By
P h . D . aided by
Else Holm Ph.D. both of St. Marys (Pa.)
With 194 Fignres
T h i r d c o m p J e t e l y r e w r i t t e n e d i t i o n of "Die technische Physik der elektrischen K o n t a k t e " by R . Holm
S p r i n g e r - V e r l a g
Berlin / Göttingen / Heidelberg
1958

Contents
Page
List of the most frequently used Symbols and abbreviations X V I
P a r t i
Stationary Contacts
§ 1. Introduction. A simplified summary of t h e theory of stationary electric contacts 1
§ 2. The contact surface 8
§ 3. The contact resistanoe. General theory 10
§ 4. Calculation of constriction resistances with constant resistivity 13
Problem A. 1 4 . - P r o b l e m B . 15. —Problem C. Elliptic a-spot. 1 5 . - P r o blem D. Circular a-spot. 16. - Problem E. 18. - Problem F . The influence of t h e elliptic shape of t h e contact area on the constriction resistance expressed b y a shape factor. 18.
§ 5. Constriction resistances when conditions deviate from those in § 4, b u t with Q still being a constant 20
Problem A. Spreading resistance. 20. — Problem B . Constriction resistance when t h e a-spot is covered with a film. 2 1 . — Problem C.
Metallic contact with m a n y a-spots. 22. — Problem D . The constriction resistance R(n,-a, l) of a coherent contact area with n insulating spots.
24. — Problem E. Distorted constriction. 24.
§ 6. Thermal constriction resistance 25
§ 7. Films on contacts 27
A. Different types of films. 27. - B. Aging of contacts. 31. - C. Tunnel resistance of contact films. 32.
§ 8. The contact surface as a function of load a n d elastic as well as plastio properties of t h e members 33
A. Elastic deformation. 33. - B. Plastic deformation. 34. - C. Influence of temperature and contact duration on t h e contact area. 37.
§ 9. The relation between contact load and resistance, particularly at moderate and high load 39
A. Introduction with description of Fig. (9.01). 39. - B . Crossed rod contacts. 42. - C. Explanation of t h e dashed lines in F i g . (9.01).
44. — D. Diversified measurements. 45. — E . Use of diagram (9.01) in practice. 46. - F . Contact preloaded with a high P . 47.
§ 10. Contact resistance on freshly cleaned contacts a t very small contact loads 48
§ 1 1 . The inductance of a current constriction. Skin effect 52
A. Inductance. 52. - B . The skin effect. 54.
§ 12. Electrodynamic repulsion in a Symmetrie contact of a non-magnetic material 55

Contents I X
§ 13. The capaoitance of a contact. Electrostatic attraction in a contact 56
Example A. Crossed rod contacts. 57. - E x a m p l e B. Quasiflat contaets.
58. — Example C. J O H N S O N - R A H B E K effect. 58.
§ 14. Measurement of t h e load bearing contact area 59
§ 15. The relationship between electric potential and temperature in a current constriotion which is Symmetrie with respect t o t h e contact surface; t h a t is, t h e rp—# relation 55
§ 16. The <p—# relation in cases of dissymmetry 70
Case A. Dissymmetry in regions of t h e constriotion which are distant from t h e contact. 70. - Case B . Contact between highly different materials.
71. — Case C. Contact between moderately different metals. 72. -
Case D. H e a t enters across A
0
. 72. - Case E. THOMSON effect is present.
73. — Case F . The q>—& relation in t h e environment of a bimetallic contact. 75.
§ 17. K O H L E S effect 75
§ 18. The influence of t h e J O U L E h e a t on constriotion resistance 78
Example A. WIEDEMAMT-FRANZ-LORENZ law is valid. 80. - Example B .
Semi-conducting material. 82. - Example C. H e a t flowing across A„.
82. — Example D. I n d e p e n d a n t of whether W I E D E M A N N - F R A N Z - L O R E N Z law is valid. 84.
§ 19. Distribution of t h e temperature in a Symmetrie constriotion with ciroular contact surface a t given current §5
Example A. Q and h are constant. 85. - E x a m p l e B . Corresponding t o
Example A in § 18. 86. - Example C. Corresponding to E x a m p l e B in § 18. 86.
§ 20. Temperature distribution in t h e constriotion of a contact with circular contact surface and members with very different conduotivities 87
Example A. 88. - Example B . 89.
