202 



ELECTRICAL MEASUREMENTS 



1 X 10 9 and of approximately 1,000 ohms resistance will be 

 satisfactory. This galvanometer must have a good law of 

 deflection; that is, the deflection must be proportional to the 

 current, and it must have a definite zero reading. It should 

 be so supported that it is free from mechanical vibration and 

 thoroughly insulated electrically. Ci is an ordinary four-part 

 commutator for reversing the galvanometer current, as it is 

 necessary to keep the deflections in the same direction on account 

 of possible irregularities of the zero reading due to the coil being 



FIG. 114. Universal shunt box. 



slightly magnetic. The shunt S should be of the Ayrton uni- 

 versal type, for by selecting one of the proper resistance the 

 galvanometer may be critically damped, thus enabling the 

 readings to be taken in the shortest possible time. With this 

 type of shunt, when used in the manner indicated, the damping 

 of the galvanometer is independent of the multiplying power, 

 consequently the instrument will not be overdamped even 

 though it be heavily shunted. Also, though there will usually 

 be some thermal electromotive forces due to inequalities of 

 temperature in the galvanometer circuit, G, Ci, and S, no dif- 



