PRELIMINARY NOTIONS. 9 



tbe passage of the current. Whatever may be the conductor 

 connecting two bodies charged with electricity at different 

 potentials, a certain time must lapse before the equilibrium is 

 established. The quantity of electricity flowing in one second 

 between two points, the electromotive force of the current being 

 maintained constant, depends on the resistance of the conductor 

 connecting these two points. The same principle can be ex- 

 pressed under the following form : The resistance of conductors 

 is proportional to the speed of the current. 



From the foregoing it will be seen that the electric resistance 

 is the property possessed by conductors to cause variations in 

 the intensity of a current produced by a given electromotive 

 force. The resistance of a body is directly proportional to its 

 length and inversely to its section and to its conductivity.* 



The specific resistance of a substance is the electric resistance 

 of that substance estimated in relation to the units of length and 

 of volume ; it is that which would be offered by a prismatic 

 conductor, made of the said substance, and the section of 

 which would be equal to the unit of surface. Adopting the 

 centimetre as the unit of length it would be that of a cubic 

 centimetre, two opposite surfaces of which were maintained at 

 constant potentials. The specific resistance can also be esti- 

 mated in relation to the unit of mass ; it is then the resistance 

 of a conductor, the mass of which weighs one gramme, and of a 

 section of one square centimetre. 



The resistance is frequently expressed, in theoretical calcu- 

 lations, by the relation of a time to a length, that is to say, by 

 the converse of a speed. 



CONDUCTIVITY. Conductivity is, as its name indicates, the 

 property possessed by substances to act as vehicles of electricity ; 

 it is the converse of resistance. Calling E the resistance, the 



conductivity will be represented by-.~, and, since the resistance 



JTV 



corresponds to the converse of a speed, the conductivity may be 

 expressed in a speed or by the relation of a length to a time. 



THEOKETICAL ELECTEICAL UNITS. In order to compare 

 electric currents, units must be used. The number of these 



* ' Des Grandeurs electriques,' par E. E. Blavier. 



