kundt and Rontgen's Experiments. Oa May 13, 1879, a paper by Messrs. Kiindt and Rontgen was read before the Munich. Academy,* in which the authors announce that they have succeeded in measuring the magnetic rotative powers of various gases. The gases were examined at a pressure of 250 atmospheres. They were contained in a copper tube 1'5 metres long, three millims. internal and ten millims. external diameter. The glass ends of the tube were kept in position by a steel screw press at each end. This arrangement was so successful that when the tube was filled with gas at a pressure of 250 atmospheres, no appreciable loss occurred even in twelve hours. It was found that the pressure so strained the glass ends that they became doubly refracting. For this reason plates of tourmaline were used as the polarizer and analyzer, and were placed in the tube inside its ends. One was fixed to the end which was clamped, and the other was rotated by turning the whole tube with the exception of the end to which the analyzing tourmaline was fixed. Six large helices, connected as one, were placed outside the tube. The current was measured by leading it through a small helix, surrounding a short tube containing bisulphide of carbon, and observing the rotation. The actions of the two helices were compared by removing the gas tube and placing in the large helix a long tube containing bisulphide of carbon. The amount of rotation was measured by a mirror and telescope. The compression was commenced by means of an ordinary pump, and completed by forcing glycerine by means of a hydraulic press into a large iron reservoir connected to the gas tube. As no manometer would accurately measure these higli pressures, they were computed by a comparison of the volume of the tube with the volume occupied by a known fraction of the compressed gas after expansion. * " Ueber die electro-magiictisclie Drclning der PoliirizationsebeiiD dcs Liclitcs in dcm Gasen," von A. Kuiidt und W. C. Rontgen. SitziuK'sber. ■i.\\ iliinclien, 1879, ii. " Jbitli. Diys.," p. 1-18. Rotation in Gases — Kilndt and Rontgen. 239 It was found ttat the gas was opaque for several hours after being compressed. This was caused by the fact that the gas was heated by compression, heated the sides of the tube, and caused unequal refractions. To hasten the cooling, the tube was surrounded by a jacket through which flowed a stream of cold water. Results. It was found that — (1) Atmospheric air, oxygen* nitrogen, carbonic oxide, carbonic acid, coal gas, ethi/l, and marsh gas, all turn the plane of polarization in the direction of the magnetizing current — that is, that their rotation is in the same direction as that of water and bisulphide of carbon. (2) That the amount of rotation, under the same circumstances, varies greatly in different gases. (3) That in any gas, under similar circumstances, the rotation is proportional to the density. The following is a general table of the results : — Gas. Pressures uaed. Rotations measured. =5 number of atmo- ephei-es by whicb the gas would have to be compressed to make it act as powerfully as liquid bisulphide of carbon. --\ = ratio of rotation in the pas at 760min.&0°C to that in liquid bisulphide of carbon. CO = rotation in circular measure in diflferent experiments with different current Btrengtbs. of the green thal- lium ray between two points whose magnetic poten- tials differ by unity in the pas at 0° C.& Atmo- spheres. 61' to r 41' 700 mm. Hydrogen 121 to 210 7253 •0001379 ^■20268xlO-» Oxygen 70 to 237 38' to 2° 5' 6793 •0001-174 4-49220x10-9 Air 144 to 227 55' to 1° 53' 6J95 ■0001819 5 •64364x10-9 Carbonic Oxide 172 to 222 1° 61' to 3° 3' 3863 ■0002689 7-89031 X 10-9 Marsh gas 113 to 190 2° 32' to 50 15' 2481 ■0001031 12-28499x10-9 * We see that this result does not agree ■with M. Becquerel's. 240 Electro- Optics. Professor Rontgen, in a letter dated April 28, 1880, has sent me the results (not yet published) of a further investigation of tlie same subject by himself and Prof. Kiiadt. They have compared the action of the gases with the action of a tube of water of equal length. The water tube was compared with bisulphide of carbon. They found the following numbers. S and (n have the same meanings as in the above table. s Oxygen- Nitrogen. ■000127 Air. Hydrogen. Carbonic Oxide. •000109 ■000127 •000132 •000232 a) 3-322x10-9 3-870x10-9 3-870x10-' 4-023x10-' 7-060x10 9 Effect op Eaeth's Magnetism. From a comparison of their results with those of myself and of M. Becquerel, Messrs. Kiindt and Rontgan calculate what would be the rotation produced in atmospheric air by the action of the earth's magnetism. They find that light travelling in a north and south direction would have to pass through 25-3 kilometres, or 158 miles, to be rotated 1°. Rotation in Air caused by Earth's Magnetism — Becqueeel. On Nov. 17, 1879, M. Henri Becquerel announced to the French Academy * that he had succeeded in observing a rotation caused by the action of the earth-'s magnetism on the atmosphere. It is known that the light of the sky is polarized in a plane, sensibly coinciding with one passing through the sun, the observer, and the point of the sky observed, and we will in future call this plane the " plane of the sun." M. Becquerel first discusses the fact which he has discovered, that this coincidence is not exact ; he then goes on to show that, if no disturbing cause interfered, the plane of polarization and the plane of the sun would coincide when the sun is vertical. It is found, however, that there is a small angle between the planes, and this angle is caused by the action of the earth's magnetism on the air. * " Comptes Eendus," torn, kxiix. p. 838. Maxwell's Theory of Magnetic Rotation. Tiie following determinations were made : — 241 1 Poiiits looked at. Apparent zenith distance. Obserred rotation. Maximum probable error. Point on the Southern \ horizon . . ) Point on the Northern \ horizon . . J Point on the South ) magnetic meridian ) 85°-0' 86° 26' (•85°-2n' 0°-22' 0°-24' 0°-42' 0°59' + 5' ±5' + 12' ±15' No rotation could be observed in a region perpendicular to the compass needle. Theory. The explanation of the phenomena of magnetic rotation of polarized light is still exceedingly obscure ; and it is not likely that any complete explanation can be offered until we know a great deal more of the nature both of magnetism and light than we do at present. Prof. Maxwell * has given a provisional expla- nation which is based on the following reasoning : — We know that two uniform circular vibrations of the same amplitude, having the same periodic time, and in the same plane, but revolving in opposite directions, are equivalent to a rectilinear vibration whose direction passes through the points where two particles describing the given circular paths in the given manner would pass each, other. We also know that if we accelerate either vibration, we turn the direction of the equivalent rectilinear vibration in the direction of the added motion. f This acceleration may be pro- duced by a, motion of the medium in which the vibrations take place. Such motion, which must be rotatory, may be either a motion of the medium or of sensible portions of it as a whole, or it may be motions of the molecules of the medium. * "Electricity," 811, vol. ii. p. 402. f I have not given any explanation of the theory of the composition of vibrations, as it would be out of place in a work on Electricity. There is an very clear explanation of it in Mr. Spottiswoode's work on Polarization, before mentioned. To this the student is referred, as it is impossible to understand the relations between electricity and light without a previous knowledge of the elementary phenomena of polarization. 242 Electro-Optics. Common experien'ce shows us that there is no motion of sen- sible portions of the medium ; any rotatory motion, then, which exists must be rotations of the moleculss on their own axes. Such motions, though not able to produce an effect on any substance perceptible by ordinary methods, may possibly be rendered sensible by the delicacy of the mode of investigation afforded us by the use of polarized light. The theory which ascribes magnetic and other effects to the rotatory motion of the molecules is called " the theory of molecular vortices.'" — For an account of it the reader is referred to Maxwell's " Electricity '' (arts. 822-831), vol. ii. pp. 408-417 — ^the theory being far too complex and too purely mathematical to be discussed in this work. Here, how- ever, are two quotations from Prof. Maxwell which I may fairly introduce : — "We have been hitherto obliged to use language which is perhaps too suggestive of the ordinary hypothesis of motion in the undulatory theory. It is easy, however, to state our result in a form free from this hypothesis. " Whatever light is, at each point of space there is something going on, whether displacement, or rotation, or something not yet imagined, but which is certainly of the nature of a ' vector ' or directed quantity, the direction of which is normal to the direction of the ray. This is completely proved by the pheno- mena of interference. " In the case of circularly-polarized light, the magnitude of this vector remains always the same, but its direction rotates round the direction of the ray, so as to complete a revolution in the periodic time of the wave. " The uncertainty which exists as to whether this vector is in the plane of polarization or perpendicular to it, does not extend to our knowledge of the direction in which it rotates in right- handed or in left-handed circularly-polarized light respectively. The direction and the angular velocity of this vector are perfectly known, though the physical nature of the vector and its absolute direction at a given instant are uncertain. " When a ray of circularly-polarized light falls on a medium under the action of magnetic force, its propagation within the medium is affected by the relation of the direction of rotation of the light to the direction of the magnetic force. From this we Maxwell's Theory of Magnetic Rotation. 