20 HEYL & BRIGGS— THE EARTH INDUCTOR COMPASS. 



known as the dial switchboard, which is an important part of this 

 invention. 



Such a switchboard may assume various forms. The form 

 adopted in practice is very simple in actual construction, although not 

 so simple in mathematical theory as certain more complicated practical 

 forms that preceded it in the course of its development. Its theory 

 is illustrated in Figs. i-8. 



In Fig. I we have a closed circuit of resistance, the sine and cosine 

 voltages being connected to the circle at four equally spaced points 

 zt S, ± C. The galvanometer connections are made at any two 

 diametrically opposite points P, Q, of the circle. Indicating, as in 

 Fig. 2, positive potential by a line drawn radially outward from the 

 resistance circle, and negative potential by a line drawn inward, and 

 supposing the vessel to lie in an east and west line, so that the sine 

 voltage is a maximum and the cosine voltage zero, we have a distri- 

 bution of potential around the circle indicated by the dotted line. The 

 only diametrically opposite points which will be at equal potentials 

 are those at the ends of the horizontal diameter. This, then, will be 

 the null position for an east and west course. 



If, on the other hand, the sine voltage is zero and the cosine 

 voltage a maximum, as is the case in a north-south course, the null 

 position will be that of the vertical diameter. 



If the vessel lies in a northwest and southeast line, 6 = — 45 '^ 

 (or 135°), sin 6 is negative and cos 9 positive, both, however, being 

 equal in absolute numerical value. Maintaining the connections 

 shown in Fig. i, but changing the sign of S, the distribution of 

 potential will be as shown in Fig. 3. Points + 6^ and -f- C are at 

 equal potentials ; between them we have a level of potential, and no 

 current flows in this quadrant. The same is true of the quadrant 

 between — 5^ and — C. But between -\- S and — C (and + C and 



— 5") a fall of potential exists. Symmetry indicates that the only 

 equipotential points for the galvanometer leads lie on a line making 



— 45° with the vertical diameter; that is, pointing northwest with 

 reference to the cardinal points previously determined (Fig. 2). 



Let the vessel now veer a little farther to the west ; the sine voltage 

 will increase (negatively) and the cosine voltage decrease (posi- 



