ELECTRICAL MEASUREMENTS. 



LECTURE II. 



THE measurements connected with electrical currents 

 dynamical electricity are chiefly three. We have 1. 

 The measurement of the Strength of Currents, or, to express 

 it more shortly, the measurement of electric currents ; 

 2. The measurement of Electromotive Force ; and 3. The 

 measurement of the Resistance of Conductors. The measure- 

 ment of resistance or of conducting power comes to the 

 same thing, for the relation between these two properties 

 is a reciprocal one. If we know the conducting power of 

 any piece of wire or other conductor, we know that the 

 resistance is equal to 1 -^ the conducting power or the 

 conducting power is equal to 1 -f- the resistance ; so that 

 if the conducting power of a wire is one-half, its resistance 

 is two ; and if the resistance is one-half, the conducting 

 power is two, and so on. One is the reciprocal of the other, 

 so that whether we speak of the measurement of resistance 

 or of conducting power it is essentially the same thing. A 

 statement of the one is a statement in another form of the 

 other. 



What I shall try to explain to-day, as far as time and 

 circumstances permit, is the methods by which absolute 

 measures of these three quantities can be obtained. 

 When I speak of the absolute measure of a current, I 

 mean a measurement which tells us, not that one current, 

 for instance, is twice or three times as strong as some 

 other current, but which tells us the actual quantity of 

 electricity conveyed by the current in unit of time. The 

 idea attached to the term strength of current in the case 

 of electricity is exactly comparable to the idea attached to 



