ELECTROSTATIC SYSTEM. 589 



Electrostatic capacity is therefore a length, as we have already 

 seen (73). 



Strength of current [/']. The strength of a current is the quantity 

 of electricity which traverses the section of a wire in unit time, 

 or the quotient of a quantity ^, by the time which it takes to 

 traverse this section, We have accordingly 



or 



Resistance \r\. The resistance of a conductor is defined by 

 Ohm's law (204). It is the quotient of the electromotive force 

 between two points, by the strength of the current, which gives 



-., and 



Electrostatic resistance is therefore the inverse of a velocity, 

 as has already been proved (206). 



Quantity of magnetism [/]. In the electrostatic system, the 

 quantity of magnetism is defined by the condition that the action 

 of a magnetic pole of mass q', on a portion of current whose strength 

 is / and whose length / is very small, at a distance d from the pole, 

 and perpendicular to the right line which joins its centre to the 

 pole, shall be defined by the elementary law (458) 



or 



from which is deduced 



Magnetic density [o-'J. The surface density being the quantity of 

 magnetism for unit surface, we have 





