HEYL & BRIGGS-THE EARTH INDUCTOR COMPASS. 19 



In general, since 



w sin ^ -{- w cos ^ = o 

 if tan e = — (n/m), the galvanometer reading may be made zero in 



.•f-C 



-c 



Fjq.y 



any desired direction by combining, additively or subtractively, suit- 

 able fractions or multiples of the voltages from the two pairs of 

 brushes. This is the underlying principle of the course-setting device 



