56 ELECTRICAL MEASUREMENTS 



be made without opening the case, by turning a slotted head 

 at the front of the base. The pointer is an aluminum tube 

 flattened at the index end. The moving parts are balanced by 

 three adjustable counterweights on short arms which project at 

 right angles to the axis of rotation and are in the plane of motion 

 of the pointer; adjustment is made by moving the weights along 

 the arms. Parallax is eliminated by the use of a mirror beneath 

 the pointer 



Line 



Line 



FIG. 22. Diagram for Weston moving-coil ammeter. 



The graduation of the scale is practically uniform, but no par- 

 ticular law of deflection is assumed. The principal points are 

 determined by comparison with a standard instrument and the 

 subdivision is done by a dividing engine. 



A small resistance coil is included in the galvanometer circuit, 

 and the final adjustment is made by altering its resistance. 



For self-contained portable instruments, the present practice 

 of the Weston Instrument Co. is to use in milliammeters, up to 



