THE MEASUREMENT OF CURRENT 51 



at any desired height by means of set screws. The lower ends 

 of these rods are provided with levelling screws. The weight 

 of the arrangement is about 45 pounds. 



To make the necessary adjustments the device is levelled and 

 the galvanometer put in place, levelled and firmly clamped in posi- 

 tion. The axis of the suspended system should be in the vertical 

 axis of the arrangement, for symmetry is important. 



The platform is now raised until the point of attachment of 

 the suspension fiber to the frame of the instrument is in the 

 plane passing through the necks of the hooks. It is then clamped 

 in position. 



The whole device, galvanometer and all, (it may be necessary 

 to remove the suspended system) is now hung by one of the hooks 

 and the weights adjusted until the rods which are normally in a 

 vertical position are truly horizontal; the weights should be at 

 equal distances from the upper ends of the rods. These adjust- 

 ments insure that the point of attachment of the fiber and the 

 center of gravity of the whole arrangement are at the same point 

 and in the plane of support. 



The device may now be put in position, the three suspension 

 wires, of equal length, attached to the hooks, and levelling 

 screws raised, leaving the arrangement freely suspended. It is 

 well, for convenience of adjustment, to attach the wires to the 

 frame at their lower ends by small turn-buckles. Complete 

 shielding from draughts is essential. To prevent serious results 

 arising from the breaking of the suspension wires a shelf should 

 be placed immediately below the device. 



SHUNTS 



In using galvanometers, it is often found either that the instru- 

 ments are too sensitive or that their carrying capacities are 

 insufficient. In such cases, shunts placed between the terminals 

 of the galvanometer and acting as bypasses for the current, are 

 employed (see Fig. 20). 



When zero methods are used, shunts are resorted to for the 

 purpose of protecting the galvanometers during preliminary ad- 

 justments. Much time is thus saved, for the violence of the 

 deflection and consequently the time necessary for the needle to 



