24 HEYL & BRIGGS— THE EARTH INDUCTOR COMPASS. 



Which shows that for a square frame 4> = 9, at least for the first 

 quadrant. It may readily be shown that the same relation holds in 

 the second quadrant as well, as indeed is indicated by conditions of 

 symmetry. 



Upon this theory may be based a very simple practical construction 

 for the dial switchboard, illustrated in Fig. 9 (photograph). 



Fig. ga shows the switchboard base with dial removed, showing 

 the square resistance frame. This frame has an inside diameter of 



6.5 cm. and each turn of wire is nearly 2 cm. long. It is wound with 

 No. 30 constantan wire, and each arm has a resistance of about 37 

 ohms. The inner edge of the square frame stands slightly above the 



