16 HEYL & BRIGGS— THE EARTH INDUCTOR COMPASS. 



ances to which it has been subjected. It is not difficult to give the 

 plane such a motion as to set the compass card spinning on its pivot 

 so that before its motion subsides sufficiently to allow of even an 

 approximate reading the plane has traveled two or three miles. The 

 great speed of the plane (ordinarily from seventy to one hundred 

 miles an hour) and the comparatively sharp and sudden turns some- 

 times executed produce a set of conditions of an order entirely dif- 

 ferent from those to which a navigator of the water is accustomed. 

 The attempt to meet these disturbances by damping the compass card 

 is not a satisfactory solution; for damping, while it diminishes dis- 

 turbance, also decreases sensitivity, none too great at best. 



The difficulty of the situation is perhaps best shown by the fact 

 that the Great War, which produced, under stress of necessity, so 

 many inventions, closed without having brought out any satisfactory 

 form of airplane compass on either side of the conflict. 



For satisfactory service under conditions of this nature the earth 

 inductor in connection with a galvanometer possesses a fundamental 

 advantage over the magnetic needle. Unlike the latter, it has no 

 memory. From instant to instant it furnishes an electromotive force 

 determined by its orientation with respect to the earth's field, irre- 

 spective of its past or present state of translatory motion. This 

 advantage has not been unrecognized by previous workers (Dunoyer: 

 British Patent 4609 of 1907; Chabot: British Patent 9912 of 1903; 

 Bliss: U. S. Patent 1,047,157 of Dec. 17, 1912). It may be noted 

 that no one of these proposed devices possessed sufficient practi- 

 cability to bring it into use during the war. 



In all previous attempts at the construction of a compass of this 

 type, the current developed in the rotating coil, amplified if necessary, 

 was caused to pass through a galvanometer, and the course of the 

 vessel indicated by the amount of deflection produced. The instru- 

 ment described in the present memoir differs from all previous at- 

 tempts in the following respects : 



I. It employs a null method for its indications, and therefore 

 enjoys all the advantages of sensitivity characteristic of null methods 

 as a class. As long as the vessel lies in the course predetermined by 

 the pilot, no deflection is produced in the galvanometer. 