§ 2 1 . Resistance — voltage oharaoteristics of clean Symmetrie contacts. Softening a n d melting voltages 90
§ 22. Development of t h e temperature in a current constriotion 95
A. Introduetion. 95. — B. Remarks ooncerning t h e diagrams. 98. -
C. Moving contact. 101. - D. Temperature development in a cylinder.
103. — E . Cooling of a previously heated contact region. 104. -
F . Examples. 104.
§ 23. The growth of tarnish Ulms on metals 105
A. Fundamentals of t h e theory. 105. - B. Passivating films. HO. -
C. Tarnishing of various base contact materials. HO. - D. Tarnishing of noble metals. 115.
§ 24. W a t e r films, local cells and rusting 116
A. Thickness of water films. 116. - B . Rusting by means of electrochemical attaek. Local cells. 117.
§ 25. Thermoelectric effects 118
§ 26. Observations on t h e tunnel effect 121
A. Introduetion. Method. 121. - B. Observations giving a. 125. -
C. Measured a compared with t h e theoretical value. 127. - D. Super conduetivity of contacts. 128.

X Contents
§ 27. Fritting of tarnish films 130
A. Introduction. 130. — B . A-fritting. General appearance of t h e process. 131. — C. Cessation voltage of fritting. 132 — D . The l'rit bridges.
137. - E . B-fritting. 139. - F . The eoherer. 141. - G. Defritting. 141. -
H . Initial stage of fritting. 141. — I. Recent investigations of fritting.
142.
§ 28. 2?i/-characteristics of contacts with t h i n alien films 146
§ 29. Adherence in dry contacts which are not heated t o any influential extent by t h e current 150
§ 30. Adherence in contacts t h a t are heated by t h e current passing through t h e m . Besistance welding 154
§ 3 1 . About stationary contacts in practice 158
A. General survey of t h e role t h a t contact films play in praetical contacts.
159. — B . Measurements in F . L. on contacts m a d e without impact or sliding. 162. — C. Clamped contacts. Screw. 165.
§ 32. Dimensioning a contact with respect t o its heating 169
§ 33. Contact effects in carbon microphones 172
§ 34. Contact noise in a stationary contact 183
§ 35. Contact with semiconductors. Rectification. Transistors. Static electrification 188
A. Introduction. 188. —B. Contact b e t w e e n a m e t a l a n d asemiconductor and its rectifying property. 188. — C. The p—n-junction. 190. —
D. Types of semiconducting rectifiers. 191. — E . Remarks about transistors. 194. — F . R e m a r k concerning t h e contact between Silicon carbide crystals. 195. — G. Static electrification. 195.
§ 36. Carbon-pile rheostats. Electric resistance of pressed powders 196
P a r t I I
Sliding Contacts j 37. Survey concerning friction a n d wear 199
A. Introduction. 199. — B . The friction work resulting from plastic deformation. 200. — C. The adherence term of t h e friction. 201. —.
D. T h e increase of t h e contact area during sliding. 203. — E . Coulombs law of friction. 204. — F . About t h e characteristic value of /i sa 1 on freshly cleaned metal contacts in air. 205. — G. Friction in bimetallic cont a c t s . 206. — H . Friction between non-conducting members. 207. —
I. Devices for t h e investigation of friction. 207. — J . Rolling friction.
207. — K . The influence of adhesion wear on friction. 209. — L. The work of t h e adhesion friction. 209. i 38. Early observations on t h e high friction in clean metallic contacts in vacuum, a n d t h e influence of admitted gases 210 i 39. Boundary lubrication 212
A. Features of b o u n d a r y lubrication. 212. — B . Discussion of two competing theories of b o u n d a r y lubrication. 216. — C. Lubricating practice. 221. — D. Beilby layer. 221. - E. Properties required of bearing materials. 222. — F . Ball bearings. 224.

Contents X I
§ 40. Theory of friotion a n d wear on carbon contacts. Lubrication by means of solid lubricants as graphite a n d molybdenum disulphide 224
A. Introduction. 224. — B . Friction of carbon brushes as dependent on t h e orientation of graphite basal planes. 226. - C. High altitude effect on brush wear. 227. - D . Adjuvants. 229. - E . Graphite and molybden u m sulfide powder as lubricant. 229.
