42 A. M. Mayer — Experimental Proof of Ohm's Law. 



Art. III. — An experimental proof of Ohm's Law : pre- 

 ceded by a short account of the discovery and subsequent 

 verification of the law ; by Alfred M. Mayer. 



I purpose giving in this paper a sinrple and direct experi- 

 mental proof of Ohm's law, C = — J. Generally a mere for- 

 mal statement of this law with illustrations are given in text 

 books on Physics, and the student is left to infer that its truth 

 is shown by the cumulative evidence given by the immense 

 number of quantitative relations in electrical actions which the 

 law associates, and by the experience that deductions made on 

 the basis of this law agree in measure with the results of ex- 

 periments. The latter fact is certainly one of the best proofs 

 of the truth of the law ; but, nevertheless, the relations 

 between C, E and E, are not directly and simultaneously shown 



to be exactly expressed by C=— . It is true that some works 



give experiments to show this relation but they are so difficult 

 to perform by reason of the difficulty of maintaining constant 

 C, E and R, that the results of the experiments only approxi- 

 mate to those required by the law. 



Ohm was led to the conception of this law by assuming 

 that the flow of electricity in a voltaic circuit is similar to 

 the flow of heat by conduction in a rod of indefinite extent. 

 Also, his assumptions that the actions of two electrified parti- 

 cles are directly as their distance and that the electricity is 

 uniformly dense over each cross section of a conducting wire 

 were directly opposed to the laws and facts well established by 

 Coulomb for statical electricity. It is not surprising that 

 scientific men were slow in adopting the views and theory of 

 Ohm. In his memoir (Die Galvanische Kette mathematisch 

 bearbitet von Dr. G. S. Ohm : Berlin, 1827), he states : * 

 " Three laws, of which the first expresses the mode of distri- 

 bution of the electricity within one and the same body, 

 the second the mode of dispersion of the electricity in the 

 surrounding atmosphere, and the third the mode of appearance 

 of the electricity at the place of contact of two heterogeneous 

 bodies, form the basis of the entire memoir, and at the same 

 time contain everything that does not lay claim to being com- 

 pletely established. The two latter are purely experimental 

 laws ; but the first, from its nature, is, at least partly theo- 

 retical. 



* See translation, published in vol. ii, of Taylor's Scientific Memoirs, p. 402. 

 London, 1841. 



