1 92 PROPAGATION OF ELECTRICITY. 



The ratio - represents the resistance of a cube equal to unity 



parallel to one edge ; it may be called the specific resistance of the 

 conductor. 



The resistance of a conductor is a magnitude of the same kind 

 as the inverse of a velocity in mechanics. 



For we have 



V -V 



M MJ 



The difference of potential V l - V 2 is equal to the quotient of an 

 electrical mass M by a length a-, the strength of a current, or the 

 flow during unit time, is equal to the ratio of the quantity of elec- 

 tricity M', which flows in the time /, to the corresponding time. We 

 have then 



The quotient is an abstract number, and the ratio - is a 



M / 



velocity. The resistance r is therefore the inverse of a velocity. 



206. We may, indeed, imagine an experiment in which this 

 velocity would have a physical meaning. 



Consider a sphere, of radius R, charged with a mass of electricity 

 M, and let us suppose this sphere connected with the earth by a 

 conductor of resistance r. 



M 



The potential of this sphere is equal to ; it diminishes as soon 



is. 



as it is connected with the earth ; but if the sphere contracts at the 

 same time as the charge diminishes, it may happen that the potential 

 remains constant 



This condition being realised, then if ^M is the loss of charge of 

 the sphere, and aTR the diminution of the radius in the time dt, 



M_M 

 ~~lf ~ R- 



but from Ohm's law, 



dlli^-dt. 