243 conclude^ by the reasoning of Art. 820 [Maxwell's ' Electricity '] that in the medium^ when vmder the action of magnetic force, some rotatory motion is going on^ the axis of rotation being in the direction of the magnetic forces ; and that the rate of propa- gation of circularly-polarized light, when the direction of its vibratory rotation and the direction of the magnetic rotation of thfi medium are the same, is different from the rate of propagation when these directions are opposite. " The only resemblance which we can trace between a medium through, which circularly-polarized light is propagated, and a medium through which lines of magnetic force pass, is that in both there is a motion of rotation about an axis. But here the resemblance stops, for the rotation in the optical phenomenon is that of the vector which represents the disturbance. " This vector is always perpendicular to the direction of the ray, and rotates about it a known number of times in a second. In the magnetic phenomenon, that which rotates has no pro- perties by which its sides can be distinguished, so that we cannot determine how many times it rotates in a second. " There is nothing, therefore^ in the magnetic phenomenon which corresponds to the wave-length and the wave-propagation in the optical phenomenon. A medium in which a constant magnetic force is acting is not, in consequence of that force, tilled with waves travelling in one direction, as when light is propagated through it. The only resemblance between the op- tical and the magnetic phenomenon is that, at each point of the medium, something exists of the nature of an angular velocity about an axis in the direction of the magnetic force.'' * The second quotation follows a mathematical discussion of the theory of molecular vortices. " I think we have good evidence for the opinion that some phenomenon of rotation is going on in the magnetic field ; that this rotation is performed by a great number of very small por- tions of matter, each rotating on its own axis, this axis being parallel to the direction of the magnetic force, and that the rota- tions of these different vortices are made to depend on one another by means of some kind of mechanism connecting them. " The attempt which I have made to imagine a working model of this mechanism must be taken for no more than it really is — * " Electricity," 821, vol. ii. p. 407. 244 Electro-Optics. a demonstration that mechanism may be imagined capable of producing a connection mechanically equivalent to the actual connection of the parts of the electro-magnetic field. The problem of determining the mechanism required to establish a given species of connection between the motions of the parts of a system always admits of an infinite number of solutions. " Of these, some may be more clumsy or more complex than others, but all must satisfy the conditions of mechanism in gene- ral. The following results of the theory, however, are of higher value : — " (1) Magnetic force is the effect of the centrifugal force of the •vortices. " (2) Electro-magnetic induction of currents is the effect of the forces called into play when the velocity of the vortices is changing. " (3) Electro-motive force arises from the stress on the con- necting mechanism. " (4) Electric displacement arises from the elastic yielding of the connecting mechanism.''-' * * "Electricity," 831, vol. ii. p. 416. CHAPTER XLVII. DU. KERIl's DISCOVERIES, Relation between Statical Electricity and Polarized Light. In the Phil. Mag. for Nov. 1875, Dr. Kerr, of Glasgow, announced a very remarkable discovery. He finds that when glass and certain other dielectrics are subjected to an intense electric strain, they acquire the power of double refraction, and hence convert plane into elliptically polarized light. Pig. 248. In his first experiments a piece of thick plate-glass (fig. 248) had two holes drilled from its edges parallel to its faces, to within 5 inch of one another. Wires inserted in these were connected to an induction coil, and light polarized in a plane at 45° (fig. 249) to a a line of electric strain. b 6, b b' direction of optical vibrations. Ray of light perpendicular to plane of paper. Fig. 249. the line of the wires passed perpendicularly through the glass, and was received in a second Nicol placed so as to extinguish it. 246 Electro-Optics. When the coil is vvorted, sparks pass in the air between the ordinary discharging rods, and the glass is subjected to a stress which increases as the discharging points are drawn further apart. As soon as the points are far enough apart for the stress to be considerable^ the light reappears and cannot be extinguished by any rotation of the Nicol. If the angle between the plane of polarization and the line of electric stress differs from 45°, the effect diminishes and it becomes zero when the anffle is 0° or 90°. In a second paper,* Dr. Kerr announces that he has succeeded in obtaining the effect with the following liquids : — Bisulphide of carbon, benzol, paraffin, and kerosene oils, oil of turpentine and olive oil. Since Dr. Kerr's discovery was first announced, I have, by the use of larger apparatus (fig. 250) than that which then was at Dr. Kerr^s disposal, succeeded in so magnifying the effect as to enable me to project it on a screen by the electric light, so that it was clearly seen by a large audience at the Royal Institution on Feb. 6, 1879. The images of the points were about \\ inches apart, and a patch of white light about 3 inches across appeared on the screen when the coil was worked. f * Phil. Mag., Dee. 1875, p. 446. t When rehearsirg the experiment the day before the lecture, the electro- static stress was accidentally allowed to become strong enough to perforate the glass. Immediately before perforation there occurred some very remark- able effects. First appeared a patch of orange-brown light about 6 or 7 inches diameter This at once resolved itself into a series of four or five irregular concentric rings, dark and orange-brown, the outer one being perhaps 14 inches iu diameter. In about two seconds more these vanished and were succeeded by a huge black cross about 3 feet across, seen on a faintly Inminous ground. The arms Electro-Optic Powers of Liquids — Kerr. 247 In the Phil. Mag. for August and September, 1879, Dr. Kerr describes a later and more extended series of experiments on the same subject. All the dielectrics were liquids, and were contained in a specially constructed cell ; and the electricity was obtained from a common plate machine. Dr. Kerr concludes his papers with the following summary of results : — " (1) When an insulating liquid is traversed by electrostatic force, it exerts a purely birefringent action upon transmitted light. In relation to this action, liquids are divisible into two classes, the positive and the negative. " (2) Positive liquids act as glass extended in adirection parallel to the lines of electric force, or as plates of quartz or other positive uniaxals with axes parallel to the lines of force. Bisulphide of carbon is the best example. " (3) Negative liquids act as glass compressed in a direction parallel to the lines of force, or as plates of Iceland spar or other negative uniaxals with axes parallel to the lines of force. Oil of colza is one of the best examples. " (4) In the following table the positive liquids are arranged as , nearly as possible in the descending order of electro-optic power, the larger and clearer intervals being marked by separating lines. The negative liquids are not so arranged ; but colza and seal oils are certainly among the strongest, and linseed is the weakest. of ttie cross were along the planes of polarization, and therefore (the experiment being arranged according to Dr. Kerr's directions) were at 45° to the line of stress. The glass then gave way, and all the phenomena disappeared except the extreme ends of the cross ; and the discharge through the hole, where the glass had been perforated, was alone seen. The phenomena was seen by Mr. Cottrell, by Mr. Valter (the second assistant), and by myself. A fresh glass plate was at once drilled, in hopes of repeating the experiment in the lecture next day, but, owing to sparks spring- ing round, we did not succeed in perforating the glass, and therefore saw only the faint return of light described by Dr. Kerr. I have since made a great many more experiments, and have destroyed a good many expensive glasses, but in every case perforation has occurred suddenly, instead of gradually, and therefoi-e I have never succeeded in repro- ducing the effects. Proc. Roy. Soc, Feb. 13, 1879. Compare vol. ii. p. 253. 