HEYL & BRIGGS— THE EARTH INDUCTOR COMPASS. 31 



The total weight of the apparatus as shown in Fig. lo is about 

 five and a half (5^) kilograms. 



Excessive tilt is prevented by a guard ring (not shown in Fig. 10) 

 supported by guy wires in such a position as to intercept the motion 

 of the lower projecting end of the axle 6 when the angle of tilt 

 exceeds 20°, an angle not usually reached in ordinary flying. The 

 great demand for a compass comes from cross-country and cloud- 

 flying — not from acrobatic maneuvers. 



Air tests of this instrument have been made during its develop- 

 ment at one of the flying fields in this country, the latest series being 

 completed during the week of October 24, 192 1. This last series of 

 tests has demonstrated that the instrument as here described is reliable 

 in the air; that the needle of the galvanometer can be read during 

 pitching to a half millimeter, corresponding to two and one half 

 (2^) degrees change of course. It appears to be impossible for a 

 pilot to roll his plane without also slightly veering it sufficiently to 

 cause an oscillation of the needle of about one (i) millimeter on 

 either side. The needle is unaiTected by a vertical drop or " bump." 

 On rounding a curve centrifugal force deflects the needle; but such 

 force diminishes, on leveling up, at a rate sufficiently slow to allow 

 the damped pendulum to become stationary by the time it reaches the 

 vertical. A sharp veer of the plane will cause a slight oscillation of 

 the needle, which is damped out in about three (3) seconds. Due to 

 the constant slight vibration of the plane, the damping of the pendu- 

 lum produces no loss of sensitivity, the pendulum being shaken into 

 the vertical from even a small displacement. 



In the opinion of those pilots who have tested the instrument 

 some form of electrical drive would be preferable to wind power, as 

 affording a higher speed and greater sensitivity, and being more uni- 

 form. Many planes carry electrical generators. No disturbance of 

 the compass would arise from the rotating field of an A.C. motor 

 mounted on the same axle, as the flux due to the stray field would 

 rotate at the same speed as the earth inductor, and be therefore in- 

 variable with respect to the coil. 



Certain mechanical improvements in detail have been shown by 

 the latest air tests to be advisable. An invention, like a work of art, 



