636 BELL SYSTEM TECHNICAL JOURNAL 



\ft M tlie tciuieiuy of inoderii atomic theory is to make it seem likiK 

 that tlic atoms lili ahiiost the wliole space within tiic metal. 



Another \va\' to a\ni(l the (lifticullx witli the specitic heats consists 

 in assimiing tliat the liigii natural s|)eecl with which the electrons 

 {\y at)oiit is altogether independent of temperature; the burden of 

 making cr as expressed in C.i) \ar\' in the proper manner with temper- 

 ature is then laid upon /, which, Wicn suggested, should be supposed 

 to vary inversely as the amplitude of \ibration of the atoms — that 

 is, a free electron collides with an atom only if and when it is in vibra- 

 tion, and the chance of a collision increases with the amplitude of the 

 vibration. The variation of resistance with pressure may then be 

 explained, so far as the usual sign goes, by saying that when an ordi- 

 nary metal is compressed the amplitude of oscillation of its atoms 

 diminishes, though the temperature remain the same; the frequency 

 of oscillation must then vary inversely as the amplitude, to keep 

 the average energy of oscillation constant; there is some reason for 

 expecting this to happen. Bridgman's theory somewhat resembles 

 this one, except that the electrons are supposed to glide through the 

 atoms and collide with the gaps; gaps between atoms are compara- 

 tively unusual, and occur chiefly when two atoms are vibrating with 

 great amplitudes in opposite senses, so that the variation of con- 

 ductivity with pressure again has the proper sign. But to explain 

 the behavior of the three metals of which the resistance increases with 

 pressure and with tension, Bridgman went back to the idea that in 

 these the electrons glide through the interspaces. 



As I have gi\en only the phenomena of conduction which the 

 electron-theory explains with difticult\-, I must in justice mention 

 the ones on which its reputation chiefly depends. In the hrsl place 

 it is a theory of thermal conduction as well as electrical conduction; 

 the electrons in the hotter |)arl of a metal maintained at an uneven 

 temperature are a.ssumed to li.nc ,i i;rcater average energy than the 

 electrons in the cooler part, so that ihcv- dil'fuse down the tem[)erature- 

 gradient and realize a coiuiclion-cuncnt of heat. The tlieiM\- le.uls 

 to as definite a mmuTir.ii value of tin- one coiuhictiv itv' as of the 

 other. ,md the r.il io ol I'ieclrical to t hernial coiuiuctivitv' is predicted as 



a imiversal constant for all metals, multiplied into the ab.solute tem- 

 (lerature, and devoid of the quantities m and / which have caused us 

 so much trouble. This is one of the predictions which is nearly 

 enough true to be impressive; the ratio X/trT does indeed vary sur- 



