342 Prof. Oliver Lodge on the Seat of the 
symmetry disappears, there must be constraint and accom- 
modation ; 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* 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 Seebeck or 
true contact-force—the cause of thermoelectricity 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 nonconductors, 
like glass and silk and ebonite, and it becomes enormous. 
But when one uses nonconductors we cannot expect to excite 
currents flowing in closed circuits ; we can only expect elec- 
trical displacement and electrostatic phenomena ; and indeed 
it is no such easy matter for electricity to move in such sub- 
stances, 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 
determine 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 contactt. 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. 
Measurement of contact-force between insulators is beset 
* I do not say necessarily on positive electricity. It seems a complica- 
tion ; 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. 
+ Mr. Joseph Thomson (Proc. Roy. Soc. 1876) endeavoured to extend 
ordinary contact methods to nonconductors. 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. 
