ISBN: 3540038752
TITLE: Electric Contacts
AUTHOR: Holm, Ragnar
TOC:

List of frequently used symbols and abbreviations XIII
Part I
Stationary Contacts
 1. Introduction. A simplified summary of the theory of stationary electric contacts 1
 2. The contact surface 7
 3. The contact resistance. General theory 9
 4. Calculation of constriction resistances with constant resistivity ** in an isotropic material 11
Problem A. 13 - Problem B. 14 - Problem C. Constriction resistance of an elliptic spot. 14 - Problem D. 15 - Problem E. 17 - Problem F. The influence of the elliptic shape of the contact area on the constriction resistance expressed by a shape factor. 18
 5. Constriction resistances when conditions deviate from those in  4, but with ** still a constant 19
Problem A. 20 - Problem B. The constriction resistance of one member when the a-spot is covered with a film. 20 - Problem C. Multispot metallic contact referred to a semi-infinite member. 21 - Problem D. 23 - Problem H. 26 - Problem I. 26
 6. Introduction to thin films on contacts. Contact cleaning 27
 7. The load bearing contact area as a function of load and elastic and plastic properties of the members 29
A. Introduction. 29 - B. Flat Contact. 30 -C. Experimental determination of A. 32 - D. Persistence of asperities in indentations. 37 - E. Influence of temperature and contact duration on the contact area. 37 - F. Special application of Eq. (I, 23) concerning creep in contacts. 39
 8. The relation between contact load and resistance, particulary at moderate and high load 40
A. Introduction with description of Fig. (8.01). 40 - B. Crossed rod contacts. 42 - C. Explanation of the dashed lines in Fig. (8.01). 44 - D. Diversified resistance measurements. 46 - E. Use of Diagram (8.01) in practice. 47 - F. Practically clean contacts, preloaded with a high P. 47
 9. Contact resistance on freshly cleaned rods in air at very small contact loads 48
A. Observations on gold and silver. 48 - B. Observations on carbon contacts. 50 - C. Method of wire probes for exploring contact surfaces. 52
 10. The inductance of a current constriction. Skin effect 52
A. Inductance. 52 - B. The skin effect. 54
 11. Electrodynamic repulsion in a symmetric contact of non-magnetic material 55
 12. The capacitance of a contact. Electrostatic attraction in a contact 57
Example A. Crossed rod metallic contact. 58 - Example B. 59 - Example C. JOHNSEN-RAHBEK effect. 59
 13. The relationship between electric potential and temperature in a current constriction which is symmetric with respect to the contact surface; that is, the phi theta-relation 60
A. Introduction. 60 - B. Proof of the phi theta-relation in a symmetric constriction. 61. C. Justification of assumptions. 63 - The -relation in metallic constrictions. 63
 14. The phi theta-relation in cases of dissymmetry 64
Case A. 64 - Case B. Contact between electrodes differing in ** and lambda. 65 - Case C. 65 - Case D. Heat flows across contact surface. 67 - Case E. 67 - Case F. Thomson effect. 67 - Case G. 68
 15. Influence of a thin film in the contact on the phi theta-relation. KOHLER effect 69
 16. The influence of the JOULE heat on constriction resistences in symmetric contacts 71
Example A with W-F law. 74 - Example B. Heat flows across contact surface. 75 - -Example C. Integration of Eq. (16.09) without using WIEDEMANN-FRANZ law. 77
 17. Distribution of the temperature in a symmetric constriction with circular contact surface at givven current 78
Example A. 78 - Example B. 79 - Example C. 80
 18. The (equilibrium) temperature distribution in the constriction of a contact between two metals with different conductivities, both obeying WIEDEMANN-FRANZ law. Thermoelectric effects 80
A. Features of the thermoelectric effects. 80 - B. The problem of temperature distribution in the case of PELTIER heat in the contacts. 82
 19. Temperature distribution in the constriction of a contact between members of very different conductivities 85
 20 Resistance-voltage characteristics of clean symmetric contacts. Softening and melting voltages 87
A. Introduction. 87 - B. Discussion of Diagram (20.01). 87 - C. Softening. 88 - D. Melting. 89 - E. Measurements of RU-characteristics. 91
 21. Development of the temperature in a metallic current constriction 92
A. Introduction. 92 - B. Survey of the problems. 93 - C. Discussion of the diagrams for the different cases. 96 - Case A. Symmetric contact heated by the current throughout the constriction. 96 - Case B. Contact heated by the current. The members have very different conductivities. 96 - Case S. A semi-infinite body at rest, with a circular heat source. 97 - Case (A and S) for very small z-values. 98 - Case M. Heat source moving on the surface of a semi-infinite body. 99 - D. Cooling of a previously heated contact constriction. 100 - Examples. 100
