THE COPLEY MEDALJ&T OF 1871. 321 



magnet and a mass of annealed iron, is directly propor- 

 tional to the square of the strength of the magnetizing 

 current; while the attraction exerted between an electro- 

 magnet and the pole of a permanent steel magnet varies 

 simply as the strength of the current. These investiga- 

 tions were conducted independently of, though a little 

 subsequently to the celebrated inquiries of Henry, Jacobi, 

 and Letiz 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 circuit; and also propor- 

 tional to the square of the quantity of transmitted 

 electricity. This is a law of primary importance. In 

 another paper, presented to, but declined 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 pro- 

 portional 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 con- 

 clusions of the highest importance to electro-chemistry. 



It was during these inquiries, which are marked through- 

 out by rare sagacity and originality, that the great idea of 

 establishing quantitative relations between Mechanical 

 Energy and Heat arose and assumed definite form in his 

 mind. In 1843 Mr. Joule read before 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 Faraday advised Mr. Joule not to submit 

 the paper to the Royal Society. But its drift and results 

 are summed up in these memorable words by the author, 

 written some time subsequently: "In that paper it was 

 demonstrated experimentally, that the mechanical power 

 exerted in turning a magneto-electric machine is converted 



