THE COPLEY MEDALIST 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 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 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. 
