HALL. — ELECTRIC CONDUCTION AND THERMOELECTRIC ACTION. 97 



Appendix. 



Sir J. J. Thomson, 22 after discussing at length various aspects and 

 consequences of the free electron theory of electric conduction, comes 

 "to the conclusion that the mechanism by which we have supposed 

 the electric current to be conveyed through a conductor is at most 

 only a part and not the whole of the process of metallic ' conduction.' " 

 He continues, "One reason for this conclusion is the large changes 

 which take place in the electrical resistance of some metals on fusion, 

 changes which do not seem to be accompanied by any corresponding 

 change in thermoelectric quality. Thus the conductivities of tin, 

 zinc, and lead at their melting points are, when the metals are in the 

 solid state, about twice what they are in the liquid." 



He then goes on to say that the free electron theory, as previously 

 developed by himself and others, would predict for these substances 

 " a Peltier effect between the solid and the liquid metal about half the 

 magnitude of that between bismuth and antimony, and thus, as these 

 effects go, exceedingly large. Now Fitzgerald, Minarelli and Ober- 

 meyer, as quoted by G. Wiedemann, ' E/e/rfn'c^a£,' II, p. 289, could 

 detect no sudden change in thermo-electric circuits with these metals 

 when they passed from the solid to the liquid state." 



This passage from Thomson, only vaguely remembered, had little 

 if anything to do with the course of my speculations and argument as 

 developed in the paper just concluded; but at the end, being strongly 

 of the opinion that the part played by the free electrons in metallic 

 conduction is a very subordinate one, and that the spaces between the 

 atoms are the great obstacles to electric flow, I naturally turned with 

 curiosity to the question whether the change of resistance in the 

 fusion of metals can be accounted for by the mere change of volume, 

 and so of mean distance between adjacent atoms, which occurs in this 

 change of physical state. 



Examination of the available data in this field of inquiry speedily 

 showed one important fact; that, of the twelve metals for which we 

 have trustworthy information concerning change of volume and 

 change of resistance in fusion, ten, cadmium, caesium,, copper, lead, 

 mercury, potassium, sodium, thallium, tin, and zinc, increase in both 

 volume and resistivity during fusion, while two, bismuth and gallium, 

 decrease in both properties. Antimony, which decreases in resistance 

 during fusion, may increase slightly in volume during the same change; 

 but this is still in doubt. Northrup, who has been doing excellent 



22 Corpuscular Theory of Matter, p. 75. 



