in a Magnetic Field at Different Temperatures. 697 



moving negatively charged corpuscles in statistical equi- 

 librium with the surrounding atoms or clusters o£ atoms, 

 requires that the mean free path o£ these corpuscles should 

 increase as the temperature is reduced. 



Sir J. J. Thomson * points out that for most pure metals 

 X should vary almost inversely as the absolute temperature, 

 though the fact that the resistance of cadmium and zinc 

 increases rather faster than the absolute temperature as well 

 as that the Thomson effect is in each case positive, would 

 cause the mean free path to show a somewhat larger 

 variation with temperature than this inverse first power law- 

 requires. 



By the aid of the expression 



r 6 VI \mj v z 



given some years ago by Prof. Thomson t for the change of 

 resistance in a magnetic field, we can determine X for the 



different temperatures at which — has been found. 



It will be seen from this equation that if X varies as , 



1 Br 1 ^ 



jj 2 — should vary as Qr 



The experiments with cadmium and zinc described in this 

 paper indicate that at temperatures between 15° and 100° 0. 



, — varies somewhat more rapidly than ^ though less 



than Tfc, whilst at the temperature of liquid air the variation 



1 



is rather less than „. v On the whole, therefore, the variations 



of X as found from the experiments here described agree 

 fairly well with 1 hose required on the same hypothesis by con- 

 sidering the variation of resistance with temperature and 

 the Thomson effect. 



The absolute values of X at a temperature of 1G° (J. 



obtained from the above equation by putting =* WxlO 7 



and d = 10 7 + are 



for cadmium X = 33 x lO" 6 



and for zinc X = 1*9 x 10 ~ 6 . 



* ' Corpuscular Theory of Matter, 1 p. SO. 



t Rapport* piisenUs au Vonyrha Internationale de FJit/s'que, iii. 

 p. 138 (1900). 



| ' Corpuscular Theory of Matter,' pp. ]0 and 52. 



