170 SCIENTIFIC APPARATUS. 



circumference of a circle of unit radius, produces a magnetic force 

 equal to unity at the centre of the circle. Now when the same 

 quantity of electricity is measured, on the one hand, by a method 

 based on electrostatic phenomena, or, on the other hand, by a 

 method based on electromagnetic phenomena, it is found that the 

 numerical value obtained in the former case is, in round numbers, 

 3 X io 10 (thirty thousand million) times as great as the numerical 

 value obtained in the second case. In other words, the quantity 

 of electricity which is denoted by unity in calculations based on 

 electromagnetic phenomena is nearly thirty thousand million times 

 as great as the quantity denoted by unity when electrostatic pheno- 

 mena are taken as the basis of measurement. An experimental 

 determination of the ratio between these two units was first made 

 by Weber and R. Kohlrausch (1857); subsequent determinations 

 have been made by Sir William Thomson (1868 and 1873), and 

 by Clerk Maxwell (1869). 



A more definite idea of the nature of the relation between these 

 two units of electric quantity may perhaps be derived from the 

 following considerations. Suppose an electrostatic unit of posi- 

 tive electricity to be carried round the circumference of a circle of 

 one centimetre radius with a uniform velocity of v centimetres 

 per second ; the effect at the centre of the circle will be the same 

 as if a conductor v centimetres long were wrapped round the 

 circle and traversed by a current conveying one electrostatic unit 

 of electricity past each point of it in one second. But a con- 

 ductor of length v conveying one unit of electricity per second 

 would produce the same effect as a conductor of unit length con- 

 veying v units of electricity per second; therefore, in order that 

 the magnetic force at the centre of the circle may be unity, the 

 numerical measure of the velocity v must be equal to the number 

 of electrostatic units in one electromagnetic unit. 



The various effects which a current of electricity can produce 

 represent the various forms under which the work done by it can 

 show itself. It has been already said (page 168) that the work 