248 5 Electro -Optics. Positive liiquids. Bisulphide of carbon. Negative Liquids. Fixed oils of vegetable origin Colza, Cumol. Sweet almonds. Paraffin oil (sp. gr. -890). Carbon dicbloride. Olive, Poppy-seed, Eape-seed, Xylol. Toluol. Nut, Mustard-seed, Cymol. Benzol. Linseed. Fixed oils of animal origin : — Seal, Amylene. Codliver, Lard, Keatsfoot. Paraffin-oil (sp. gr. -814. Sperm-oil. Terebene. Bromtoluol. Valeric acid ? " The birefringent actions of these twenty-six dielectrics have been observed repeatedly ; they are perfectly regular, and, to sense, perfectly pure. Valerie acid alone is so faint as to be doubtful. " (5) All the negative liquids yet known belong to the class of the fixed oils. Sperm -oil holds an exceptional place, being clearly positive. "(6) The influence of density on electro-optic power is marked and certain in the case of the paraffin oils, increase of density being accompanied by increase of electro-optic power. "' (7) In bisulphide of carbon and several other liquids, electro- optic measurements are manageable through long ranges of potential and optical effect. " (8) Stannic chloride exerts a very strong optical action under electrostatic force ; but the character of the effect is not yet certainly known. " (9) Of the forty or more liquids yet examined in the plate cellj there are none that exhibit any moderate degree of insulatino- power except the twenty-seven now named in (4), (8). This appears to justify the generality of the statement made in (1). " (10) ^^'hen nitrobenzol is traversed by an intense electric current, it exerts a purely birefringent action on transmitted light. The action is similar to that of a positive uniaxal plate with axis parallel to the line of discharge.'^ Fig.l. Hg,2. Eg.3. K.g.4. Fife.5. F%,6. Plate LII. — Rontgen's Repetition of Kerr's Electro - Optic Experiments. 'ftneeiitBnKisJ)ay&Sfinj3iL- Rontgens Electro-Optic Experiments. 249 Peof. Eontgen's Experiments. — Plate LII. On December 31, 1879, Prof. W. C. Eontgen of Geissengave an account * of a repetition of Dr. Kerr^s experiments which lie has made on a large scale. Prof. Eontgen used large Nicols, a powerful lime-light, and a glass cell twelve centims. high, six wide, and three thick, filled with bisulphide of carbon. The electrodes were so arranged that the lines of force were vertical. Magnificent effects were observed. When the electric machine was worked, the light returned so intensely that the eye could not bear to look at it. These beautiful effects were obtained, not only when the planes of the crossed Nicols were at 45° to the vertical, but when they were parallel and perpendicular to it. This at first sight appears not to agree with what Dr. Kerr has stated, namely, that there is no restitution when the planes are parallel or perpendicular to the lines of force, and a maximum when they are at 45° to them. An examination of Plate LII. will, however, show us that this discrepancy is only apparent. Figs. 1, 3, 5, show the effects obtained with different electrodes when the planes of the crossed Nicols were at 45° to the vertical — called " Position I." of the Nicols. Figs. 2, 4, 6, show the corresponding effects when the planes were vertical and horizontal — called " Position II/-' of the Nicols. Fig. 1. — The electrodes consisted of a brass plate at the bottom of the cell, connected to earth, and a brass ball one centim. in diameter above it, connected to the electrical machine. We see that the greater portion of the field is illuminated, the middle most brightly. There are, however, two dark "tails" (Schwanze) which curve downwards from the sides of the ball. The directions of these tails pass through the centre of the ball, and leave its surface at angles of ± 45° with the vertical. When the liquid in the cell is not quite clean, the dust particles arrange themselves along the lines of force and allow the direction of the latter to be seen. The lines of force are * " TJeber die von Herrn Kerr gefundene neue Beziehung zwischen Liclit nnd Elektricitat," xix. Ber. d. Oberh. Gesellsoh. f. Natur- u. Heilk, p. 1. 250 Electro-Optics. vertical at the centre of the field ; but as they start from each portion of the ball at right angles to its surface,* and curve round to the plate, they must be in all directions at different parts near the edge of the field. The dark tails are the " locus " of poinls where the tangent to the lines of force is parallel or perpendicular to the plane of polarization. Fig. 1 is complementary to fig. I. The lines of force and the plane of polarization are parallel or perpendicular to each other at the centre of the field, and consequently there is avertical dark line; at the sides they are at 45°, and therefore those regions are brightly illuminated. In Dr. Kerr's experiments a small field only was used, and, consequently, when the planes of his Nicols were vertical and horizontal, he saw no light, because he was looking at the region occupied by the vertical dark band in fig. 2. Thus Prof. Rontgen's experiments entirely confirm Dr. Kerr's discovery, that the electricity exercises its maximum effect on the light when the line of force and the plane of polarization make an angle of 43°, and that there is no effect whatever when they coincide or are at right angles. Effect of Strained Glass Compensatou. Wlien glass is so strained as to make it act like a crystal of opposite sign to the dielectric, it produces reversed effects, or, if used with the dielectric, it more or less compensates the electro - optic effect according to the amount of strain. When a piece of glass is inserted in the beam of light pro- ducing fig. 1, and is compressed vertically, the tails gradually go together and finally join, and form one band similar to that in fig. 2. If the compression is still further continued, the tail is apparently drawn up to the ball, and at last disappears. When the glass is compressed horizontally, the reverse appearance is produced, the middle of the field becomes brighter, and the sides darker, and the tails bend outward. When the Nicols are in Position II., no effect is produced by compressing the compensator in a vertical or horizontal direction. * For the surface of a conductor is an equipotential surface, and the lines of force are perpendicular to it. Vol. i. pp. 29, 30. Rontgens Electro-Optic Experiments. 251 When it is placed at an angle of 45°, it causes tlie field to become unsymraetrical. Figs. 3 and 4. — The lower electrode consists of a ball one centim. diameter, the upper of a stout brass wire. Fig. 3 and 4 represent respectively the effects observed with the Nieols in Positions I. and II. The dark tails in both eases trace out the " locus " of points where the lines of force are parallel and perpendicular to the planes of polarization. Figs. 5 and 6 are the corresponding pair of appearances seen with electrodes, consisting of stout rectangular brass rods. The dark brushes are again the same " locus." Various other electrodes were tried with similar results. Other Dxelecteics. The effect was also obtained with cod-liver oil. It acted more feebly than bisulphide, and in the opposite direction. Bisulphide of carbon acted like glass extended along the lines of force, cod- liver oil like glass compressed in the same direction. Thus the classification of fluids into positive and negative, similar to positive and negative crystals, is completely established. Turpentine acted like cod-liver oil. Experiments on paetiallt-condt7cting LiquiDs. The effects were obtained with nitrorbenzol, sulphuric ether, glycerine, and distilled water, but only when an air-spark was interposed in one of the wires, and the machine connected to a Ley den jar. A flash of light was then seen in the field corresponding to each spark. This result is especially interesting as showing that there is a momentary state of strain in conductors before it is relieved by the yielding of the material. Exhausted Tube. Prof. Rontgen tried passing the light across a vacuum tube which was so highly exhausted that no discharge could pass. No effect was observed even when a very strong difference of potential was maintained at the terminals. Moving Liquid. Prof. Rontgen found that he was able to partially imitate the 252 Electro-Optics. optical effects of electric strain by driving the fluid in a strong stream through the cell. Different Dielectrics. In comparing different liquids. Prof. Kontgen found, agree- ably to Dr. Kerr's more extended results, that dielectrics under electric strain act on light like uniaxal crystals with the line of electric strain for axis, and that, like crystals, they vary from strong to weak, and from positive to negative.