212 214] ELECTROMAGNETISM. 417 



a million. These units form a consistent system, so that electrical 

 relations involving quantities measured in these units require no 

 numerical factors. For instance, a current of one ampere is 

 produced when an electromotive force of one volt is impressed in 

 a circuit whose resistance is one ohm, and the activity of one watt 

 thereby exerted dissipates energy at the rate of one joule per 

 second. 



214. Electrostatic compared with Practical Units. From 

 the above definitions with the value of v given and equation (9') 

 we find 



1 C.G.S. electrostatic unit of Potential = 300 Volts. 

 From (10') 



1 Farad contains 9'10 U C.G.S. electrostatic units of Capacity. 



The electrostatic unit of capacity is the unit of length, accordingly 



1 Microfarad = 900,000 cm. of Capacity. 



A sphere of nine kilometers radius in free space would have a 

 capacity of one microfarad. From (6) 



1 Coulomb = 3'10 9 C.G.S. electrostatic units of Electric Charge. 

 From (ii 7 ) 



900,000 Megohms = 1 C.G.S. electrostatic unit of Resistance. 



From equation (7) we may find the dimensions of e and m, when 

 those of e and /* are settled upon. Any convention that may be 

 made gives us a possible system of units. It must be noticed, 

 however, that there is always a relation between the dimensions of 

 e and p, From equations (4) and (5) 



Squaring this and dividing by 



we obtain 



w. E. 27 



