.. ON OHM'S LAW. 



motive ferae increase* without limit, the current increases slower and slower, 

 so H the "resistance," M defined by Ohm's law, would increase with the 

 electromotive force. On the other hand, Schuster* has described experiments 

 which lead him to suspect a deviation from Ohm's law, but in the opposite 

 direction, the resistance being smaller for great currents than for small ones. 



Lorenti*, of Leyden, has also proposed a theory according to which Ohm's 

 law would cease to be true for rapidly varying currents. The rapidity of 

 variation, however, which, as he supposes, would cause a perceptible deviation 

 from Ohm's law, must be comparable with the rate of vibration of light, so 

 that it would be impossible by any experiments other than optical ones to 

 test this theory. 



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 direc- 

 tion, 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 

 molecular 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 know- 

 ledge of the dynamical theory of bodies than we at present possess. It is 

 only by experiment that we can ascertain the laws of processes of which we 

 do not understand the dynamical theory. 



We therefore define, as the resistance of a conductor, the ratio of the 

 numerical value of the electromotive force to that of the strength of the 

 current, and we have to determine by experiment the conditions which affect 

 the value of this ratio. 



Thus if E denotes the electromotive force acting from one electrode of the 

 conductor to the other, C the strength of the current flowing through the 

 conductor, and R the resistance of the current, we have by definition 



B -c- 



and if H is the heat generated in the time t, and if J is the dynamical 

 equivalent of heat, we have by the principle of conservation of energy 



* Report of British Auociation, 1874. 



t Over de Terugkaatging en Brdcing van het Lie/a. Leiden, 1875. 



