Till-: MEASUREMENT OF CURRENT 57 



1,500 milliainp., a drop of approximately 150 millivolts at full- 

 detlection. In ammeters having ranges from 2 to 150 amp., 

 the drop is about 50 millivolts; for ranges from 200 to 500 amp., 

 it is 35 millivolts. In all external shunts of the new type the 

 drop is approximately 100 millivolts and in the switchboard 

 shunts it is about 50 millivolts. 



In self-contained instruments the shunts are mounted in the 

 base which, if the current capacity is considerable, is properly 

 ventilated. For large currents the best procedure is to separate 

 the millivoltmeter and the shunt and mount them in different 

 cases; this allows the shunt to be designed so that the heat is 

 readily dissipated. An external-shunt instrument gives a flexible 

 arrangement very convenient for general testing, for shunts of 

 different ranges may be used with the same millivoltmeter. 



FIQ. 23. Switchboard shunt. 



In using any form of shunt and millivoltmeter, it is necessary 

 to calibrate and to use the instrument with the same set of leads 

 connecting the shunt and the millivoltmeter and to avoid all 

 extraneous resistances in the leads due to imperfect contacts at 

 the terminals. 



The moving-coil principle is now universally employed in the 

 best makes of direct-current instruments. Separate millivolt- 

 meters and shunts are universally used in direct-current switch- 

 board work. The shunts are put at any convenient point in the 

 busbars and small leads are run to the indicating part which is 

 on the front of the switchboard. This greatly simplifies the con- 

 struction of the board and reduces expense. 



Fig. 23 shows a switchboard shunt. It will be noted that the 

 resistance strips are very short and are soldered into massive 



