ON ohm's law. 39 



tho quantity of electricity that passes through any section of the conductor in 



unit time, measured either by a galvanometer or by a voltameter, bo C,»thcn, 



E 

 according to Ohm's law*, - is directly proportional to the length of tho con- 



ductor, and inversely proportional to the area of its section. 



The coefficient of proportionality for a definite-^ substance depends merely 

 on the temperature of the substance ; for unit length and unit section of a 



given substance the value of the ratio — for a given temperature is called the 



\j 



specific resistance of the substance for that temperature, and is one of the 



most important of its physical constants. 



This law has been directly verified by its discoverer, and by Becquerel, 

 Davy, Fechncr, Kohlrausch, and others ; and indirectly it has been verified 

 for a great variety of substances with a degree of accuracy approached in few 

 physical measurements. 



Lately, in discussing some experiments of his own, Dr. Schuster has raised 

 the question whether after all Ohm's law is only an approximation, the limit 



of whoso accuracy lies within the region of experiment. We might suppose 



■pi 

 that tho ratio -^, J was some function of C", say 



c 



where R is a constant very nearly equal to what has hitherto been called the 

 specific resistance, and S is a small constant which, according to Dr. Schuster's 



suggestion, would be positive. It is clear that — call only be an even function 



\j 



of C, unless we admit unilateral conductivity, for which there is no experi- 

 mental evidence in a purely metallic circuit. 



A Committee of the British Association, appointed to consider the subject, 

 were of opinion that it Avas of importance to attempt a further experimental 

 verification of Ohm's law. 



At the suggestion of Professor Maxwell, the experimental details of two 

 methods of verification proposed by him were undertaken by the writer of 

 this Report. Of the two experiments representing these methods the second 

 is by far the most conclusive. It not only avoids the difiiculty of eliminating 

 temperature effects, which to a certain extent interfere with the first experi- 

 ment, but it pushes tho verification of Ohm's law very near the natural limit 

 of all such verifications, viz. the limit of the solid continuity of the conductor. 

 It has thus been rendered proba))le that experiment cannot detect any 

 deviation from Ohm's law, cither in the direction indicated by Dr. Schuster, 

 or in the opposite direction as suggested by Weber, even in wires that have 

 been brought by the electric current to a temperature beyond red heat. 



A third experiment was also tried by the writer of this Report ; its result 

 agreed with the others, but, owing to certain peculiarities, it is less conclusive 

 than they are. It led, however, to interesting results of another kind, which 



* The current is Bupposecl to be steady. 



t By definite is meant in a given physical condition, except as regards E.M.F. and flow 

 of E, and temperature. The last is excepted because we are brought face to face with 

 possible temperature variations iii the first experiment. 



X We suppose tlie conductor to be of unit length and unit section. It is of course the 

 specific resistance which is in question ; and this, if variable, will depend on the current 

 per unit of section. 



