602 THE POPULAR SCIENCE MONTHLY 



electrolytes and gases, and confines it to those who contributed to the 

 subject by the study of solids only, it includes the names of Cavendish 

 1750-1837, Priestley 1733-1804, Children 1777-1852, W. S. Harris 

 1792-1867, Davy 1778-1829, Barlow 1776-1862, J. Cumming 1777- 

 1861, A. Becquerel 1788-1878, Ohm 1789-1854, G-. Fechner 1801-1887, 

 Pouillet 1790-1868, Lenz 1804-1865, S. H. Christie 1784-1865, Ritchie 

 1814-1895, C. M. Despretz 1792-1863, Kirchhoff 1824-1887, J. M. 

 Gangain 1810-1878, Weber 1804-1891, Maxwell 1831-1878, A. 

 Schuster 1851- and G. Chrystal 1851-. 



The first person on record to investigate the relation between electro- 

 motive-force, current and resistance, afterwards formulated as part I. 

 of Ohm's law, was Henry Cavendish, of England. His work was done 

 prior to 1775, but remained totally unknown to the world until the publi- 

 cation of the Cavendish Researches by Maxwell, in 1879. Besides certain 

 experiments on the relative conductivity of the human body, of iron and 

 copper wire, and of various liquids, he made four series of experiments 

 to determine "what power of the velocity the resistance is proportional 

 to." In these experiments he employed a collection of wide and narrow 

 glass tubes filled with a salt solution. As a source of current he used 

 the discharge from a Leyden jar. The experiment consisted in adjust- 

 ing the length of the column of liquid in the tube under test until it 

 permitted the passage of a discharge of the same strength as that 

 through a second tube selected as a standard. Under these conditions 

 the resistances of the two tubes were regarded as equal. His method of 

 determining equality of discharge was to place his own body in circuit 

 with the condenser and test-tube, and then to judge by the sensation 

 experienced. This is perhaps the only case on record where the human 

 body has been used as a quantitative instrument in electrical measure- 

 ments. 



As' a result of these experiments Cavendish concluded that the 

 "resistance," in his sense of the word, varied as the 1.08, 1.03, 0.976 and 

 1.00 power of the "velocity" in the respective experiments. Maxwell 

 tells us that by "velocity" Cavendish meant current and by "resist- 

 ance " the total force opposing the current. This would make the Caven- 

 dish resistance equal to the total fall of potential around the circuit and 

 is equivalent to saying that the resistance, in the modern sense of the 

 word, is independent of the current. In his fourth experiment, which 

 was the one most carefully performed, the result is in exact accordance 

 with the modern view, and considering the crudity of his method all 

 four results may be said to check within a reasonable margin of error. 

 The work of Cavendish was on this basis regarded by Maxwell as an 

 experimental proof of Ohm's law, and it was in this light that he left 

 the matter in editing the Cavendish papers. No one since then seems to 

 have done anything further than quote Maxwell. 



Nevertheless, a closer examination indicates that Maxwell's state- 