* Bisulphide of carbon acts as a positive crystal. De. Kerb's Electko-optic Law. In March, 1880,t Dr. Kerr published the results of a series of quantitative measurements of the electro-optic effect. His experiments were all made on bisulphide of carbon, but there seems but little doubt that the law established by experiments on it is of general and universal application. The following ]aw was found to bold with absolute accuracy : — The intensity of electro-optic action of a given dielectric — that is, the quantity of optical effect (or the difference of retardations of the ordinary and extraordinary rays) per unit of thickness of the dielectric — varies directly as the square of the resultant electric force. Insteumexts^The Cell. The cell (fig. 251) consisted of a block of plate glass ten inches by six inches, built up of three slabs placed vertically, and having a joint thickness of exactly 3J-| inches. Fig. 251 shows an end view of the block. The inner rectangle represents a tunnel passing right through it. The ends of the tunnel are closed by panes of thin, clear plate glass. Outside the panes are pieces of thick india-rubber cloth, and outside the cloths are stout mahogany planks, longer than the ends of the cell, and connected together by stout screw bolts at each end. There are holes in the planks and cloths, somewhat larger than the ends of the tunnel. * All uniaxal crystals divide the ray of light into two— the "ordinary" and "extraordinary." Whichever ray travels slowest is most refracted. Crystals in which the ordinary ray goes fastest are called positive ; those in which the extraordinary ray goes fastest are called negative. See Spottis- wooJf, "Polarization of Light," p. 92. ■ t Phil. Mag., 1880, i. p. 157. Kerrs Electro-Optic Law — Chromatic Effects. 253 By tightening tlie screw bolts, the cell could be perfectly closed without the use of any cement whatever. Kg. 261. The shaded pieces represent the conductors. The lower one is a wide, thick brass plate resting on the floor of the cellj the upper is a narrower plate supported from glass rafters, to which it is attached by bullet-headed screws. Both plates are the full length of the tunnel, viz. 3-i-|- inches. The dotted lines in fig. 251 represent three borings. The connecting wires to the upper and lower plates pass respectively through the vertical boring, and through the narrow one on the right. The larger boring on the left is used to fill and empty the cell. Chromatic Effects. In the preliminary work with this cell, some very fine chromatic effects were observed. The following is Dr. Kerr's account of them : — " Here, as in all the following experiments, the cell is charged with rather less than a pint of clean bisulphide of carbon. No other optical pieces are employed at present than the charged cell and a couple of Nicol's prisms. " A beam of light from a bright cloud is reflected horizontally into the room through an opening in the window-shutter, passes through the first Nicol, then perpendicularly through the plate of liquid, then through the ocular Nicol. The pieces are so levelled and directed that the observer at the polariscope looks 40 254 Electro-Optics. fairly through the deep slit that separates the two conductors in the cell. I may mention again that the dimensions of the slit are about one-twelfth of an inch^ one inch, and four inches — the first dimension lying vertically as the lines of force, and the last lying horizontally along the line of sight. The two Nicols are fi-xed, the first with its principal section at 45° to the vertical, and the second at extinction, which is here quite pure. Wires are led permanently from the two conductors — from the lower to earth, and from the external knob of the upper to the prime conductor. To give greater steadiness and distinctness to the progress of the optical efiect, the wire from prime conductor to cell is put in permanent contact with the knob of a Leyden jar, whose outer coating is uninsulated. " When the machine is set in motion at a moderate rate, the potential of the upper conductor rises slowly, and the black space between the two conductors is illuminated, the light passing gradually through impure black, faintly bluish grey, faint white, and so forward, up to a sensibly pure and brilliant white. Thus far there is nothing new, except that the highest potential yet reached is comparatively low, while the optical effect is very large, and already far beyond neutralization by the action of any hand compensator of strained glass. " As the potential of the prime conductor still rises, the pola- riscope gives a fine progression of chromatic effects, which descend regularly and continuously through a certain range of Newton's scale. The luminous band between the conductors passes first from white to a bright straw-colour, which deepens gradually to a rich, yellow, then passes through orange to a deep brown, then to a pure and dense red, then to purple and very deep violet, then to a rich and full blue, then to green. All the colours are beautifully dense and pure, certainly as fine as any that I have ever seen in experiments with crystals in the polariscope. " Generally about the point last named of the scale of colours, at or near the green of the second order, the process terminates in spark-discharge through the liquid. Sometimes, but not frequently in my observations, it terminates at an earlier stage, to run its regular course at the next trial. The irregularity appears to be due to an accidental precipitation of discharge by the action of solid particles, impurities in the liquid. Kerr's Electro- Optic Law. 255 "Through this whole range of effect, from the pale blue or impure black of the first order to the green of the second order, the plate of electrically-charged liquid acts regularly as a uniaxal ci-ystal, as a plate of quartz with optic axis parallel to the lines of force, the plate increasing in thickness continuously and very rapidly as the potential rises." Instruments continued — Electeometee. To measure potential, a Thomson long-range electrometer was used. This is a modification of the absolute electrometer described in voL i. page 55.* The Jamin CojiPENSATOE.t To measure the optical effect, a Jamin compensator was used. This consists of two crystals of quartz (fig. 252), cut into two prisms, ABC, DCB. The light passes through in the direction MN. The axis of one of the crystals is in the plane of the paper, that of the other is at right angles to it. One of the prisms is fixed in a tube. The other can. be moved by means of a micrometer screw. Black bands are seen in the field of view. By turning the screw the retardation is altered, and the black bands move. A fine silk thread is stretched across the field of view. We adjust one band on the thread, and note on the scale of the micrometer what number of divisions it is necessary to move the micrometer screw to bring the next band to the thread. The difference of retardation corresponding to the difference of distance between two bands can be calculated mathematically, * See Thomson, " Electro-statics and Magnetism," $ 383, p. 306. T Jamin, " Cours de Physique," torn. iii. pp. 623, 639. 256 Electro- Optics. and we can thus calculate the amount of retardation corresponding to one scale division. The compensator is placed in the path of the ray which has passed through the electro-optic cell, and adjusted so that a black band lies on the thread. On the machine being worked, ihe band is displaced. The compensator screw is then turned till the band returns to its original position. The number of scale divisions through which it is moved give the retardation caused by the electric strain. Simultaneous observations of the electrometer and compen- sator give the retardation caused by different differences of potential. The general arrangement of the apparatus is as in fiff. 253. HL -£ M w N Kg. 253. B is a mirror which reflects the light into the apparatus, M N are the Nicols, P the cell, C the compensator, D a lens, and Q the position of the observer's eye. Calculation. Let V be the difference of potentials as measured by the electrometer^ D the distance between the plates, R the resultant electric force, Q the quantity of optical effect. We then know that for a small field, and for portions of it not near its edges, E. is proportional to V and inversely proportional to D, and we may write, V E = D" or But we have stated that Q is proportional to 11^ Q '^ R" <^ g5* (1) * The sign a meiins " is proportional to." Proof of Kerr" s Electro-optic Law. 257 If the values of Q are calculated by this formulaj the agree- ment or disagreement of the results with the observed values of Q will be a test of the accuracy of the law stated on page 253. The following results were obtained for different potentials, and with different distances between the plates : — D measured . 1 2 2 3 3 4 4 V measured . 60 90 120 90 120 120 150 Q measured . 63 36 64 16 27 15 24 Q calculated . 