§ 4 1 . Measurements on specific friction force 231
§ 42. Stick-slip motion. The temperature in currentless slidmg contacts 234
A. Stick-slip or jerky motion. 234. - B . T h e t e m p e r a t u r e in currentless sliding contacts. 235.
§ 43. Statistical study of t h e electric conduction and t h e friction of sliding eontacts. Radio-noise in sliding contacts 237
§ 44. Friction wear in metallic contacts without current 242
A. Introduction. 242. - B . Size and frequency of wear fragments appearing during periods of adhesive wear. 243. — C. Details of t h e formation of wear detritus. 244. - D. W h y liquids, even t h e deposit from air humidity, are able t o influence wear without greatly affecting t h e friction coefficient. 245. - E. The work necessary t o break off a wear fragment. 245. — F . Formula for Classification of types of friction wear. 246. — G. Friction wear in currentless sliding contacts represented b y 2 • 10
5
Z, which is calculated employing t h e hardness, H, of t h e softer member. 248. - H . Wear in sliding contacts of measuring a p p a r a t u s .
253.
§ 45. Electrical Performance of carbon brushes on rings a n d commutators when arcing is excluded 254
A. Introduction. 254. — B . Early investigations on t h e d a r k collector film. 255. - C. Chemical analysis of t h e collector film. 259. — D . Structure of t h e film. 259. - E . Oscillographic investigations of frittings of t h e collector film. 259. — F . More about t h e film generation particularly in t h e " s t a t i c " state. 262. - G. Two brushes in t h e same track. 264.
§ 46. The t e m p e r a t u r e in a contact between a carbon brush and a copper ring or commutator ^"5
A. Introduction. 265. - B . Supertemperature of t h e contact surface above t h e t e m p e r a t u r e of t h e bulk of t h e ring. 266. - C. The tempera t u r e in t h e hottest section of t h e brush. 267.
§ 47. W e a r and friction in t h e brush-ring contact 269
A. Introduction. 269. - B . Influence of t h e current on t h e wear in absence of arcs. 269. - C. Numerical d a t a of brush qualities. 272. -
D . Abrasion of t h e slip ring. 273. - E . Wear in t h e brush-commutator contact in case of arcing. 274. - F . Friction between an electrographite brush a n d copper ring. 275.
§ 48. Commutation problems
2 7 6
A. Introduction. 276. - B . Computation of cardinal conditions for good commutation. 279. - C. Arcing. 281. - D. Numerical example referring t o a d-c generator with 2 poles for about 50 a m p . 283. —
E . Brush contacting several segments simultaneously. 284. — F . Socalled short-circuit currents in t h e brush. 285. - G. The importance of t h e elasticity of t h e brush for t h e commutation. 287. - H . The appropriate value of the. emf V t o be induced in an a r m a t u r e winding by t h e field of t h e interpoles. 288.
§ 49. Current collectors for trolley cars 288

X I I Contents
P a r t I I I
Electric Phenomena in Switchini* Contacts
§ 50. Definitions and high power breakers 290
A. Introduction. 290. — B . Breaking of a-o 290. — C. Vacuum Switches.
292. — D. Direct current switohes. 292. — E . Quenching circuits. 293. —
F . Material transfer a t switohing. 293.
§ 51. Ignition of ares in Switches 293
A. Introduction. 293. — B . Electric breakdown in t h e gap between clean metallic electrodes a t atmospheric pressure. 293. — C. Ignition of an arc in a closing contact. 295. — D. Drawn arcs. Floating. 297. —
E . Reignition of t h e arc. 299.
§ 52. Discharge transients 302
§ 53. Vl-characteristics of t h e stationary arc in air, and their use for calculating t h e duration of short arcs 304
A. Introduction. 304. — B . Observations on breaking contacts. 304.
— C. The use of Vl-characteristics for determination of arc durations.
310. — D. Simplified Vl-characteristics in normal atmosphere. 311. —
E. Determination of I m
a n d V m
by aid of one oscillogram a n d Diagr. X L
314. — F . Use of t h e resistance line together with arc-characteristics for t h e problem of how a constant current is shared between an arc and an ohmic resistance. both in parallel. 314. — G. Survey of t h e method of applying t h e resistance line together with arc characteristics. Condition of stability. 315. — H . Semiconducting resistors parallel t o t h e arc. 315. -
I. Vacuum arc. 316.
§ 54. Electric Oscillations generated by d—c arcs 316
§ 55. Bouncing 31g
§ 56. Mechanical erosion a n d tarnishing phenomena t h a t are typical for sliding and switching contacts 321
A. Introduction. 321. — B . Mechanical material transfer. 322. — C. Catalytic effects in switching contacts. 323. — D. Frictional oxidation.
323. - E. Oxidation in t h e arc. 324.
