AN" ABSOLUTE MEASUREMENT OF THE THOMSON 

 EFFECT IN COPPER. 



By R. O. King, B. A. Sc, 



• ExJiihition of 1851 Scholar. 



Presented April 13, 1898, by E. H. Hall. 



The experiments described in this paper were carried out at the Jeffer- 

 son Physical Laboratory of Harvard University, under the supervision of 

 Dr. E. H. Hall. A grant of one hundred dollars was obtained from the 

 Rum ford Fund of the American Academy of Arts and Sciences for the 

 purchase of apparatus. The principle of the method employed in 

 the experiments was devised by Prof. H. L. Calleudar, M. A., F. R. S., 

 of McGill University, and depends on the measurement of change of 

 temperature by means of change of electrical resistance. 



Historical. 



In 1856 Lord Kelvin published* the details of an experiment which 

 showed the existence of a new phenomenon, which has since been called 

 the Thomson Effect. Aside from the interest always attached to a dis- 

 covery, the Thomson effect possessed an additional interest due to the fact 

 that, two years before, Lord Kelvin had inferred its existence by the 

 application of the laws of thermo-dynaraics to thermo-electric circuits. 

 In spite of the great interest taken in the Thomson Effect at that time, 

 very few attempts have been made to obtain an absolute measurement of 

 it, or to examine its dependence on temperature conditions. This can 

 probably be explained by its extreme smallness and difficulty of measure- 

 ment. 



Lord Kelvin in his original experiments sought only to prove the 

 existence of the " electric convection of heat." The metals used in the 

 principal experiments were iron and copper. The effect in iron was 

 found to be considerably greater than in copper. No absolute measure- 

 ments of the effect were attempted. 



* Royal Society of London, Phil. Trans., 1856. 

 VOL. XXXIII. — 23 