631 35-5 63-1 15-8 28 15-8 24-6 Further comment is superfluous. "Kerr's Electro-optic Law " is entirely established.* Dr. Kerr concludes his paper as follows : — - "In conclusion, I observe that the principal result of the experiments, what I have called the Law of Squares, may be correctly stated in several very different forms. The quantity of optical effect, per unit of thickness of the dielectric, varies either — " (1) Directly as the square of the resultant electric force, or " (2) Directly as the energy of the electric field per unit of volume, or " (3) Directly as the mutual attraction of the two conductors that limit the field, or " (4) Directly as the electric tension of the dielectric, a quantity that was conceived long ago very clearly by Faraday, and introduced afterwards definitely into the Mathematical Theory of Electricity by Professor Clerk Maxwell. " Faraday's and Clerk Maxwell's views as to the action of the dielectric in the transmission of electro-static force, and as to the state of molecular constraint that is associated with and essential to that action, are very strongly confirmed by the new facts of electro-optics. The dioptric action of an electrically- charged medium is closely related to the electric stress of the medium, the axis of double refraction coinciding in every ease with the line of electric tension, and the intensity of double * An experiment is given to show that the effects are unaltered when the sign ol V is changed and its numerical value remains constant. 258 Electro-Optics. refraction varying, certainly in bisulphide of carbon and pro- bably in all other dielectrics, directly and simply as the intensity of the tension." We cannot imagine a more complete proof than Dr. Kerr's experiments afford that electric induction is a " state of strain" in the dielectric. 259 CHAPTER XLVIII. Dii. Kerr's Discoveries {continued). ROTATION OF THE PLANE OF POLARIZATION OF LIGHT REFLECTED FROM THE POLE OF A MAGNET. Dr. Kerr has also made another most remarkable discovery.* He finds that — " When plane-polarized light is reflected regularly from either pole of an electro-magnet of iron, the plane of polarization is turned through a sensible angle in a direction contrary to the nominal direction of the magnetizing current — so that the true south pole [the north-pointing pole] of polished iron, acting as a reflector, turns the plane of polarization right-handedly.'' The experimental arrangements were as follows : — ■ L (fig. 254) is the source of light. Fig. 254. E the observer's eye. A and B the first and second Nicols. C a wedge of soft iron. The light was reflected from one highly polished pole of a horse-shoe magnet. The first Nicol is so arranged that the plane of polarization is either parallel or perpendicular to the plane of incidence, because * Phil. Mag. May, 1877. 26o Electro-Optics. in any other case tte light becomes eUiptieallj polarized by reflection. It has always been found necessary to employ, what Dr. Kerr calls, a " sub-magnet/' namely, a wedge (C) of soft iron kept by slips of wood at a distance of about -5^ inch from the iron surface, so that there is just room for the light to pass. Dr. Kerr believes that the only action of a sub-magnet is to cause a greater concentration of magnetic force on the iron mirror. This point, however, requires further investigation. In all cases of oblique incidence it was found that the effect on the polariscope was mixed, being partly due to the magnetic force, and partly to metallic reflection. The effect of the latter was to convert the plane-polarized light into light more or less elliptically polarized, and which was therefore not extinguishable by any rotation of the second Xicol. To obtain the pure magnetic effect. Dr. Kerr arranged for the incidence of the light to be normal to the mirror as shown in fig. 255. The sub-magnet C was perforated and the light was re- El x^ — ^^ Fig. 255. fleeted down on to the polished pole F by means of an unsUvered glass mirror M, through which the reflected light passes through the second Xicol to E. TVith this arrangement the rotation due to the magnetic force is seen alone, and is quite distinct. The magnet used consisted of an iron core, two inches diameter and ten inches long, surrounded by about 400 turns of wire. It was worked by six small Grove's cells. The rotations were very small. Nothing, however, could be more conclusive than Dr. Kerr's Light reflected from the Pole of a Alagnet — Kerr. 261 paper. In it every possible doubt which could be east upoa the reality of the phenomenon seems already answered.* Enough is not yet known about the laws of the new phenomena to enable us to discuss their theory. Repetition of the Expeeihents. The present writer has repeated Dr. Kerr's experimentsj using an iron cylinder, 2 feet 4 inches long, and 2j inches diam.eterj on which were placed the two helices belonging to the electro-magnet described in vol. ii. page 13. The Jellett analyzer (vol. ii. p. 229) was used to measure the rotation. The following readings of the plane of polarization were taken : — Correit direct. Corrent reversed. 271 55 271 57 271 30 -71 27 271 54 271 52 Mean double rotation . 271 23 271 2G . 26' 45" I have not as yet been able to get any distinct effect without the sub-magnet. Until this can be done, " absolute " measures of the amount of rotation due to a given strength of pole will not be possible. Light Reflected from the Side of a !Magxet. Dr. Kerr findsf that the plane of polarization of light is also rotated when the light is reflected from the side of a magnet. Pig. 256 represents a plan of his apparatus. A block of iron, AB, is laid on the poles of a horse-shoe magnet ; L is the lamp, P a metal screen with a sUt in it, C the point where reflection takes place, XX' the Xieols, E the obser\'-er's eye. • The phenomenon is certainly not an air-rotation, which nntil now has escaped notice. Xot only do all the experiments negative this hypothesis, bat in a direct observation which I have made I found that a mao-net which will impress a double rotation of 7' on light reflected from its pole, has no effect whatever upon light passed without reflection through perforated poles, though if the effect had been an air-rotation, it should have been in this case four times as much as before, or some 2S'. t Phil. :Mag. 1S73, i., p. 161. 262 Electro-Optic's. It is found that the only positions of the Nicols which would give pure extinction, are those where the principal section of the first is parallel or perpendicular to the plane of incidence. Fig. 268. Dr. Kerr found that, when the magnet was exeited, rotation took place in a direction whose relation to that of the magnetizing current depended on the angle and plane of incidence. First Exjieriment. — Plane of polarization of first Nicol parallel to plane of incidence. The plane of polarization was rotated in the same direction as the magnetizing current. By the " angle of incidence ' is meant \ angle LCE (fig. 356). " The intensity of the optical effects of magnetization varies very noticeably with the angle of incidence. About incidence 85° the effects are very faint, but perfectly regular and much better than merely sensible; about incidence 75° they are more distinct and very sensibly stronger; about incidences 60° and 65° they are comparatively clear and strong, a good deal stronger than at 75°; about incidence 45° they are still pretty strong, but very sensibly fainter than at 60°; about incidence 30° they are again very faint, about the same as at 85°.'''' Second Experiment. — Plane of polarization of first Nicol per- pendicular to the plane of incidence. At about incidence 85° the effects were exactly the same as in the first experiment ; then the effect diminished, and at about 75° it entirely disappeared; after 75° it began to re-appear, but the rotation was in the opposite direction; about incidence 60° the efiPect was comparatively clear and strong, though sensibly fainter than that obtained in the first experiment, at the same incidence; about 30° it is faint, but still distinct and clearly stronger than the contrary effect obtained at 85°. StIMlIAET. Thus, when the plane of polarization is parallel to the plane of Light reflected from the Side of a Magnet — Kerr. 263 iQcidence, the rotation is always in the same direction as the magnetizing current for all incidences. When it is perpendicular^ the rotation is in the same direction as the magnetizing current for incidences between 85° aud 75°, and in the opposite direction for incidences between 75° and 30°. No sub-magnet was used in these experiments. 