ON ohm's law. 49 



On the whole, therefore, we cannot conclude that there was any denation 

 fronti Ohm's law under the circumstances of this experiment. It is hardly 

 worth while to estimate the value of this experiment quantitatively, as the 

 second experiment now to be described is so far superior in this respect. 



Second Experiment. 

 Introduction, by Prof. Maxwell. 



The service rendered to electrical science by Dr. G. S. Ohm can only be 

 rightly estimated when we compare the language of those writers on electri- 

 city who were ignorant of Ohm's law with that of those who have understood 

 and adopted it. 



By the former, electric currents arc said to vary as regards both their 

 " quantity " and their " intensity," two qualities the nature of which was 

 very imperfecly explained by tedious aud vague expositions. 



In the writings of the latter, after the elementary terms " Electromotive 

 Force," " Strength of Current," and " Electric Resistance " have been defined, 

 the whole doctrine of currents becomes distinct and plain. 



Ohm's law may be stated thus ; — 



The electromotive force which must act on a homogeneous conductor in 

 order to maintain a given steady current through it, is numerically equal to 

 the product of the resistance of the conductor into the strength of the current 

 through it. If, therefore, we define the resistance of a conductor as the 

 ratio of the numerical value of the electromotive force to the numerical 

 value of the strength of the current, Ohm's law asserts that this ratio ig 

 constant — that is, that its value does not depend on that of the electro- 

 motive force or of the current. 



The resistance, as thus defined, depends on the nature and form of the 

 conductor, and on its physical condition as regards temperature, strain, &c. ; 

 but if Ohm's law is true, it does not dejiend on the strength of the current. 



Ohm's law must, at least at present, be considered a purely emjiirical 

 one. No attempt to deduce it from pure dynamical principles has as yet 

 been successful ; indeed Weber's latest theoretical investigations * on this 

 subject have led him to suspect that Ohm's law is not true, but that, as the 

 electromotive force increases without limit, the current increases slower and 

 slower, so that the " resistance," as defined by Ohm's law, would increase 

 with the electromotive force. On the other hand, Schuster t 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. 



LorentzJ, 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 



* Pogg. Anu. 1875. t Eeport of British Association, 1874. 



X Over de Terugkaatsiug en Breking van het Licht. Leiden, 1875. 

 1876. B 