§ 57. Methods to suppress or minimize arcing during switching 326
A. Introduction. 326. — B . Quenching b y means of a resistor. 326. —
C. Capacitive quenching. 327. — D. Arc quenching in t h e contact rectifier. 328. - E . Weakening of t h e arc between barriers, magnetic blowout. 329. - F . Motion of an arc in a magnetic field. 330.
§ 58. Arc duration in contact making with voltage below 200 t o 300 volts . . . 331
A. Introduction. 331. — B . Calculations with respect t o t h e wiring
Diagram (58.02) with initially charged capacity. 331. - C. Inductance
1= 0, a n d consequently ß= 0 in t h e circuit (58.02). 334. - D. Floating. 335. - E . Empirical formula for t a
, t h e life of t h e arc, in t h e circuit (58.02). 336. - F . B a t t e r y instead of C, equivalent t o C = oo. 337.
§ 59. Arc duration on breaking contact. Single circuit 338
A. Ohmic circuit according t o wiring diagram of Fig. (59.01). Operation in air. 338. — B. Ohmic circuit. Operation in a vacuum. 338. —
C. Inductive circuit according t o wiring diagram in Fig. (59.04). Capacity of t h e leads neglected. Operation in air. 338. — D. The q u a n t i t y of electricity, q, t h a t flows through a drawn arc with t h e life time t a
.
341. — E. Influence of t h e capacity of t h e leads. 342.

Contents X I I I
§ 60. Are duration and other phenomena in an arc quenching cireuit aecording t o wiring Diagram (60.01) 342
A. General equations for t h e quenching cireuit when quenching an arc on breaking contact: r in position a. 343. — B . Case of V'„ ss constant.
343. — C. V a
sa V m
and arc current /„ considerably greater t h a n I m
.
344. — D. F„ differiug slightly from V m
, a n d t h e short cireuit current,
I k
, lying in t h e ränge between I m
and a few amp. 345. — E . Note concerning t h e position of r. 347. - F . Empirical formula for t h e calculation of t h e arc life at constant opening velocity of t h e cireuit of Fig. (60.01) under such conditions t h a t t h e arc generates oscillations. 347. — G. (Kondition for no breakdownof t h e gap between t h e separatingelectrodes. 349.—
H . Calculation of t h e voltage F s
between t h e electrodes as funetion of t h e time, t, in ease of no arc. 350.
§ 6 1 . Quenching of arcs b y resistance parallel t o t h e operating contact or parallel t o t h e induetive coil 352
A. Quenching with r in positon a. 352. - B . Arc quenching with r in position b. 354.
§ 62. Distinct types of arcs in relay contacts 354
A. Introduction. 354. - B . The power balance in t h e short arc determines t h e prevalence of t h e evaporation from the anode. 355. — C. The plasma arc. 356. - D . Remarks. 357.
§ 63. Material transfer in switching contacts 358
A. Definition of t h e major types of material transfer. 358. — B. Material transfer at contact opening. 359. - C. Material transfer at contact make. 363.
§ 64. Measurement of t h e material transfer in switching contacts, particularly with normal electrodes 363
A. Introduction with definition of electrode types a n d Symbols. 363. —
B. Determination of t h e a m o u n t of material transfer t h a t is produced b y plasma arcs between normal electrodes. 364. — C. Method t o determine cu c
and m a
in general. 365. — D. Measurements of bridge transfer in opening contacts. 366. — E . Determination of t h e coefficient y t
, which aecording t o E q . (63.10) characterizes t h e material transfer in short arcs between normal electrodes. 368. - F . T h e final length, s m
, of t h e short arc.
370. — G. Abnormal, short arcs with zero or a small material transfer from t h e anode. 371. — H . The amount of material transfer during fioating. 372. — I. The disintegration of t h e cathode in glow discharges.
373 — J . Material transfer a t contact closure without bouncing. 374.
§ 65. Bridge material transfer in t h e shape of pips and spires 375
§ 66. Theory of t h e eleotric material transfer in switching contacts. History of this theory 376
A. History. 376. — B . Explanation of t h e material transfer caused b y t h e arc. 378. - C. Present theory of t h e bridge transfer. 379 - D. THOMSON
. effect. 379. - E . P E L T I E R effect. 383. - F . K O H L E B effect. 384. -
G. Comparison between calculated a n d observed magnitudes of t h e bridge transfer. 388.
§ 67. Numerical example on t h e calculation of material transfer for a silver contact with capacitive arc quench 387
§ 68. Mercury Switches 388

X I V Contents
§ 69. Application of statistics t o surety of contact make 390
A. Introduction. 390. — B . Testing surety of contact make with Single macro contacts. 391. — C. Effect of twin contacts. 393.
