574 BRIDGMAN. 



of it should throw much additional Hght on the mechanism. The 

 old form of electron theory due to Drude was not competent to 

 explain the pressure effect, although it dealt fairly well wnth the 

 temperature effect. If the experimental facts with regard to pres- 

 sure had been sufficiently known it would not have required con- 

 siderations of specific heat to show that Drude's theory cannot 

 be correct. Only quite recently has the theoretical bearing of the 

 effect of pressure begun to be discussed^. I hope in this paper to 

 present enough material for a more extended discussion, and in the 

 latter part of the paper I shall try to indicate some of the significance 

 of the results. Even in its present incomplete form, however, the 

 electron theory will evidently need other data than the effect of pres- 

 sure and temperature on resistance. One of the most important 

 of such other effects is that of pressure on thermo-electromotive 

 force; I hope to give such data in a succeeding paper for some of the 

 metals investigated here. 



The effect of pressure on electrical resistance has of course been 

 measured a number of times before. The data here cover a con- 

 siderably wider range, both of material and pressure. None of 

 the previous work covers a pressure range of more than 3000 kg., 

 and except for a single isolated instance with 100*%) error, I know 

 of no measurements of the variation of pressure coefficient with 

 temperature. The data here bring put the to me unexpected fact 

 that the variation of pressure coefficient with temperature is very 

 much less than the variation of resistance itself. I have also tried 

 to improve on previous work in respect to the purity of the ma- 

 terials. Where possible I have given chemical analyses, and in all 

 cases the temperature coefficient. The temperature coefficient is a 

 very good indication of the purit}' of the metal, being almost always 

 higher for the purer material. The pressure coefficient shows no 

 such consistent variation, but may sometimes increase and some- 

 times decrease in the presence of impurity. 



The most important previous work on the effect of pressure 

 has been done by Lisell ^ up to 3000 kg., Lafay ^ to 2000 kg., Barus * 



1 E. Griineisen, Verb. D. Phys. Ges. 15, 186-200 (1913). 



2 E. Lisell, Om Tryckets Inflytande pk det Elektriska Ledningsmotst§,ndet 

 hos Metaller samt en Ny Metod att Mata Hoga Trvck, Upsala, (1902). 



3 Lafay, Ann. de Chim. et Phys. 19, 289-296 (1910). C. R. 149, 506-569, 

 (1909). 



4 C. Barus, Bull. U. S. Geol. Sur. No. 92 (1892). Amer. Jour. Sci. 40, 

 219 (1890). 



