28 HEYL & BRIGGS— THE EARTH INDUCTOR COMPASS. 



process. This allows sufficient looseness to permit response to a 

 small angle of tilt, and also sufficient damping to insure that the oscil- 

 lations do not continue beyond the time required for the plane to 

 regain its level. 



On rounding a curve centrifugal force will, of course, deflect the 

 pendulum somewhat from the vertical; but such centrifugal force is 

 removed as gradually as it is applied, and by the time the plane comes 

 again into a straight course the axis is stationary in a vertical 

 position. 



The natural time of swing of the armature pendulum as arranged 

 is, as has been said, about one third of a second. The time of roll 

 or pitch of even the smallest planes is several seconds, and for the 

 larger planes still longer. The disturbances arising from the driving 

 mechanism have a period of about one twentieth of a second. The 

 pendulum is sufficiently massive to resist forced vibrations of the 

 latter period; and the oscillations of the plane itself are too remote 

 in period to produce any sensible effect. A certain amount of gyro- 

 static action at the usual speed of revolution (1,200 r. p. m.) con- 

 tributes materially to the stabilizing action. Excess of gyrostatic 

 action is undesirable, as its effect is to lengthen the period of swing 

 and bring it too near that of the oscillations of the plane. 



Actual experiment is necessary to appreciate the very satisfactory 

 degree of stability possible of attainment by an apparatus of this 

 nature. 



It might be supposed that there would be a small quadrantal error 

 in an electrical system such as described, due to the fact that the 

 armature is sending out a current which is not constant for different 

 azimuths of the vessel, and consequently the reaction of the armature 

 on the earth's field would be variable. When one set of brushes alone 

 is functioning the electromotive force E is applied to opposite corners 

 of the square frame (Fig. 7). li R be the resistance of one arm of 

 the frame, the equivalent resistance of the whole frame is also R, 

 and the current output of the armature is E/(R -\- r), where r is the 

 internal resistance of the armature. If, as in Fig. 3, both pairs of 

 brushes are equally active, the voltage of each pair is o.yE. The 

 resistance in the square frame encountered by each voltage is that of 



