124 



ELECTRICAL MEASUREMENTS 



or cables, the universal shunt (page 52) should be usc.l. The 

 advantage of this arrangement is that the resistance through 

 which the damping current, set up by the motion of the; coil, 

 must flow is always the same. Consequently, X does not vary, 

 even though the multiplying power of the shunt be changed. 

 Another advantage is that the total shunt resistance, r, may be 

 made so great that the instrument is not over-damped, even 

 though it is heavily shunted. 



Obviously the universal shunt loses its advantages, if the con- 

 denser be replaced by a closed circuit, as an exploring coil for 

 magnetic work. 



FIG. 56. Flux meter. 



The Flux Meter. The flux meter, 6 as its name indicates, is 

 designed for measuring the flux in magnetic circuits. This 

 instrument is essentially a moving coil ballistic galvanometc r in 

 which the restoring moment is reduced to a minimum by the 

 removal of the controlling spring. In a perfect instrument the 

 restoring couple would be zero; practically it is difficult to reduce 

 the controlling action of the necessary leads to the movable coil 

 to a negligible amount and usually after the movable system has 

 been deflected it very gradually sinks back towards zero. 



Fig. 56 shows the instrument and the suspended system. 



