[From the British Association Report, 1876.] 



LXXXI. On Ohm's Laio. 



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 electricity 

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

 and adopted it. 



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

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

 imperfectly explained by tedious and 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 is constant 

 that is, that its value does not depend on that of the electromotive 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 depend on the strength of the current. 



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

 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 electro- 



* Pogg. Ann. 1875. 



