in Alloys of Bismuth and Tin. 183 



of the Thomson effect, the temperature of any point in the rods 

 changes, one rod experiencing an evolution, the other an absorption 

 of heat. After an interval ranging from 30 minutes to 2 hours, 

 according to the magnitude of the temperature change, the 

 temperatures again become steady and the value of the com- 

 pensating current is again observed. 



The current through the rods is then again reversed and so on. 



The change of temperature measured in this way is then four 

 times that due to the Thomson effect. 



The second part of any experiment consists in measuring the 

 rise in temperature produced by a current in the rods. 



In this case the rods are at the same temperature throughout, 

 and by means of a lead soldered to the strip of copper joining 

 them the current is sent through each in turn. 



The current is sent first through one rod and when the 

 temperature has become steady the difference of temperature of 

 corresponding points of the two rods measured as in the former 

 experiment ; this circuit is then broken and the current passed in 

 the same direction through the other rod. Sufficient time having 

 elapsed for the temperatures to become steady another reading of 

 the compensating current is taken and the current changed back 

 again to the first rod, but flowing in this case in the opposite 

 direction. 



The change of temperature thus observed is then twice that 

 due to the evolution of heat in either rod separately. 



The evolution or absorption of heat in the first experiment is 

 obtained on the assumption that the heat produced is proportional 

 to the change of temperature, and the value of the " specific heat " 

 calculated. 



For if H 1 is the heat produced or absorbed per unit length per 

 unit time when a current flows along a temperature gradient, then 



H ^°^fx ergS ' 



where G x is the current in c.G.S. electro-magnetic units, 



cr the specific heat of electricity, 



, dd , 

 and -j- the temperature gradient, 



n dd 

 or E x = — — ergs, 



when G ± is measured in amperes. 



Also if H 2 is the heat evolved per unit length per unit time by 

 a current C 2 , according to Joule's law 



H 2 = C£ r ergs, 



