164 ELECTRICAL MEASUREMENTS 



may be determined. M and N are called the balance or ratio 

 arms and P the rheostat arm of the bridge. 



Auxiliary Apparatus. Besides the bridge box, the other neces- 

 sary pieces of apparatus are the battery (usually two or three 

 cells), the keys K B and K 0) the galvanometer, shunt, and com- 

 mutator, see Fig. 89. 



Keys. Keys K B and K a are usually combined to form what is 

 called a bridge key, which when depressed throws in first the 

 battery and then the galvanometer (to eliminate the effects of 

 inductance and capacity). To avoid the chance of leakage to the 

 galvanometer and also thermo-electric effects, care must be taken 

 when manipulating this key not to touch the metal work. The 

 commutator H is used to reverse the battery current and so to 

 eliminate the effects of thermo-electromotive forces. 



The Galvanometer. The galvanometer should be one which is 

 not affected by variations of the local field and, if possible, should 

 be critically damped; either a shielded Thomson or a D' Arson val 

 instrument may be used. 



In selecting a D'Arsonval galvanometer for bridge work, the 

 peculiarities of the instrument should be considered ; for, suppose 

 the resistance in the bridge arms between the galvanometer 

 terminals is low and that the instrument is one, which, for critical 

 damping, requires that it be in series with a high external resist- 

 ance. On the passage of the current, as soon as the coil begins 

 to move, an e.m.f . will be set up in the circuit and the motion will 

 be damped. Consequently instead of a sharp, decided movement 

 of the index the motion will be so deliberate as to greatly increase 

 the difficulty of deciding when the bridge is in balance. Obviously 

 it is impossible to select a galvanometer which will be critically 

 damped for all combinations of the bridge arms, but with care a 

 good working compromise may be secured. In general, with a 

 given sensitivity, the shorter the period of the instrument, the 

 more satisfactory will its action be. 



The Shunt. The shunt, S, is a bypass for the current and is 

 placed between the galvanometer terminals. It is used during 

 preliminary adjustments to protect the galvanometer against 

 currents of abnormal strength. By means of the movable arm 

 the value of the shunt resistance may be altered so that as the 

 adjustment of the bridge nears perfection, a greater proportion 



