When the Chinese emperor Ho-ang-ti found himself on the plains of Tchoulou in 2364 BCE "confounded by the fog of the enemy" he found one element of modern air warfare, conceived another. The "fog" was perhaps the first smoke screen used in battle. His innovation was the Tchi-nan, chariot of the south, or compass chariot which consisted of a figure on a magnetized base mounted on a cart. The earth's magnetic flux caused the figure to point south and the other cardinal points were indicated. From this, we trust, the Emperor got his bearings in the man-made fog, routed his foe.
On a voyage from Tripoli to Alexandria, in the 12th century, Eilak Kibkjaki reported that dark and stormy nights were made less treacherous by mounting a magnetic needle in a sliver of wood or a bit of cornstalk floating in a bowl of water. And these compass bowls served as navigational aids for several centuries. Later, Columbus' voyage in 1492 proved that the earth was round, that a land of milk and honey existed to the west of Spain, and that compass needles deflected in various parts of the world.
But it was not until the French physicist, Leon Foucault, climaxed years of research into the earth's rotation by building a gyroscope with which he could observe the ceaseless, onward movement of the earth and demonstrate that the earth revolved while the gyro remained static. In 1910, about sixty years after Foucault's demonstration, the gyro compass made its appearance on ships where the need for a nonmagnetic compass increased with conversion to steel structures in marine equipment.
Bulky, heavy, slow to assume initial stability, the original gyro compass was nevertheless effective and acceptable to mariners because it did provide a directional, meridian-seeking device not affected by rolling turns or magnetic variations. Sometime later, the gyro compass, adopted from the marine type, made its appearance in aircraft, and proved itself stable in rough air and lateral turns. However, its inability to remain stable for long periods of time, the fact that it required initial setting by magnetic compass and frequent correction in flight, soon made the gyro compass a navigational aid and not a primary directional instrument for airmen. As airplane speeds increased, the gyro's value decreased, for it was simple enough to reset the compass every half-hour in a boat traveling twenty knots an hour but impossible to perform the same task in navy fighters traveling 300 knots per hour.
It remained for the research men at Bendix Aviation Corporation to devise the first new compass principle in more than four centuries. A curious electro-mechanical hybrid embracing some of the features of the simple magnetic compass, the directional gyroscope, and the earth inductor compass which led Lindbergh across the Atlantic, the new instrument is known as the Flux-Gate compass, is so unique in structure that were the enemy to capture one day he would logically assume that some of the parts were missing.
To attempt monosyllabic explanation of the complex device would be foolhardy, of little value to either the technician or the casual observer. There are, however, several prosaic analogies which may be drawn to point the primary functions and operation of the new instrument. The Flux-Gate, or magnetic azimuth sensitive element, from which the instrument takes its name, is outwardly nothing more than three strips of magnetic material mounted on a triangular Bakelite plate. Measuring only three inches on a side, this equilateral triangle provides the key to the new compass' myriad advantages. In flight, the plane may travel in any direction but the sides of the Flux-Gate triangle will always remain at a known angle to the magnetic flux of the earth. Traveling from the North to South Poles, this magnetic field has, for more than four centuries, pulled the magnetic compass needle northward in leading men of the sea and air from place to place.
In operation, the fixed coil system of the Flux-Gate is augmented by the magnetic force of the earth itself, varying in strength as the plane flies parallel with, or tangent to, the north-south flux between the earth's poles. In other words, the magnetic force affecting the plane's compass will be weaker or stronger just as the force of water pulsing through a pipe laid in line with a river bed is greater than the hydro power resulting when the pipe is laid at angles to the current. Thus, when a plane bearing the Flux-Gate flies directly North or South, the known magnetic force of the little gate is amplified by the addition of magnetic flux from the earth.
Because revelation of all Flux-Gate advantages might benefit the enemy at this time, the more important features must remain unknown temporarily. There are, however, several Flux-Gate attributes which must be self-apparent to Nazi and Nipponese navigators. So their mention here can hardly affect the plans of either our Allies or the Axis. For example, the magnetic compass becomes inoperative within 20° of either Pole, because the lines of magnetic flux are, at this point, absorbed by the earth. Conversely, the flux lines are strongest at the Equator, decreasing in strength with flight into either of the temperate zones. The Flux-Gate compass, on the other hand, continues to point directions to within 3° of the Poles. It has, therefore, reduced the navigational No-Man's Land from 1,200 to 180 miles. Obviously, current conversation about flights via the short great circle routes "over the Pole" have been a bit premature. But the Flux-Gate may well make such flights factual, not fanciful.
The Flux-Gate Compass, through extended engineering effort, has also been made practical for remote compass indications, with as many as eight remote compass dials duplicating the reading of the master instrument. Application of this feature is apparent. The recording unit can be installed in that part of the plane where conductors, ferrous alloys, structural points of the plane produce the least compass deviation because of their magnetic activity. In most planes the best location probably would be near the tail. As a result, the navigator, bombardier, and pilot of a plane can each watch the compass even though the tail-gunner may be closest to the Flux-Gate unit. When one considers that changes in radio installations, other magnetic equipment generally are made in the forward section of the plane, it is obvious that field servicing and correction for deviation is cut to a minimum by the remote installation.
This, briefly, is the story behind the first compass change since the lode-stone made its imprint on civilization. Further details are already being disseminated to Army and Navy personnel through special technical orders, schools like one at Bendix Philadelphia plant. Additional information on the Flux-Gate compass will appear in this magazine when its publication becomes valueless to our common enemy. No story can ever tell everything that happened during the seven years of research which led to perfection of the Flux-Gate compass just seven months ago, when one of the instruments fashioned by Bendix engineers was kept in a vault because it was their first one to work perfectly under all conditions. Now, Philadelphia girls, few of whom know or care about the Flux-Gate's technical aspects, are turning them out at a monthly rate which would startle the most blase production man. Nor are they pampered and protected by vaults or bell-glass. They're Built to show men the way to go home straight from an Immelmann, or a half-roll.
This article was originally published in the December, 1943 - January, 1944, issue of Air Tech magazine, vol 3, no 6, pp 36-37, 44.
Photos credited to Bendix Aviation Corp, Radio Corp of America.
The PDF of this article includes diagrams and descriptions of the operation of the Flux Gate Compass in the modes of "Airplane flying straight" and "Airplane changing direction."