 22. Growth of films on metals used for contacts, near room temperature 102
A. Introduction. 102 - B. Rate of adsorption. 103 - C. Development of tarnish form a chemisorbed monolayer. 104 - D. Protective films. 107 - E. Tarnishing of some relatively poor contact metals. 108 - F. Tarnishing of noble metals. 109 - G. Contact breating. 111
 23. Growth of visible oxide films on metals at moderate to high temperature. Decomposition at still higher temperature 111
 24. Water film, local cells and rusting 114
A. Thickness of water films. 114 - B. Rusting by means of electrochemical attack. 115
 25. Alien solidified films on contacts 116
 26. Tunnel effect 118
A. Introduction. 118 - B. General results in the case of room temperature and less. 122 - C. Details of observations on the tunnele effect at room and lower temperature. 123 - D. Evaluation of film thickness, based on measured sigma, for the case of small 8???. 125 - E. Observations on tunneling at room temperature through relatively thick films, 25 ALPHA and more. 126 - F. Tunneling between dissimilar metal members. 128 - G. The superconductivity of contacts, particulary contacts with a film which is penetrable via tunnel effect. 128 - H. Type III, field emission. Influence of relatively high temperatures on the tunnel effect. - 130 - I. Tunnel effect when both electrodes are of the same semiconducting material. 131 - J. Tunnel current across a gap that surrounds a metallic contact consisting of a circular spot with the radius alpha. 132 - K. Tunneling within a semiconductor. 134
 27. Fritting of tarnish films 135
A. Introduction. 135 - B. Theory of fritting. 135 - C. Widening of an a-spot by electrical forces. 137 - D. Cessation of a normal A-fritting. 139 - E. A- and B-fritting; definition. 139 - F. Observations on A-fritting. 140 - G. Observations on B-frittings. 146 - H. Applied Fritting. 147 - I. Metallic or quasi-metallic contact through an opening in a relatively-thin contact film. 148 - J. Polarity dependence of frittings of tarnish films on silver and copper. 150
 28. Adherence in dry contacts which are not heated to any influential extent by the current 153
A. Introduction and theory. 153 - B. Experimental verifications. 155 - C. Location of the weld break. 158 - D. The practical significance of the adherence in non-heated contacts. 159
 29. Adherence in contacts that are heated by the current passing through them. Resistance welding 160
 30. About stationary contacts in practice 164
A. Introduction. 164 - B. Type I, permanent contacts. 165 - C. Separable contacts of high mechanical load. 166 - D. Resistance measurements of elamped contacts with force produced by a screw. 167. - Medium and low load contacts in air. Printed circuit boards. 169 - F. Medium and low loads contacts in sealed chambers. 171 - G. Microcontacts. 172 - H. Stored lubricated contacts. 174 - I. Contacts in measuring apparatus. 175 - J. Contact assembly. 175
 31. Dimensioning a contact with respect to its heating 176
 32 Contact effects in carbon granular microphones 180
A. Introduction. 180 - B. Configuration of the transmitter. 180 - C. Calculations of the a.c. power of a microphone. 181 - D. Deterioration by "burning" of microphone granules during the life of the transmitter. 184
 33. Contact with semiconductors. Rectification. Static electrification 185
A. Introduction. 185 - B. Contact between a metal and a semiconductor, and its rectification property. 185 - The p-n-junction. 187 - D. The silicon controlled rectifier. 188 - E. Remark concerning the contact between silicon controlled rectifier. 188 - E. Remark concerning the contact between silicon carbide crystals. 189 - F. Static electrification. 189
 34. Carbon-pile rheostats. Electric resistance of pressed metal powders 190
Part II
Thermal Contacts
 35. Thermic metallic contacts 193
A. Introduction. 193 - B. Bilateral heat current. 193 - C. Thermal resistance of nominally flat bolted contacts. Examples. 194
Part III
Sliding Contacts
 36. Survey of fundamentals 199
A. Introduction. 199 - B. The geometry of a nominally flat sliding contact. 199 - C. Experiments. Type II. 202 - D. Experiments when Type I would be approached. 204
 37. Sliding contacts in air 205
 38. Boundary lubrication 209
A. Features of boundary lubrication. 209 - B. Discussion of two competing theories of boundary lubrication. 213. - C. Lubricating practice. 218 - D. The BEILBY layer. 218 - E. Boundary-lubricated separable contacts including some sliding metal-metal contacts. 219 - F. Properties required of bearing materials. 220 - G. Ball bearings. 221
 39. Theory of friction and wear of carbon contacts. Lubrication by means of solid lubricants as graphite and molybdenum disulfide 221
A. Introduction. 221 - B. Bond strength in a slinding contact between graphite members. 223 - C. The high altitude effect. 225 - D. Adjuvants. 227
