MEASUREMENT OF A QUANTITY OF ELECTRICITY. 553 



CHAPTER VIII. 

 RATIO OF UNITS. 



1129. VARIOUS METHODS. The ratio a of the electromagnetic 

 unit of electricity to the electrostatic unit (610) is the ratio of a 

 length to a time, and is therefore of the same nature as a velocity ; 

 it is a quantity the absolute value of which is independent of the 

 choice of the fundamental units of length, mass, and time. The 

 experimental determination of this ratio leads then to the com- 

 parison of a length and of a time ; and in particular, as a resistance 

 is equal in electromagnetic measure to a velocity, it may be ex- 

 pressed as a function of the resistance of a circuit. 



It is evident, moreover, from the series of equalities 



E 17 f& 



that there are as many methods of determining this constant a as 

 there are quantities which can be measured at once in electrostatic 

 and in electromagnetic units. Nevertheless, these methods are not 

 entirely distinct, and often lead to the same experimental measure- 

 ments ; we may reduce them to three principal methods, corre- 

 sponding to the only quantities which can be measured directly in 

 electrostatic units that is to say, an electrical charge, a difference 

 of potential, or a capacity. 



1130. MEASUREMENT OF A QUANTITY OF ELECTRICITY. The 

 former researches on this subject are due to Weber and to Kohl- 

 rausch ;* we shall mention them in detail, in consequence of their 



* WEBER and KOHLRAUSCH. Electrodyn. Maasbestim., Abh. der Konigl. 

 Sachs. Gesellschaft der Wissensch., Vol. v., p. 219. 1856. 



