508 Prof. Maxwell and Mr. P. Jenkin on the Elementary 



electricity in electromagnetic measure is exceedingly small ; but 

 when the capacity of the conductor is great, as in the case of 

 long submarine cables, the charge may be considerable. By 

 making use of the electromotive force produced by the friction of 

 unlike substances, the charge or electrification even of small 

 bodies may be made to produce visible effects. The electricity 

 in a charge is not essentially in motion, as is the case with the 

 electricity in a current. In other words, a charge may be per- 

 manently maintained without the performance of work. Elec- 

 tricity in this condition is therefore frequently spoken of as sta- 

 tical electricity; and its effects, to distinguish them from those 

 produced by currents, may be called statical effects. The pecu- 

 liar properties of electrically charged bodies are these : — 



1. When one body is charged positively (14), some other 

 body or bodies must be charged negatively to the same extent. 



2. Two bodies repel one another when both are charged 

 positively, or both negatively, and attract when oppositely 

 charged. 



3. These forces are inversely proportional to the square of the 

 distance of the attracting or repelling charges of electricity. 



4. If a body electrified in any given invariable manner be 

 placed in the neighbourhood of any number of electrified bodies, 

 it will experience a force which is the resultant of the forces that 

 would be separately exerted upon it by the different bodies if 

 they were placed in succession in the positions which they 

 actually occupy, without any alteration in their electrical con- 

 ditions. 



From these propositions it follows that, at a given distance, 

 the force,/*, with which two small electrified bodies repel one 

 another is proportional to the product of the charges, q and q v 

 upon them ; but when the distance varies, this force, /, is in- 

 versely proportional to the square of the distance, d, between 

 them. Hence 



/=f (17) 



When q and q 1 are of dissimilar signs, /becomes negative; i. e. 

 there is an attraction, and not a repulsion. This equation is 

 incompatible with the electromagnetic definitions given in Part 

 III., and, if it be allowed to be fundamental, gives a new defi- 

 nition of the unit quantity of electricity, as that quantity which, 

 if placed at unit distance from another equal quantity of the 

 same kind, repels it with unit force. 



34. Electrostatic System of Units. — This new measurement 

 of quantity forms the foundation of a distinct system or series 

 of units, which may be called the electrostatic units; and mea- 



