Page:Popular Science Monthly Volume 83.djvu/606

602 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 17771861, 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 electromotive-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 publication 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 adjusting 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 measurements.

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 "resistance" the total force opposing the current. This would make the Cavendish 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. Xo one since then seems to have done anything further than quote Maxwell.

Nevertheless, a closer examination indicates that Maxwell's