THE MEASUREMENT OF CURRENT IB 



Galvanometer Constant. It is natural to wind the coils so 

 that their cross-sections will be rectangular. Let n' be the num- 

 ber of turns of wire per square centimeter of cross-section, the 

 other quantities being denned by Fig. 8. Then, if the coil is 

 wound throughout with the same size of wire the field at its 

 center due to unit current is 



cot. 



G = 4im'6'log e 



cot 



FIG. 8. Pertaining to demonstration for galvanometer constant. 



For, consider the filament of current, dx dy, the coordinates of 

 whose trace on the plane of the paper are x and y. The magnetic 

 force at the center of the coil due to the length ds of this filament 

 when conveying unit current, will by the elementary laws of 

 electromagnetic action be perpendicular to the radius vector of 

 the point, x, y, and have for its value 



n'dsdydx 



When resolved along the axis this becomes 



