252 ELECTROSTATICS. [PT. II. CH. VI. 



is the work that must be done against the electrical forces in bring- 

 ing a unit of positive electricity from an infinite distance to the 

 given point. Positive electricity tends to move from places of 

 greater to places of less potential, and negative electricity the 

 contrary. 



The unit of electricity must be defined as the amount of 

 electricity, which if concentrated on a very small body, will repel 

 a body similarly charged and placed at unit distance from it in 

 vacuo, with unit force. This unit is the basis of the electrostatic 

 system of electrical units. In the c.G.s. system, the unit of elec- 

 tricity repels a similar unit at a distance of one centimeter with a 

 force of one dyne. 



It is necessary, as shown by Experiment VIII, to specify the 

 medium in defining the unit. If air were adopted instead of 

 vacuum, that is, ether, the difference would be so slight* as to 

 escape detection by all but the most refined electrostatic measure- 

 ments at present in use, hence we may with very slight error 

 consider all experiments to be made in air. 



The dimensions of the unit of electricity are found from the 

 equation of force 



= Force, 



and the C.G.s. unit of electricity is 



1 gm.^ cm.* sec." 1 . 

 The dimensions of p, <r, V are found from 



T . Electrification [e] r ^\T 



[p] = Volume density = - Volume - = ^ = [M*L 



r -i a r j -j. Electrification [e] 

 M = Surface density = - = = 



[V] = Potential = 



Distance [L] 



. * Less than one part in a thousand. 



