HEYL & BRIGGS— THE EARTH INDUCTOR COMPASS. 17 



2. A course-setting device of a novel type is employed. By turn- 

 ing a movable dial carrying compass graduations to the desired course- 

 mark, the electrical connection of the galvanometer to the earth in- 

 ductor is so arranged that the galvanometer will read zero only when 

 the vessel is in the desired line. This device enables the pilot to 

 control a compass situated at a safe distance from magnetic disturb- 

 ances without the use of a moving mechanical connection. 



3. This course-setting device possesses a feature which enables 

 the pilot to distinguish between north and south, or, in general, 

 between the two opposite directions which the vessel rnay take in 

 any line. 



4. A method is provided for eliminating the errors due to rolling 

 and pitching, arising from the vertical component of the earth's field. 



5. By the judicious use of iron in the core of the coil, the size of 

 the earth inductor may be greatly reduced and the current output 

 increased without the introduction of any sensible error. 



In the course of its development this instrument has naturally 

 passed through several forms. Only the final form will here be 

 described. 



Current is generated by the rotation in the earth's horizontal field 

 of an armature rotating about a vertical axis. A commutator and 

 four collecting brushes, spaced at 90°, take off current from the 

 armature. For simplicity we may first suppose the vessel to lie in 

 the magnetic meridian. The brush system as a whole may then be 

 turned to such a position that one pair of brushes furnishes no 

 electromotive force and the other pair a maximum. The brush sys- 

 tem may now be fixed in this position with respect to the vessel. If 

 now the vessel be turned through an angle 9 from the meridian, the 

 two pairs of brushes will furnish electromotive forces proportional 

 respectively to sin 6 and cos 0. In this it is assumed that 6 is meas- 

 ured positively in a clockwise direction from the meridian, and that 

 the commutator connections are so made that the algebraic sign of 

 the voltage at each pair of brushes will be that proper for the sine 

 or cosine of the angle in whatever quadrant that angle may be located. 

 For this reason the two pairs of brushes will be henceforth referred 

 to as the sine and cosine brushes, respectively. 



PROC. AMER. PHIL. SOC , VOL. LXI., B, AUG. lO, I922. 



