442 Messrs. Trowbridge and Penrose on 



i. e. heat is absorbed by a current in passing from hot to cold, 

 and evolved in passing from cold to hot. The above results 

 were confirmed by many similar experiments, as will be seen 

 later. 



It was next determined to find whether the Thomson Effect 

 was reversible — that is, whether the heat absorbed by a current 

 in passing across a section of temperature t was equal to the 

 heat evolved by the same current when passing in the opposite 

 direction across the same section. This subject has import- 

 ant bearings on the thermodynamical theory of thermoelec- 

 tricity. 



The following method was pursued : — Both ends of the nickel 

 were at the temperature of the air, 15° C. The current from 

 six Grove cells was passed as before, and the deflections of the 

 galvanometer were observed every half-minute. The apparatus 

 was arranged exactly as before. Column I. of the accompany- 

 ing table gives the deflections. One end of the nickel was 

 now placed in melting ice. After one hour it reached a con- 

 dition of thermal ecprilibrium, and the current from the Grove 

 cells was passed alternately in opposite directions. The deflec- 

 tions are given in II. and III. 



If the deflections in II. and III. are subtracted from the cor- 

 responding deflections in I., we get the amount of deflection 

 due to the Thomson Effect. It will be observed that all the 

 deflections in II. are less than those in I., and those in III. are 

 greater, as they obviously should be. The only inaccuracy in 

 this determination is due to the fact that we neglected the 

 alteration in electrical resistance of the nickel due to the slight 

 change in temperature. 



I. 



Deflections 

 taken every 

 5 minute. 



II. 

 H-C. 



Deflections 



every 

 ^ minute. 



III 

 C-H. 



Deflections 



every 

 -j- minute. 



I.-II. 



I.-1II. 



1-8 

 2-6 

 2-9 

 3-1 



18 

 2-4 

 2-65 



2-75 



2-0 

 2-8 

 315 

 325 



00 

 02 

 0-25 

 035 



-02 

 -02 

 -0-26 



-0-15 



The numbers in these tables are obviously too small to draw 

 any conclusions. They, however, confirm the preceding results 

 as to the direction of the Thomson Effect; and tend rather to 

 prove than disprove the reversibility of the effect. The experi- 

 ment was repeated several times, but with no better result. 



Experiments were also made to test the influence of mag- 