 40. Stick-slip motion. The temperature in currentless sliding contacts 228
A. Stick-slip or jerky motion. 228 - B. The temperature in currentless sliding contacts. 230
 41. Frictional wear in metallic contacts without current A. Types of wear. 232 - B. Details of the formation of wear detritus. 234 - C. The influence of adhesive wear on friction in clean metallic contacts. 235 - D. Why liquids, even the deposit from air humidity, are able to strongly influence wear, without appreciably affecting the friction coefficient. 235 - E. Frictional wear in currentless sliding contacts; represented by ZETA which is calculated according to Eq. (41.03) as a function of the hardness, ETA, of the softer member. 236 - F. Size and frequence of wear fragments appearing during periods of adhesive wear. 240 - G. Behavior of sliding contacts of measuring apparatus. 242
 42. Electrical performance of carbon brushes on rings and commutators when arcing is excluded 242
A. Introduction. 242 - B. Early investigations about the conduction mechanism in the brush-ring (cooper) contact. 243 - C. Polarity effect in the contact between a graphite brush and a copper ring. 245
 43. The temperature in the sliding contact between a carbon brush and a copper ring or commutator 249
 44. Friction and wear with a carbon-brush collector contact 251
A. Mechanical wear with sliprings, no current. 252 - B. Influence of the current on the wear (absence of arcs). 252 - C. Brush wear during commutation with arcs. 253 - D. Abrasion of the slip ring. 257 - E. Friction between an electrographite brush and a ring. 257 - F. Brush lifting by hydrodynamic air pressure. 259
 45. Theory of commutation with special regard to voltage flashes and arcs 260
A. Fundamental equations. 260 - B. A first integral. 262 - C. Arcing during unbalanced commutation. 263 - D. Energy dissipated in arcs and voltage flashes. 264 - E. Each brush makes contact with more than two segments. 266 - G. About so-called short-circuit current in the brush. 267
 46. Current collectors for trolley cars 268
 47. Electric noise in contacts 269
A. The thermal noise. 269 - B. Shot noise. 270 - C. The specific contact noise. 270 - D. Contact trembling or agitation. 272 - E. Noise generated by sudden changes in, or interruptions of, the contact. 272
Part IV
Electric Phenomens in Switching Contacts
 48. Introduction 274
 49. Ignition of arcs in switches 275
A. Introduction. 275 - B. Electrical breakdown in the gap between metallic electrodes at atmospheric pressure. 275 - C. Drawing of arcs. 278. - D. Arc ignition followed by floating. 278
 50. VI-charcteristics of the stationary arc in air; their use for calculation of the duration of drawn short arcs 279
A. Introduction. 279 - B. Observations on breaking contacts in an ohmic circuit. 281 - C. Simplified VI-characteristics for different metals with known Im and for small currents. 283 - D. Use of the resistance line together with arc characteristics for the problem of how a constant current is shared between an arc and ohmic resistance both in parallel. 285
51 Vacuum arc; particulary its extinction 286
A. Introduction. 286 - B. The duration of the vacuum arc. 286. - C. Current chopping by vacuum arcs. 289
 52. Interrupting an a.c. current 290
A. Introduction. 290 - B. Vacuum switch. 291 - C. High power current breakers. 291 - D. Medium and low duty breakers. 295 - E. Contact rectifier and synchronous switch. 296
 53. Breaking direct current 297
 54. Electric oscillation generated by d.c. arcs. 298
 55. Bouncing 300
 56. Material transfer in switching contacts. A survey 304
A. Definiton of the major types of material transfer. 304 - B. Concerning the amount of arc transfer, particulary from the cathode. 306 - C. Arc quenching. 311 - D. Mechanical wear produced by the hammering effect of switching contacts. 312 313
 57. Discharge transients 313
 58. Arc duration contact closure with voltages below 200 to 300 V 316
A. Introduction. 316 - B. Calculation with respect to the wiring diagram (58.02) with initially charged capacity. 316 - C. Inductance jota = 0, (thus beta = 0) in the circuit (58.02). 319 - D. A battery instead of C, equivalent to C = ***; finite 1. 320
 59. Floating 320
 60. Arc duration on breaking contact without quenching. Circumstances common to Communication Engineering 323
A. Ohmic circuit according to Fig. (60.01). Arc extinction in air during a steady increase of the gap. 323 - B. Inductive circuit in air according to Fig. (60.04). Capacity of the leads is neglected. 323 - C. The quantity of electricity that flows through a drawn arc. 325
 61. Quenching a drawn arc by means of a capacitor, parallel to the contact 325
A. General theory. 325 - B. A short description of the process. 327 - C. Note concerning the position of r. 328
 62 Capacitive quenching when an arc with a very small duration or no arc is drawn 329
