UNIT OF RESISTANCE. 307 



CHAPTER III. 

 COMPARISON OF RESISTANCES. 



919. The resistance of a linear conductor is the quotient of the 

 difference of potential at the two ends by the strength of the current 

 which traverses it. The measurement of a resistance in absolute 

 values requires then the knowledge of an electromotive force and 

 of a current ; but in order to compare two resistances, it is sufficient 

 to compare the currents which correspond to the same electromotive 

 force, or the electromotive forces which correspond to the same 

 current. 



We shall only concern ourselves for the present with comparative 

 measurements. They are of special importance ; for the resistance 

 is the only electrical quantity which" it is possible to represent by 

 standards of great fixity and of easy use, as for lengths and weights. 

 Resistances are comparable with masses. A current is as necessary 

 for the comparison of two resistances, as a force such as gravity is for 

 comparing two masses. In either case the ratio obtained is inde- 

 pendent of the intensity of the action put in play, provided it is 

 the same for both terms. 



920. UNIT OF RESISTANCE. LEGAL OHM. The standard of 

 resistance may be chosen arbitrarily. Jacobi* proposed to use a 

 copper wire of known dimensions, and, in order to avoid errors 

 arising from the unequal purity of the metal, he proposed to dis- 

 tribute specimens of this wire among physicists. 



For a long time telegraphists took as unit a kilometre or a 

 mile of copper wire of known diameter, but at present more exact 

 measurements are required for industrial purposes; for the least 

 trace of foreign substances, and physical changes such as tempering 

 or twisting, so greatly modify the conductivity of a metal that 

 the. nature and dimensions of a wire are insufficient to define the 

 resistance ; the temperature, moreover, has a considerable influence. 



* JACOBI. Comptes rcndus, Vol. xxxm., p. 277. 1851. 



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