264 Electro-Optics. CHAPTER XLIX. SELENIUM. The only other direct action between electricity and light which remains to be mentioned is the alteration of the conducting power of selenium with light. Selenium is an exceedingly bad conductor, its resistance being about 3.8 x 10" times that of copper. It is found, however, that, when exposed to light, its resistance alters. Prof. W. G. Adams has found* that the change in the resistance of selenium is directly as ike square root of the illuminating power. On May 18, 1876, Prof. W. G. Adams and Mr. R. E. Day communicated to the Royal Society a paper containing the results of a year's experimenting with selenium. The following extracts from that paper sum up what is now known on the subject : — " It was observed that, with the same piece of selenium at the same temperature, the resistance diminished as the battery power was increased. Also it was found that the electrical resistance of the rod of selenium was different for currents going through it in different directions. Thus, if two plati- num wires be melted into the selenium at two points, A and B, and the resistance of the selenium be balanced by the Wheat- stone's bridge arrangement, the positive pole of the battery being connected to the electrode A, then, on reversing the current so that the negative pole of the battery was now connected to the electrode A, the numerical value of the balancing resistance required was always found to be different from that previously obtained. "If the electrical conductivity of selenium followed the • Proc. Eoy. Soc, vol. xxv., 1876, p. 113. Selenium — Adams and Day. 265 ordinary law of metallic conductioiij this would not be the case; and hence it seemed probable that a careful investigation of these points would lead to some important results. " In the experiments recorded in this paper, the objects which the authors have had specially in view have been : — • '' (1) To examine the character of the electrical conductivity of selenium when kept in the dark. " (2) To determine whether light could actually generate an electric current in the selenium. " Several pieces of selenium were prepared as follows : — A small piece varying from \ inch to 1 inch in length was broken oflF a stick of vitreous selenium. A platinum wire was then taken and bent round into a small ring at one end, and the remainder of the wire turned up at right angles to this ring. The rings of two such wires were then heated in the flame of a spirit lamp, and pressed into the ends of the little cylinder of selenium, thus forming platinum electrodes. The whole was then annealed. "A few preliminary experiments were made to determine whether the change of resistance with change of direction of the current had any connection with the position of the selenium or the direction of the current with regard to the magnetic meridian. No such connection was found to exist. " Prom the results obtained from a great many experiments made to determine the diminution of resistance with increased battery power, and the change of resistance with a change of the direction of the current, the following conclusions were drawn : — " (1) That, on the whole, there is a general diminution of resistance in the selenium as the battery power is increased. " (2) The first current through the selenium, if a strong one, causes a permanent set of the molecules, in consequence of which the passage of the current through the selenium during the remainder of the experiments is more resisted in that direction than it is when passing in the opposite direction. " (3) The passage of the current in any direction produces a set of the molecules which facilitates the subsequent passage of a current in the opposite, but obstructs it in the same direction. Hence, when two currents are sent through successively, after a very small interval, in the same direction the resistance observed 266 Electro-Optics. in the second case even with the higher battery power is often equal to or greater than it was before. " The results of these experiments seem to indicate that the conductivity of selenium is electrolytic. A number of experiments were undertaken in order to discover whether, after the passage of an electric current through a piece of selenium, any distinct evidence of polarization could be detected. It was then found that, afbcr passing the current from a voltaic battery for some time through the selenium, and after having disengaged the electrodes from the battery, and connected them with a galvano- meter, a current, in some cases of considerable intensity, in the opposite direction to that of the original battery current, passed through the galvanometer. This proved that the passage of the battery current sets up polarization in the selenium. "All the results hitherto described were obtained with the selenium kept in the dark." The authors then tried to discover whether, on exposing the selenium to light during the passage of the polarization current, any change in the intensity of that current would be produced. They found that in several cases there was a distinct change; in most instances the action of the light assisted the passage of the current ; but in one case they found that the effect of light was not only to bring the deflection ot the galvanometer down to zero, but also to send it up considerably on the other side. " Here there seemed to be a case of light actually prodrwing an electro-motive force within the selenium, which in this ease was opposed to and could overbalance the electro-motive force due to polarization. " The question at once presented itself as to whether it would • be possible to gtart a current in the selenium merely iy the action of light. Accordingly, the same piece of selenium was connected directly with the galvanometer. While unexposed, there was no action whatever. On exposing the tube to the light of a candle, there was at once a strong deflection of the galvanometer needle. On screening off the light, the deflection at once came back to zero. "This experiment was repeated in various ways and with light from different sources, the results clearly proving that by the action (f light alone lee can start and maintain an electrical current in the selenium. Selenium — Adams and Day. 267 " All the pieces of selenium hitherto used had repeatedly had electrical currents passing through them, and it therefore seemed desirable to examine the effect of exposure to light on pieces of selenium which had never before had an electrical current sent through them. ''Accordingly, three pieces were prepared as nearly alike as possible, and were annealed. Two of them were found on trial to be sensitive to light — that is to say, light impinging on them produced an electrical current. The third piece, however, showed no signs of sensitiveness. Hence it appears that three pieces which were made up from the same stick, which are of the same length, and were annealed at the same time, may, owing to some slight difference in their molecular condition, be very different as to their relative sensitiveness to the action of light. " In the experiments by which the above results were obtained, the piece of selenium under examination had always been exposed as a whole to the influence of the light, so that it was not possible to tell whether any one part of a piece was more sensitive than any other.'' In order to examine into this point more full3^, the authors " used the lime-light, and then, by means of a lens, the light was brought to a focus on the particular portion of the selenium plate which was to be tested. A glass cell containing water, and having parallel sides, was interposed in the path of the beam, so as to assist in absorbing any obscure heat-ra}S. " The result of these experiments proved conclusively the following points : — " (1) That pieces of annealed selenium are in general sensitive to light, i.e.that under the action of light a difference of potential is developed between the molecules which under certain conditions can produce an electric current through the substance, " (2) That the sensitiveness is different at different parts of the same piece. " (3) That in general the direction of the current is from the less towards the more illuminated portion of the selenium, but that, owing to accidental differences in molecular arrangement, this direction is sometimes reversed. " The currents produced in the selenium by the action of light do not resemble the thermo-electric currents due" to beating of the junctions between the platinum electrode and the selenium; 268 Electro-Optics. for in many cases the current produced was most intense when the light was focussed on points of the selenium not coinciding with the junctions ; also the current was produced suddenly on exposure ; and, on shutting oflf the light, the needle at once fell to zero : the gradual action due to gradual cooling was entirely wanting. " When the light fell upon a junction, the current passed from the selenium to the platinum through the junction, which is not in accordance with the place assigned to selenium in the thermo- electric series of metals. "Experiments were next undertaken in order to examine what effect would be produced on the strength of a current which was passing through a piece of selenium in the dark when a beam of light was allowed to fall upon it. " The results obtained from these experiments were as follows : — " With pieces of selenium of low resistance, and with a weak current passing through them, — " (1) When light falls on the end of the selenium at which