A. Equations. 329 - B. Condition for no breakdown of the gap between the separating electrodes. 331 - C. Example. 331
 63. Quenching of arcs by a resistance parallel to the operating contact or to the inductive coil 332
A. Quenching with r in position a. 332 - B. Arc quenching with r in position b. 334
 64. Details about the types of arc in relay contacts and the material transfer produced by them 334
A. Survey. 334 - B. Result of measurements. 335 - C. The critical length, scr of the anode dominated arc. 336
 65. Bridge transfer and short arc transfer at contact separation 338
A. Introduction. 338 - B. A classical example of a long liquid bridge. 339 - C. Methods of determination of the rate of transfer. 341 - D. Results of measurements and their interpretation. 342
 66 Theory of the bridge transfer 347
A. Introduction. 347. - B. Bridge transfer because of the THOMSON effect. Theory. 348 - C. THOMSON effect. Comparison with measurements. 351 - D. KOHLER effect. 352 - E. Comparison with measurements. 353 - F. For completeness we calculate the material transfer as it could be produced by the PELTIER effect. 353 - G. Remark 354 - H. Remark. 354
 67. Bridge material transfer in the shape of pips and spires 354
 68. Mercury switches 356
A. Introduction 356 - B. Medium duty switches with a pool of mercury forming one electrode. 357 - C. Mercury wetted contacts. 358 - D. Mercury between the members of a sliding contact. 358
 69. Application of statistics to contact operations 359
A. Reliability of contacts. 359 - B. Some numerical results of tests. 361 - C. Effect of twin contacts. 361
 70. The choice of contact material; contact shape for practical applications 362
A. Permanent contacts. 363 - B. Microcontacts. 363 - C. Relay contacts with high repetitive operation. 364 - D. Light duty relays for medium frequencies. 364 - E. Medium duty circuit breakers and contactors. 365 - Heavy duty circuit breakers with up to thousands of amperes and volts. 365 - G. Sliding contacts for resistors and apparatus. 366
Appendices
 I. Elasticity, plasticity and hardness 367
A. Introduction. 367 - B. HERTZ' formulas for ideally elastic indetations. 367 - C. Plastic deformations. Dislocations. 369 - D. Mathematics of plastic yielding. 370 - E. Indentation in an isotropic semi-infinite body, produced by a spherical indenter (ball). 371 - F. The ball and pyramid indentation tests. Hardness. 372 - G. Influence of friction on the indentation. 377 - H. Diffusion effects. 377 - I. The work consumed by a plastic deformation. Examples. 378
 II. Electronic conduction in solids 380
A. Introduction. 380 - B. General concepts concerning conduction. 380 - C. Quantization of the electrons in a crystal and the constitution of a band. 381 - D. Influence of the temperature on the distribution (or partition) of electrons on cells in metal. 382 - E. Current carriers in semiconductors. 384 - F. Mobility of current carriers. Resistance. 386 - G. Surface potential barrier of a metal. Thermionic emission of electrons 387 - H. Equilibrium in a contact between metals A and B with different work functions PHI, PHIA > PHIE 389 - I. Metal-semiconductor contact. 390
 III. Heat conduction. WIEDEMANN-FRANZ law (WF-law) 393
A. Heat capacity. 393 - B. Thermal conduction. 395 - C. WIEDEMANN-FRANZ law (WF-law). 396
 IV. Probability. Noise 397
A. Normal distribution. 397 - B. Probability paper. 399 - C. The POISON distribution. 400 - D. Electrical noise. 402
 V. Structure, electrical and thermal conductivity of carbons 404
A. Introduction. Graphite lattice. 404 - B. Carbon grades. 406 - C. Graphitization. 407 - D. Electrical conductivity of carbons. 408 - E. Heat conductivity of carbons. 410
 VI. Hydrodynamic or thick film lubrication 412
 VII. Metal whiskers 417
A. Whiskers formed from the solid metal. 417 - B. Whiskers rapidly formed from metal vapor. 417 - C. Mechanical and magnetic properties of whiskers. 418 - D. Metallic dendrites in semiconductors. 418
 VIII. Some fundamental formulas concerning electric discharges 419
A. Introduction. Kinetic fundamentals. 419 - B. Drift velocity. 420 - C. Thermal ionization. SAHA'S equation. 420 - D. Plasma. 421 - E. Current in vacuum restricted by the space charge of the current carriers 422
 IX. Theory of the electric arc 423
A. Introduction. 423 - B. Cathode layer. 425 - C. Emission of primary electrons from the cathode. 426 - D. The ion current in the cathode layer. 429 - E. The rate of evaporation, omega, m3/coul. 429 - F. Summary of the results presented in Table (IX, 5). 430 - G. Power balance at the cathode. 430 - H. Power balance at the anode. 433 - Voltage-current characteristics of arcs. 433 - J. Pinch effect. 435
 X. Tables 436
 XI. Diagram XI in two copies, one detachable 441
Author and literature index 443
Subject index 477
Errata 483
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