HEYL & BRIGGS— THE EARTH INDUCTOR COMPASS. 25 



outer edge, and along this inner edge the insulation is removed from 

 the turns of wire. 



Fig. gb shows the assembled switchboard, front view. The dial 

 carrying the compass divisions carries on its under side the wiping 

 contacts which constitute the terminals of the galvanometer connec- 

 tions. These contacts press against the exposed portions of the wire 

 on the square frame. 



Fig. gc shows the back view of the switchboard. From the four 

 corners of the scjuare resistance frame wires run through holes i, 2, 

 J, 4. to the back of the switchboard, and thence as indicated to the 

 5" and C binding posts, which in turn are connected respectively with 

 the sine and cosine brushes. From the wiping contacts on the under 

 side of the dial wires run through the hub 5, which moves with the 

 dial, to the wiping contacts 6, 7, from which wires run to the posts G, 

 which are connected with the galvanometer. 



A simple manipulation of the dial switchboard enables the pilot 

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

 forward and backward directions in which he may be flying in any 

 line when the galvanometer reads zero. If the dial be turned slightly, 

 say to the right, the pointer of the galvanometer will move from zero ; 

 and the galvanometer connections can be made so that this motion 

 will also be to the right when the vessel is moving forward on the 

 course indicated by the initial position of the dial switchboard. If 

 the plane be flying backward on this course, the flux through the 

 armature is reversed in direction ; and, with the same connections, the 

 motion of the pointer will now be opposite to the motion of the dial. 



Contrary to what might be supposed, there is no difficulty in ob- 

 taining a galvanometer at once sufficiently sensitive and rugged to be 

 used under the conditions prevailing in an airplane. The galvanom- 

 eter used is of a standard commercial type, double-pivot spring con-, 

 struction, giving one millimeter deflection for one hundredth of a 

 milliampere. Its resistance is about 22 ohms. Being always in cir- 

 cuit with a resistance less than its critical damping resistance, the 

 motion of its pointer is always dead-beat. The vibration and jarring 

 to which the instrument is subjected in flight are not great, being 

 actually less than are to be found at the instrument board of an auto- 



