THE MEASUREMENT OF RESISTANCE 203 



ficulty will arise, as this circuit is of constant resistance; the only 

 effect will be that the deflections will be read from a zero which 

 may differ slightly from the mechanical zero of the instrument. 



R is a fixed known resistance of 100,000 ohms or J-f o megohm. 



An exceedingly convenient form of Ayrton shunt is shown in 

 Fig. 1 14. The constant resistance in the box between the galvan- 

 ometer terminals is 10,000 ohms. By means of the four handles, 

 the movable terminal may be carried from to 10,000 by steps 

 each one of which changes the multiplying power by ^fo.ooo 

 part of the value it has when all the slides are at the extreme 

 right hand, which value is taken as unity; for with the Ayrton- 

 Mather arrangement we are concerned only with relative 

 multiplying powers. The multiplying power to be used is 

 obtained by dividing 10,000 by the sum of the readings of the 

 four slides. The three posts at the rear are the shunt terminals; 

 the four in the second line connect with the resistance R, of 

 100,000 ohms, which is divided into three sections of 10,000, 

 30,000 and 60,000 ohms respectively. 



The key K 2 (Fig. 113) is kept in the position shown, by a 

 spring; when in this position no current can flow through the 

 galvanometer and the instrument is thoroughly protected. If 

 the key be held in the dotted position the galvanometer is in 

 service. The construction should be such that the circuit is 

 not broken when the key is thrown from one position to the 

 other. 



The key K\ serves to throw the cable in circuit and, when in the 

 position shown, short-circuits it, thus ensuring thorough dis- 

 charge. The battery B is connected to one side of a double- 

 pole, double-throw switch Cz, the other side being short-circuited 

 to enable discharge deflections to be taken. One of the middle 

 connections of C% is carried to the tank plate P, the other to the 

 middle post of K 2 . The battery should be fairly well insulated 

 to prevent its running down. It must be capable of giving a 

 constant e.m.f. of, possibly, 500 volts. 



It is usual to connect the negative pole of the battery to 

 the core of the cable, the idea being that with this connection 

 the effect of electrolysis is to open up any fault which may exist 

 in the insulation. Many specifications require tests with both 