§ 70. The choice of contact material and contact shape for practical applications 394
A. P e r m a n e n t contacts. 394. — B . Micro contacts. 394. — 0 . High repetitive Operation relay contacts. 395. — D . Light d u t y relays for medium frequencies. 396. — E . Medium d u t y circuit breakers a n d contactors. 396. — F . H e a v y d u t y circuit breakers with u p t o thousands of amperes a n d volts. 396. — G. Sliding contacts for resistors and apparatus. 397. - H . R e m a r k about non-welding in carbon and wolframcarbide contacts. 397.
P a r t I V
History
§ 71. History of early investigations on contacts 398
A. Contact resistance. 398. — B . Microphone and coherer. 401. — C. Current constriction. 405.
Appendices j I. Hardness, strain hardening, atomic diffusion phenomena as recovery a n d creep 407
A. Survey of t h e theory of plastic deformation of solid bodies a n d of diffusion phenomena. 407. — B . Hardness as defined b y t h e ball indent a t i o n test. 410. — C. Brittle materials. 413. — D. The work consumed by a plastic deformation. 414. j I I . Electronic conduction in solids 414
A. Energy b a n d scheme. 414. — B . Distribution of t h e electrons on t h e energy levels of a band, with special reference t o t h e conduction band of a metal. 416. — C. Potential barrier. Thermionic emission of electrons.
419. — D. Addenda about rj, n- a n d p-conduction. 420. — E . Semiconduction. 421. — P . Potential barriers and equilibrium conditions in contacts. 424. — G. The thermal conductivity, 1c, and t h e law of W I E D B -
MANTST, F B A N Z a n d L O E B N Z . 428. i I I I . Tunnel effect. Thermionic emission and field emission 429
A. Theoretical basis for t h e calculations. Classes I and I I of caloulation procedure. 429. — B . Field emission and thermionic emission enhanced b y t h e SOHOTTKY effect. 433. — C. Tunnel resistivity. 435. — D . Comparison between tunnel current and thermionic current according t o formula
(111,13). 437. — E . Tunnel effect when both electrodes are of t h e same semi-conducting material. 438. —F. R e m a r k concerning t h e field strength.
439. — G. Tunnel current across a gap t h a t surrounds a metallic contact consisting of a circular spot with t h e radius a. 439.
5IV. Structure, electric and thermal conductivity of carbons 440
A. Introduction. Graphite latice. 440. — B . Carbon grades. 442. —
C. Graphitization. 443. — D. Electric conductivity of carbons. 444. —
E . H e a t conductivity of carbons. 447.
5 V. Hydrodynamic or thick film lubrication 448

Contents X V
§ VI. Remarks about threadlike metallic formations 453
§ V I I . Some fundamental formulas concerning t h e electrio discharge 455
A. Introduotion. Kinetic fundamentals. 455. — B. Drift velooity. 456. -
C. Thermal ionization. Saha's equation. 457. - D . Plasma. 457. -
E . Current in vaouum restricted b y t h e space charge of t h e current carriers. 458.
§ V I I I . General theory of t h e arc t h a t appears in relays 459
A. Introduotion. 459. - B . Reminder of elements of t h e theory of electrio discharges i n gases. 460. - C. Thickness of t h e oathode layer a n d metal vapor pressure within it. 462. - D. Definition of t h e examples.
463. - E . Current density J
+
of t h e positive ions a n d J e
of t h e primary electrons, a t t h e cathode. 465. — F . h as a function of T and p. 465. -
G. Comparison with measurements. 467. - H . Power balanoe a t t h e cathode. 467. — I . Summary of t h e results concerning oathode phenomena in arcs between non-refractory electrodes. 469. — J . The power balance a t t h e anode. 470. — K . Cathode of refraotory materials as carbon a n d wolfram. 471. - L. W h y is t h e voltage of a short arc of t h e order of 10 V? 471. — M. Movement of t h e aro spot. 472. - N . Currentvoltage characteristics of arcs. Arc life. 473.
§ I X . Calculation of t h e size of t h e load bearing area and of t h e pressure on i t i n experiments b y B O Y D a n d R O B E R T S O N [1] 475
§ X 476
§ X I 481
Author and literature index 483
Subject index
514