What Is An Earth Inductor Compass?

It is a compass that relies on spinning coils interacting with the Earth's magnetic field lines. There was one on the Spirit of St. Louis in 1927 when Lindbergh made his famous flight to Paris. This is a basic model to demonstrate the principle (and in the process I discover a mistake in my assumptions!)

VIDEO: Earth Inductor Compass: https://youtu.be/5aGUOTizpqc

VIDEO on Earth Inductor Compass that failed:
Concept Was Right, Demo Was WRONG! Earth Inductor Compass (4K)
https://youtu.be/cZrNqPsOjO0

MATERIALS

• DC motor 28 mm dia (38mm long)
• DC motor rotor from an RS 550 motor
• Amp meter
• Power supply
• Motor/stator coupled by shrink wrap

The North Magnetic Pole is not at the geographic North Pole, right now it is near Ellesmere Island at 81.3°N 110.8°W.

From Wikipedia, the free encyclopedia
The Earth inductor compass (or simply induction compass) is a compass that determines directions using the principle of electromagnetic induction, with the Earth's magnetic field acting as the induction field for an electric generator. 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

The earth inductor compass was first patented by Donald M. Bliss in 1912 and further refined in the 1920s by Paul R. Heyl and Lyman James Briggs of the United States National Bureau of Standards,and in 1924 by Morris Titterington at the Pioneer Instrument Company in Brooklyn, New York. Designed to compensate for the weaknesses of the magnetic compass, the Earth inductor compass provided pilots with a more stable and reliable reference instrument. Over the transatlantic leg of his voyage – a distance of about 2,000 miles (3,200 km) – he was able to navigate with a cumulative error of about 10 miles (16 km) in landfall, or about one half of one percent of the distance traveled, by computing his heading at hourly intervals for a dead reckoning estimate of position.

Operation

Bliss' original design consisted of two armatures spinning on a single vertical axle. One armature was connected to commutators that were 90 degrees offset from the commutators connected to the other armature. When one set of commutators is aligned with the earth's magnetic field no current is produced, but an offset angle creates a positive or negative current in proportion to the sine of the offset angle. Since the sine of the angle peaks at 90 degrees, a reading could indicate either a certain direction or the exact opposite direction. The solution to this was a second armature with commutators offset by 90 degrees to help distinguish the two opposite directions.

The direction of travel was read by comparing the indications on two independent galvanometers, one for each armature. The galvanometers had to be calibrated with the correct headings, since the voltage was proportional to the sine of the angle. Readings could be impacted by the armature's speed of rotation and by stray magnetic fields.

Later versions simplified readings to show the offset from the intended heading, rather than the full range of compass directions. The revised design allowed the user to rotate the commutators in such a way that zero current would be produced when the craft was traveling in the intended direction. A single galvanometer was then used to show if the pilot was steering too far to the left or to the right.

Lindbergh's compass used an anemometer to spin the armature through a universal joint. The armature was mounted on gimbals to prevent it from tilting with the airplane's pitch and roll. Tilting the armature could have changed the angle of the Earth's flux to the armature, resulting in erroneous readings. The gyroscopic effect of the spinning armature also helped to keep it properly aligned.

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The arms are of 3.5 per cent, silicon steel, which has, in a field equal to the earth's horizontal component, a permeability of about four times that of ordinary soft iron. The residual magnetism is also considerably less. The four arms of the cross are carried by a central hub of the same material. The arms are 1 cm. in diameter, and measure 20 cm. from end to end of a pair of opposite arms. For a rod of these proportions the demagnetizing coefficient is small enough to allow a permeability of about ten times that of a cube of the same material.
Upon each of the four arms there are 500 turns of No. 20 B. & S. copper wire, silk enamel insulation. The winding is of the closed coil type, with a four-segment commutator. The total resistance of the wire on all four arms is 3.2 ohms, and the resistance through a pair of opposite commutator segments 0.8 ohm. At 20 revolutions per second, the electromotive force is 8 millivolts.