THERMO-ELECTRIC QUALITY UNDER PRESSURE. 383 



The magnitude of the change in the Thomson heat produced by 

 pressure compared with the total Thomson heat under atmospheric 

 conditions is of interest. The relative effect of pressure on the proper- 

 ties of most solids is comparatively small; for example, the maximum 

 effect found before on resistance was 14% for lead, and the effect on 

 volume is only a few per cent. We should expect a similar state of 

 affairs with the Thomson heat. This is true, except for the anomalous 

 metals. The total pressure effects with Al, Bi, Fe, Sn, and Zn, are 

 all of the order of magnitude of the whole effect at atmospheric condi- 

 tions, but the other metals show the sraallness of the effect to be 

 expected. The results are shown in Table XLVI. 



So far, we have considered the effects only in broad outline, but 

 when we come to consider the detailed variations with pressure and 

 temperature we find great complexity. The normal behavior of 

 e.m.f. is a regular rise with pressure and temperature; the slope of 

 the e.m.f. curves at constant pressure increasing with rising tempera- 

 ture, but the slope at constant temperature decreasing with rising 

 pressure. Fe, Al, and Sn are examples of complicated variations of 

 e.m.f. with pressure and temperature, and we have also met other 

 examples where the slope at constant pressure may decrease with 

 rising temperature, or the slope at constant temperature may increase 

 with rising pressure. As for the detail of variation of Peltier heat 

 or Thomson heat with pressure and temperature, so many different 

 types are presented that it is useless to try to enumerate them. 



The Entropy of Electricity. 



By an extension of a remark made by Professor E. H. Hall with 

 regard to the ordinary thermo-electric diagram, we may find a func- 

 tion giving the entropy of the electricity in the metal as a function of 

 pressure and temperature. 



A couple composed of two branches of the same metal, one at 

 pressure, the other at pressure p, running between 0° and t° has a 

 definite e.m.f., E. E is a function of p and t. If we add to E (p,t) 

 Epb(t), that is, the e.m.f. against lead at atmospheric pressure, we 



shall obtain a function | (p,t) such that M"^] is the Thomson heat 

 at any pressure and temperature and t ( x^ j is the Peltier heat. 



