,312 report — 1884. 



expressed by Ohm's law, E = RC, which is a carefully verified though 

 empirical statement. But, analysing R into specific resistance of material 



C 



(p) and sectional area of conductor, and permitting ourselves to regard — 



area 



as proportional to the velocity of electricity in a circuit of different thick- 

 nesses, we perceive that Ohm's law means that 



- =p x velocity. 

 ax 



Let us then postulate, between electricity and any given kind of con- 

 ducting matter, a connection which shows itself as an E.M.F. proportional 

 to the speed of their relative motion and to the specific resistance of the 

 material. Molecules of matter are not at rest, but (say) vibrating at a 

 rate depending on, or rather itself determining, the temperature. These 

 motions cannot be independent of electricity, but they result in no force 

 urging it to flow because their motions are symmetrical. But place 

 two metals in contact — one hot, the other cold ; or one copper, the other 

 i ron — a t the junction symmetry disappears, there must be consfraint and 

 accommodation ; and, in whatever precise way this acts, it seems probable 

 that it can be conceived of as having the same effect as a layer of molecules 

 moving faster on their outward journey than on their return. If any such 

 dissymmetry of velocity were produced, it would exert a propelling force 

 on electricity 1 in the direction of the greatest velocity, because the force 

 is proportional to the velocity. This is the crude and tentative way in 

 which I picture to myself the Scebeck or true contact force — the cause of 

 thermo-electricity and of the Peltier phenomenon. 



But now why is this force so small in ordinary metals ? Because it 

 depends on p, the specific resistance, and this is small. Choose badly- 

 conducting metals like bismuth and antimony, or still better selenium, 

 and tellurium, and the force will be greatly increased. Choose so-called 

 non-conductors, like glass and silk and ebonite, and it becomes enormous. 

 But when one uses non-conductors we cannot expect to excite currents 

 flowing in closed circuits ; we can only expect electrical displacement and 

 electrostatic phenomena ; and indeed it is no such easy matter for 

 electricity to move in such substances, even though the force urging 

 it be excessive ; and a little mechanical violence (friction) may be necessary 

 to help it to move. But remember that no amount of friction can de- 

 termine the motion in one direction rather than another : working a 

 pump piston exhausts no air unless there are valves. Friction may supply 

 some of the energy, but the directing force must be in the substances in 

 contact. 2 To assist the passage it is customary in electrical machines to 

 touch together a conductor and insulator rather than two insulators. T 

 doubt not that when metal touches glass the surface of contact would become 

 chilled as soon as any transfer of electricity were really produced by the 

 force ; but the heat developed, by the friction apparently necessary to aid 

 the transfer, effectually masks any chilling. 



1 I do not say necessarily on positive electricity. It seems a complication, but 

 Sir William's researches show that it is positive in some metals and negative in others. 

 In the case of lead only does the grip on both electricities seem the same. 



2 Mr. Joseph Thomson (Proc. Hoy. Soc. 1876) endeavoured to extend ordinary 

 contact methods to non-conductors. He was hardly likely to get very clear results ; 

 but he was able to find some electrical transfer as the result of mere contact, if it be 

 admitted that it is possible to apply mere contact and no sort or kind of violence, 

 a supposition which is probably inadmissible. Yet the least violence destroys all 

 novelty and sends us back to Thales. 



