12 



IIKPOIIT- IHSJ. 



I! 



oxprpssrd by OIiiu'h law, K = IIC, wliicli is a carefully vciilieil thojijuli 

 empirical statcirioiit. Hut, analj'fiiiif^ Jt into spccilic! rcsistaiico of materiul 



(fi) ami sectional area of coiuluctor.aticl permitting ourselves to regard 



arcii 

 as proportioiuil to ilio velocity of electricity in a circuit ol' difTcrcnt thick- 

 nesses, \vc ]»(M-(('ivo that Ohm's law means that 



,/r , ., 



= nXvelocny. 



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

 ducting matter, a eonr.eclion which sIidws itselt as an K.M.F. proportional 

 to the speed of their relaliv(! motion and to tiio :'|iecilic resistiinee of tin 

 material. iMolecuIes of matter are not at rest, but (say) vibrating at ii 

 rate tlcju-nding on, or rather itself dcterminiMg, the temperature. 'I'licsi- 

 motions cannot be independent- of ele(,'trieify, but they result in no foi'oi 

 urging it to How because Ww'iv motions arts .symmetrical. JUit ])laco 

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

 iron — at the junction symmetry disM))pears, there must be constraint and 

 accommodation ; and, in whaUsver preci.so way this acts, it seems })rohal)le 

 that it can bo conceived of as having the same ellect as a layer of molecults 

 moving faster on their out ward journey than on their retui'n. If any such 

 dissymmetry of velocity were pi-odnced, it would exert a propelling fonu 

 on electricity' in the direction of tho greatest velocity, because the i'orcc 

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

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

 thermo-electricity and of tho i'eltier phenomenon, 



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

 depends on f), tho specific resistance, and this is small. Choose badly- 

 conducting metals like bismuth and antimony, or still better selenium 

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

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

 But when one uses non-coiuluctors wo caniu)fc expect to excite currents 

 ilowing in closed circuits ; wo 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 urginj; 

 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 tho motion in one direction rather than another : workinj^ n 

 pump piston exhausts no air uidess there arc valves. Friction may supi)ly 

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

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

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

 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. 



' I do not say necessarily on jxi.iif ire c]cch-ic\iy. It seems a complication, Ijui 

 Sir William's rcsoarclics show that it is positive in some metals and negative in other.-. 

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



^ Jlr. Joseph Thonjson (Proc. llo)j. Sue. 1870) endeavoured to extend onlinan 

 contact methods to non-conductora. He was hardly likely to got very elcar results; 

 hut he was able to tind some electrical transfer as the result of mere contact, if it b? 

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

 a siippositi(m which is probably inadmissible. Yet the least violence destroys ail 

 novelty and sends us back to Thalcs. 



