THE EARTH INDUCTOR COMPASS.^ 



By PAUL R. HEYL and LYMAN J. BRIGGS. 



(Read April 22, 1922.) 



The instrument and accessories described in this memoir have 

 been developed with especial reference to use in vessels for the navi- 

 gation of the air. Navigating conditions in aircraft are such that 

 little or no reliance can be placed on the indications of the ordinary 

 magnetic compass. For this there are two principal reasons. 



Lack of space in airplanes forces the pilot and his instrument 

 board to be located in a region of considerable magnetic disturbance, 

 due to the proximity of the engine and other magnetic bodies. Occa- 

 sionally these are variable, such as the steering rod, which in some 

 planes is of steel. Compensation, after the manner familiar to navi- 

 gators of the water, is not always a satisfactory solution of the diffi- 

 culty. Apart from the variable disturbing elements above referred 

 to, the magnetism of the engine may change considerably during a 

 long flight. On the U. S. Army airway from Dayton, Ohio, to Wash- 

 ington, D. C, it has happened that the compass has developed an 

 error of as much as forty degrees before the trip was completed. In 

 the endeavor to eliminate such troubles the compass is sometimes 

 placed in the rear portion of the plane, and readings taken by devices 

 more or less complicated, as in the German Bamberg compass. Such 

 a plan, however, does not eliminate the second and more serious cause 

 of disturbances. 



The accelerations of an airplane are greatly in excess of those to 

 be found in a water vessel ; and the directive force of the earth's field 

 on the magnetic needle is weak. Due to the great acceleration, the 

 weak directive force, and the necessary inertia of the needle and card, 

 the magnetic compass possesses rather a long memory for disturb- 



1 A memoir to which the Magellanic Premium was awarded January 6, 

 1922, by the American Philosophical Society. 



L5 