the current from the positive pole of the battery is entering the metal, it opposes the passage of the current. "(2) When light falls on the end of the selenium at which the current is leaving the metal, it assists the passage of the current. " With jpieces of selenium of a high resistance it was found that in all cases the action of light tended to facilitate the passage of the battery current, whichever was its direction. " It was also found that in those pieces which appeared so little sensitive to light that no independent current was developed in them by exposure, yet, when a current due to an external electro- motive force was passing through them, the exposure to light facilitated the passage of the current. " The results of the experiments described in this paper furnish a possible explanation of the character of the action which takes place when light falls upon a piece of selenium which is in a more or less perfect crystalline condition. "■ When a stick of vitreous selenium has been heated to its point of softening, if it were possible to cool the whole equally and very slowly, then the whole of the molecules throughout its mass would be able to take up their natural crystalline position. Selenmm — Adams and Day. 269 and the whole would then be in a perfectly crystalline state^ and would conduct electricity and heat equally well throughout its mass. But from the nature of the process it is evident that the outer layers will cool the most rapidly, and we shall have, in passing from the outside to the centre, a series of strata in a more and more perfect crystalline condition. " Light, as we know in the case of some bodiesj tends to promote crystallization, and, when it falls on the surface of such a stick of selenium, probably tends to promote crystallization in the exterior layers, and therefore to produce a flow of energy from within outwards, which under certain circumstances appears, in the case of selenium, to produce an electric current. " The ciystallization produced in selenium by light may also account for the diminution in the resistance of the selenium when a current from a battery is passing through it, for, in changing to the crystalline state, selenium becomes a better conductor of electricity." 41 270 Ekdro-Optics. CHAPTER L. CLEEK maxwell's ELECTEO-MAGNETIC THEORY OF LIGHT. Electiuc induction is a strain of some kind ; and, when electric induction passes through space in which there is not any ordinary- matter, we agree to call the unknown something that fills the space and transmits the strain an " ether/' Light is a strain of some kind ; and when light passes through space wtere there is not any ordinary matter, we agree to call the unknown something that fills the space and transmits the strain an " ether." All men of science are agreed that light consists of vibrations of an ether or very thin fluid which fills all space, and probably permeates all bodies. Prof. Clerk Maxwell's theory is briefly this : — Electro-magnetic induction, is propagated through space hy strains or vibrations of the same ether wliich conveys the light vibrations, or, in other words, " light itself is an electro-magnetic dis- turbance." Let us examine the evidence which causes us to believe that the luminiferous and the electro-magnetic ethers are one and the same. The first point of resemblance between the modes of propaga- tion of light and of electro-magnetic induction is that in both cases it can be shown mathematically that the disturbance is at right angles to the direction of propagation. It is known that the waves of light take place in directions at right angles to the ray. Prof. Clerk Maxwell has shown that the directions of both the magnetic and electric disturbances are also at right angles to the line of force.* * They are also at right angles to each other. MaxweWs Electro-magnetic Theory of Light. 271 Fig. 267. Fig. 257 shows Prof. Maxwell's conception of a line of electric force. The vertical line is the direction of the force, and the magnetic ^^ and electric disturbances are at right angles to it. Another argument in favour of the theory is that it gives a real mathema- tical reason for the fact that all good true conductors are exceedingly opaque. A.11 metals, for instance, conduct, and are opaque. The conduction of electri- city by transparent liquids takes place in a different manner from the conduction by metals, and does not aflFeet the deduc- tion, which can be shown mathematically to be a necessary consequence of the theory, namely, that all good true con- ductors must be opaque to light.* But far more important evidence in favour of the view that the ethers are not two, but one, is obtained by comparing the velocities with which optical and electro-magnetic disturbances are propagated under different circumstances. If it can be shown that the velocity of electro-magnetic induction is sensibly the same as that of light, not only in air and vacuum, but in all transparent bodies, we shall be quite sure that there are not two ethers, but one; for it would be unreasonable to suppose that the whole of every part of space is filled with two ethers which are identical in the only properties which we can examine, but which are yet different and not the same. And, further, if the velocities nearly agree, but not quite, we must reserve our judgment ; but we may be allowed to speculate on the possibility of the same ether vibrating somewhat differently when disturbed by electricity and by light. CoMPAuisoN OF Velocities in Aie and Vacuum. The velocity of light has been measured experimentally in many ways. * It must, however, be confessed that gold, silver, and platinum, when made into very thin plates, are not nearly so opaque as they should be according to the theory. 272 Electro- Optics. The most recent experiments are those made by Prof. Cornu,* in 1374, who found that in vacuo — V = S-OOl X 10'° centims. per second. The following are the results of older observations : — Pizeau . . . . 314 X 10'° Astronomical observations . 3'08 „ Foucault .... 2'98 ., Mean ... . 3-06 „ jM. Cornu's experiments are, however, so greatly superior in accuracy to any of the older ones that we shall adopt his value, namely, 3-OOJ.. Now the refractive index of air is 1-000294. The velocity of light in air is then : — ?:521^=S.O031 X 10-° Now the mean value f of the most recent determinations of the ratio of electro-static and electro-magnetic units gives us for the velocity of electro-magnetic induction in air — •B = 2-9857 X 10'° We may therefore say that t/ie velocities in air of ligJd and of electro-magnetic indiiction are sensibly equal. Velocities in otheu Media. The velocity of light in any medium of refractive index /u, is — velocity in air Prof. Clerk Maxwell has proved mathematically that the velocity of electro-magnetic induction in any medium is — velocity in air where K is the specific inductive capacity for electro-static induction as defined in vol. i., page 69. Now, if the velocity of light is equal to that of electro- magnetic induction in all transparent insulators, we should have, * " Annales de I'Observatoire de Paris," 1876. " Memoires," torn. xiii. p. Ai. t Vol. ii. p. 202. Maxwell's Electro-magnetic Theory of Light. 273 velocity of li^lit in air velocity of electro-magnetic induction in air. But we have shown that the velocities in air are equal, and hence, if the other velocities are equal, we must have — M = \/K. Wc must note that Prof. Maxwell shows that among the values of /a we must select that which corresponds to waves of infinite wave length.* Gordon's ExPEaiMiiNTS. The following table compares the values of /i and VK for various dielectrics as determined by the present writer (see vol. i , p. 118):- Dielectric. n/K. /i» = «. /iD. Mh,. X,.--vs Double extra-dense flint glass 1-778 1-672 1-710 1-757 3-527, the wave length for N i n ultra violet. Extra-dense flint glash^ 1-747 1-620 1-650 1-688 2-862 Light flint glass 1-734 1-555 1-574 1-601 Hard crown glass 1763 1-504 1-517 1-583 Parafl[]n 1-4119 1 -42201 (1-9 Sulphur 1-606 Bisulphide of carbon . 1-345 1-611 Common plate glass . 1-801 •• 1-543 * To determine the refractive index for waves of infinite length, we proceed as follows :— We have the general equation ^ = A + B (1) To determine A, a determination of the values of /x for two rays of different wave lengths X and X' are necessary and sufficient, for we have fiX= = AX' + B ^i'X"' = AX"'+ B Subtracting one equation from the other we eliminate B and obtain A = ii X^-m'X™ X" — X"^ But from (1) when X ^ oo , /:i r= A. Hence (3) (3) V- — X'= —Phil. Trans., 1879, Part I, p. 441. t G-Iadstone and Clerk Maxwell ; Maxwell's ." Electricity," J 789, vol. ii. p. 389. The melting point of my paraffin was 68° C, that of Dr. Gladstone's was less than 57° C. 2 74 Electro- Optics. The value of /it is given for the rays Hi, D, and for rays of infinite wave length. The last column shows for what wave length the refractive index would equal the square root of the specific inductive capacity. Gibson and Bakclay's Experiments. Messrs. Gibson and Barclay found for paraffin (see vol. i., p. 86) ^K = 1-405 which does not differ much from the value of ^u, given in the preceding table. Boltzmann's Expeuiments. We can either compare n/K with /x or K with /i^ In the comparison given by Prof. Boltzmann, the latter plan is adopted. The following tahle^ comparing the values of K and p}, is given by Prof. Boltzmann in his paper quoted in vol. i., page 87. K T)ipippf-iM/i \^^ From condenser method. From attraction method. 