The Earth inductor compass is a device for determining direction using the magnetic field of the Earth. The operation of the compass is based on the principle of electromagnetic induction with the Earth's magnetic field acting as the induction field for an electric generator. Because the direction of the Earth's magnetic field is aligned north-south, the electrical output of the generator will vary depending on its orientation with respect to the Earth's magnetic field. This variation in the generated voltage is measured, allowing the Earth inductor compass to determine direction.
Kevin Breen, the creator of the web site "The Beacon," describes how the compass works:
The operation of the earth-inductor compass was very simple. The controller was rotated to indicate the desired heading. This in turn rotated the brushes of the generator into such a position that there would be no flow of current when the airplane was headed in the direction indicated. The pilot accordingly turned his aircraft until the meter hand came to zero. By steering so as to keep the hand on zero the proper heading was maintained. To change to a new course, the controller was rotated so that the proper heading was indicated on the dial, and the pilot then turned the aircraft until the meter hand again came to zero. Additional controllers or meters could be installed to permit a navigation officer to assist the pilot, or to give the full use of the compass to both pilot and co-pilot. As a location could be selected for the electro-magnetic unit which was relatively free from local magnetic fields, errors from this source were reduced to a minimum, and vibration had no appreciable effect since there was no element comparable with the card of a magnetic compass which was subject to rotation. Using the earth-inductor compass aircraft could be navigated with much greater ease and accuracy than with an ordinary compass. One drawback with the inductor compass was that pilots had to ascertain which of the two potential occurrences of the 'zero' positions which were 180 degrees apart was the one which was correct for the desired course. This was accomplished by making a slight turn and noting the direction in which the pointer moved. Earth-inductor compasses were used in the Douglas DWC's during the 1924 U.S. Army Air Corps Around-the-World flight. In 1927, another equally famous flight also employed the earth inductor compass. Charles Lindbergh navigated the vast Atlantic Ocean using just such an instrument.
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