A. M. Mayer — Experimental Proof of Ohm's Law. 45 



the voltaic circuit and to the above words and to current and 

 resistance he attached precise meanings and showed the rela- 

 tions existing between those quantities. The clear definitions 

 Ohm gave of these terms marked a transition from vague ideas 

 of "quantity" and "intensity" to the clear conceptions of 

 potential, electromotive force, current and resistance. The 

 word energy he also used with clear and accurate meaning 

 as is shown in the following statement : " that the decompos- 

 ing force of the circuit is in direct proportion to the energy 

 of the current, and moreover, that it depends on a coefficient, 

 to be derived from the nature of the constituent parts and 

 their chemical equivalents." This was published in 1827, six 

 years before Faraday's researches on electrolysis. 



Neither Ohm nor his contemporaries were able to test the 

 truth of the four statements given above as embodying Ohm's 

 theory. It was reserved for Kohlrausch in 1849 to show by 

 very ingenious and accurate experiments that Ohm's state- 

 ments were true in mode and in measure. Kirchhoff* and 

 Quincke f applied with success Ohm's theory to the flow of 

 electricity in thin conducting plates, or bodies of two dimen- 

 sions, and the same was done by Smaasen ^ not only in a plane 

 but in bodies of three demensions. The most remarkable con- 

 firmation of Ohm's law was made in 1876 § by experiments, 

 suggested by Maxwell and performed by Chrystal in the 

 Cavendish Laboratory, Cambridge, " in which the testing of this 

 law seems to have been carried to the limit of experimental 

 resources." 



Though Ohm's law has thus received such ample verification 

 that it ranks with the best established laws of nature, yet, as 

 Maxwell says, " Ohm's law must, at least at present, be con- 

 sidered a purely empirical one. No attempt to deduce it from 

 pure dynamical principles has as yet been successful. . . . 

 The conduction of electricity through a resisting medium is a 

 process in which part of the energy of an electric current, 

 flowing in a definite direction, is spent in imparting to the 

 molecules of the medium that irregular agitation which we 

 call heat. To calculate from any hypothesis as to the molec- 

 ular constitution of the medium at what rate the energy of a 

 given current would be spent in this way, would require a far 

 more perfect knowledge of the dynamical theory of bodies 

 than we at present possess. It is only by experiments that we 

 can determine the laws of processes of which we do not under- 

 stand the dynamical theory." 



*Pogg. Ann., t. lxiv, 1845, and t. lxvii, 184(3. 



\ Pogg. Ann., t. xcvii, 1856. 



i Pogg. Ann., t lxiv, and t. lxxii. 



§Brit. Assoc. Kept., 1876, p. 36. 



