ELECTRICAL MEASUREMENTS. 



279 



by means of which any desired known resistance may be intro- 

 duced into a circuit. The usual construction of the resistance box 

 is as follows : A series of massive metal blocks are connected by 

 wires whose resistances are I, 2, 2, 5, 10, 10, 20, 50 ohms, etc., 

 respectively. By means of conical metal plugs which fit snugly 

 between the blocks, the blocks may be connected at pleasure, 

 leaving the resistance between them approximately equal to zero. 

 Figure 206 shows the essential features of this construction. 



Fig. 205. 



Fig. 206. 



154. Measurement of resistance by Wheatstone's bridge. 



Wheatstone's bridge consists of a net-work of conductors, as 

 shown in Fig. 206. A battery circuit branches at the points a 

 and b, and the current flows through the four resistances a, /3, 

 7 and S, as shown. A sensitive galvanometer G is connected 

 between the points c and d. When no current flows through 

 the galvanometer the four resistances a, /3, 7 and B satisfy the 

 equation 



H . 



The method of using this arrangement for the measurement of 

 current is explained in Arts. 155 and 156. 



Proof of equation (82). Let i' be the current flowing through 

 a and /3 (the same current flows through a and /3, since the 

 galvanometer current is zero) and let i" be the current flowing 

 through 7 and S. Inasmuch as there is no current flowing 



