THE COPLEY MEDALIST OF 1870. 433 



strength of the current. These investigations were 

 conducted independently of, though a little subse- 

 quently to, the celebrated enquiries of Henry, Jacobi, 

 and Lenz and Jacobi, on the same subject. 



On December 17, 1840, Mr. Joule communicated to 

 the Royal Society a paper on the production of heat 

 by Voltaic electricity. In it he announced the law that 

 the calorific effects of equal quantities of transmitted 

 electricity are proportional to the resistance overcome 

 by the current, whatever may be the length, thickness, 

 shape, or character of the metal which closes the cir- 

 cuit; and also proportional to the square of the quan- 

 tity of transmitted electricity. This is a law of primary 

 importance. In another paper, presented to, but de- 

 clined by, the Royal Society, he confirmed this law by 

 new experiments, and materially extended it. He also 

 executed experiments on the heat consequent on the 

 passage of Voltaic electricity through electrolytes, and 

 found, in all cases, that the heat evolved by the proper 

 action of any Voltaic current is proportional to the 

 square of the intensity of that current, multiplied by 

 the resistance to conduction which it experiences. 

 From this law he deduced a number of conclusions of 

 the highest importance to electro-chemistry. 



It was during these enquiries, which are marked 

 throughout by rare sagacity and originality, that the 

 great idea of establishing quantitative relations be- 

 tween Mechanical Energy and Heat arose and assumed 

 definite form in his mind. In 1843 Mr. Joule read be- 

 fore the meeting of the British Association at Cork a 

 paper ' On the Calorific Effects of Magneto-Electricity, 

 and on the Mechanical Value of Heat.' Even at the 

 present day this memoir is tough reading, and at the 

 time it was written it must have appeared hopelessly 

 entangled. This, I should think, was the reason why 



