THE MEASUREMENT OF CURRENT 53 



from the external circuit, T M , may be connected at will to points on 

 r which are commonly distant r, ^^, etc., from the fixed termi- 



nal. Referring to the figure, it will be seen that the line current 



is given by 



7? 4- (?*\r + - 

 ' ^ r)n 



n 



/ 7? -i- ^\ 



For any particular galvanometer and shunt box ( - -- j is 



constant. This factor is the multiplying power of the shunt 

 when the movable terminal is at the extreme right of r. 



It is seen that the relative multiplying power is n. The values 

 of n depend on the locations of the taps by which the movable 

 terminal,' T M is connected to r. They are independent of the 

 relative magnitudes of the resistances of the galvanometer 

 and the shunt. The box is graduated in terms of the relative 

 multiplying powers; consequently it may be used with any 

 galvanometer: hence the name universal. However, though the 

 multiplying powers are not affected, the same shunt box cannot 

 be applied indiscriminately to all galvanometers and satisfactory 

 results attained. For the maximum current through the instru- 



ment is I Q = I p , Therefore, in order that practically the 

 K -f- r 



full sensitivity of the galvanometer may be realized, r must be 

 much larger than R G ; if it is nine times the galvanometer resist- 

 ance, 90 per cent, of the sensitivity may be realized. Again, if 

 r is too small, and a moving coil galvanometer is employed, the 

 instrument will be over-damped and, therefore, sluggish in its 

 action. 



The distinct advantage of the universal shunt box is that when 

 it ig used in open-circuit work, any damping due to currents set 

 up by the motion of either the needle or the movable coil of the 

 galvanometer is constant. This is especially important when 

 capacities are being compared by means of the ballistic galva- 

 nometer. 



Also, when a universal shunt is used with a moving-coil galva- 

 nometer in a circuit of very high resistance, it is possible, by prop- 

 erly choosing r, to render the galvanometer dead beat for all 