1 Sulphur 3-84 3-90 1 4-06 1 Paraffin 2-32 f 2-30 7 l2-34i 283 Eesin . 2-55 2 48 2-38 CUYSTALLINB SuLPHUE. In the paper quoted in vol. i., page 100, Prof. Boltzmann gives the following comparison of K and /tt' along the three axes g, m, k of crystalline sulphur. Dielectric. K M= Sulphur . . -< OT 4-773 3-970 3 811 4-596 3-886 3-591 MaxwelV s Electro-magneiic Theory of Light. 275 Schillek's Experiments. In the paper quoted in vol. i., page 103, Schiller gives the following comparison : — Dielectric. K A'" Bj Blow method. By oscillation method. Paraffin, slow cooled, white 2-47 1-89 1-81 . 2-34 2-19* 1 Quickly cooled, nearly transparent 1-92 — Brown India Rubber 2-34 2-12 2-25 SiLOw's Experiments. In the paper quoted in vol. i., page lOi, Silovv finds for oil of turpentine — •v/K = 1-490. ni_.o = 1-459. Boltzmann's Comparison for Gases. In the paper quoted in vol. i., page 123, Prof. Boltzinann gives the following comparison for gases. The refractive index ' and the specific inductive capacity of vacuum are taken as unity. Dielectric — Gases at 0°0. and 760 mm. VE /« Air . Carbonic acid . Hydrogeu Carbonic oxide . Nitrous gas (N.O.) defiant gas Marsb gas 1-000295 1-000473 1-000132 1-000345 1-000497 1-000656 1-000472 1-000294 1-000449 1-000138 1-000340 1-000503 1-000678 1-000443 * There is some confusion as to the arrangement of the numbers for pMiaffin in the table given in Schiller's paper. 276 Electro-Optics. General Conclusion. An examination of the foregoing tables shows us that in some cases the velocities of light and of electro-magnetic induction are very nearly equal, but that in other cases there is a very wide difference. On the whole a sufficiently close agreement has been observed to give us fair hope that some day the discrepancies may be explained and eliminated; and meanwhile the close agreement of the velocities of light and electro-magnetic induction in air and in gases, and the numerous direct relations which exist between light and electricity leave us but little doubt that they are very closely related, and that their effects are but two forms of that common energy whose nature is unknown, but which certainly underlies all physical phenomena. ABS.— 277 —BAR. INDEX. A. Absoiute electrometers, i. 55. instruments, i. 48. measurement, theory of, i. 47. units, i. 48. Acceleration, unit of, i. 51. C.G.S. unit of, i. 52. Adams and Day on effect of light on selenium, ii. 264. Adams, Professor "W. G., on equipotential lines and surfaces and lines of flow, ii. 31. Adsiger, Peter, first observed declination, i. 162. Alternate currents, ii. 150. Amalgamated zinc, i. 206. Anion, ii. 131. Anode, ii. 131. Arrangement of cells, i. 268. Artificial strise, ii. 99. Astatic galvanometer, i. 237. needle, i. 236. Astronomical meridian, determination of, i. 177. Attraction of light bodies, i. 3. explained, i. 7. Attractions and repulsions of electrified bodies, i. 2. Aurora coincides with certain magnetic disturbances, i. 197. Axial line, ii. 14. Ayrton and Perry on contact electricity, ii. 168. on electrolytic polarization, ii. 136. on specific inductive capacity of gases, i. 130. and Perry's balistic galvanometer, i. 240. measurement of the ratio of units, ii. 198. B. Balance, the induction ; see Induction Balance. Balistic galvanometer, i. 240. Barclay and G-ibson, comparison of /li and v'K] ii. 274. on specific inductive capacity, i. 83. Barrow's circle arranged for inclination, i. 180. arranged for total force, i. 183. 278 Index. BIT. — Batteries, Mance s method of determining the resistance of, i. 267. of several cells, i. 221. Battery, Sraee's (&c.) ; see Smee's (&c.) cell. voltaic, i. 203. • theories of, i. 204 B. A. unit, i. 245, 286. Becquerel, H., on effect of terrestrial magnetism on polarized light in air, ii. 240. on index of refractive and magnetic-rotative power, ii. 233. on magnetic rotative power, ii. 233. on rotation of different rays, ii. 236. on terrestrial magnetism and light, ii. 233. Bell's telephone, i. 279. Bergmann on pyro-electricity, ii. 164. Bichromate of potash cell, the one-fluid, i. 207. two-fluid, i. 216. Bifilar suspension, i. 42. Binding screws, i. 206. Bismuth, magnetic strength of, ii. 17. Blaserna's experiments on induced currents, i. 311. Boltzmann, comparison of /*' and K, ii. 274. „ „ fi and a/K in gases, ii. 275. on specific inductive capacity attraction method, i. 90. „ „ „ condenser method, i. 87. on specific inductive capacity of gases, i. 123. theory of his attraction method, i. 135. Boroughs on the variation of the compass, i. 162. Bleaks for induction coil ; see Contact-breakers. Bridge, "Wheatstone's ; see Wheatstone's hridge. British Association unit of resistance, i. 286. Brown, T. A., on variations of the daily mean horizontal magnetic force, i. 1 97. Bunsen's cell, i. 210. Byrne's cell, i. 219. Capacity, C.G.S. unit of electrostatic, i. 68. electro-magnetic measure of, i. 245. electrostatic, i. 67. practical electro-magnetic unit of, i. 246, Carbonic acid vacua, ii. 68. Cathode, ii. 131. Cation, iL 131. Cavendish on specific inductive capacity, i. 71. Cavendish's electrometer, i. 31. — OON. Index. 279 Cavendish's law of electric force, i. 19. Cavities in paraffin, correction for in determining specific inductive capacity, i. 119. Cell, simple voltaic, i. 2U5. Smee's, (&c.) ; see Smee's (itc.) cell. Centimetre, i. 49. C.G.S. applied to electro-statics, i. 53. Changes in earth's magnetism — secular ; see Secular Changes. periodic (&c.); see Periodic (&c.) Chang3s. secular, in specific inductive capacity of glass, i. 120. Charge and potential, i. 30. Chemical efiects of the electric current, i. 223, ii. 131. Ctloride of silver cell, De La Rue's, i. 216. Christie, inventor of Wheatstone's bridge, i. 247. Circle, Barrow's ; see Barrow's Circle, i. 180. Fox ; see Fox Circle, i. 184. Circular current, action of, on a compass needle at its centre, i. 232. Clausius on electrolysis, ii. 132. Cleavage planes, effect on polarity of crystals, ii. 27. Clock contact-lDreaker for induction coil, ii. 47. Closing, induced current of, i. 314. extra current of, i. 318. Coil, the induction, ii. 42. discharge of, ii. 51. Coils, resistance ; see Resistance Coils, i. 252. sliding; see Sliding Coils, i. 257. Commutators, ping, i. 226. rapid, i. 229. roller, i. 227. spring, i. 228. Comparison of coils, ii. 2. of dimensions of units, ii. 188. experimental of units, ii. 191. of units ; summary, ii. 202. of ji and ^/K7 ii. 273. Compass needle, pull on, i. 156. Compensating resistance for shnnts, i. 265. Compression, effect of on polarity, ii. 28. Concentric spheres, capacity of two, i. 68. Conclusion, ii. 276. Condensers, i. 66. Condenser, Gibson and Barclay's sliding, i. 85. primary for induction coil, ii. 44. secondary „ „ ii. 63. Conductors, and insulators, i. 4. opaque, ii. 271. 28o Index. con. — Constant batteries, i. 207. Constants of a helix, determination of, ii. 6. Contact breakers for induction coil, ii. 44. Contact electricity, ii. 165. Conventional sign for battery, i. 223. Oornu, velocity of light, ii. 272. Coulomb's law of electric force, i. 18. ■ torsion balance, i. 33. Couple, i. 149. Crookes on radiant matter, ii. 112. Crystalline sulphur, comparison of y?- and K in, ii. 274. specific inductive capacity of, i. 100. Camming on reversal of thermo-electric current, ii. 161. Current and equivalent magnetic shell, i. 304. electric, defined, i. 203. electro-magnetic, measure of, i. 243. electro-static „ „ i. 242. in helix, calculation of the strength of, ii. 11. practical unit of, i. 245. Currents, action of, on magnets, i. 225, mutual action of two, i. 276. produced on closing and opening the circuit, i. 310. D. Daniell's cell, i. 210. Daily change in magnetic force, i. 196. Day and Adams on effect of light on selenium, ii. 264. Decennial period in changes of declination, i. 195. Declination, i. ]61. determination of, by unifilar magnetometer, i. 176. early observations of, i. 162. periodic changes in, i. 195. secular changes in, i. 193. De La Eue and MUUer on contact electricity, ii. 167. on discharge in different gases, ii. 64. — on strise, ii. 81. — on " striking distance," ii. 62. ■ and Spottiswoode on strise, ii. 69. De La Rue's battery, i. 216. De Meritens machine, ii. 153. Density, C.G.S. unit of, i. 50. electric, i. 25. Dial-pattern resistance box, i. 256. Diamagnetic bodies defined, ii. 14. list of, ii. 10. Dtamagnetism, Faraday's discovery of, ii. 15. Diamagnetic liquids, polai-ity of, ii. 26. — ELB. Index. 281 Diamagnetic polaritj', ii. 17. Dielectric, definition of, i. 70. Difference of potential (electric), i. 26. Differential galvanometer, i. 241. interruptor, Blaserna'.