Flux Gate Compass

By Hendry L Bart
Science has overcome the magnetic influence of the poles in this new compass built for air navigation.

A new scientific marvel has come out of the laboratories to back up our fighting flyers. It's a device so small that you could easily put it into your vest pocket. If you had the blueprints and the secret materials, you could probably make one in a single evening in your basement workshop. Yet this little gadget has opened up a great new frontier to our flyers. It guides them safely through what was, until yesterday, a vast no-man's-land of navigation.

More than that — it helps our bombers find distant objectives, brings home thousands who might otherwise be lost.

This tiny device of wire, plastic and metal bears the curious name of the "flux gate," and is so called because it may be considered to allow the flux lines of the earth's magnetism to flow through it. The flux gate is the secret of the amazing new gyro flux gate compass, one of the most surprising and permanently useful inventions to come out of this war. Because the Germans and the Japs could not duplicate the performance of this compass, much less get it into mass production during the war, its story can be told without fear of aiding the enemy.

Three years ago the Government asked the Bendix Aviation Corporation to try to develop a compass that would not go haywire when a plane flew closer than 1,200 miles to either of the earth's magnetic poles. Anywhere within that distance the best magnetic compasses are unreliable. If you look at a globe you can easily see just what that means to flight in North America. The North Magnetic Pole (not to be confused with the geographic North Pole) is located on Boothia Peninsula, northwest of Hudson Bay. With that as a center, draw a circle to the scale of a 1,200 miles radius. Your line encircles an area of nearly 5,000,000 square miles where many a plane and its crew has disappeared, never to be found. It takes in most of Canada, more than half of Greenland, a corner of Alaska, all of the Arctic Archipelago, and lots of water.

The Government wanted that no-man's-land opened up. To do so would clip thousands of miles from the distance between certain points on this continent and points in North Asia, It would speed up by millions of hour's in total our shipment of men and material by air around the globe.

The Government had asked only for a compass that would open up the polar regions to round-the-world navigation. They got a great deal more than that. The new compass will give the plane's true heading even while the plane climbs, dives or banks up to a dizzy angle.

A magnetic compass has a float card that goes into a wild dance when the plane maneuvers. A pilot cannot get his direction until he levels off on a straight course. Even then the card swings uncertainly left and right, "hunting," as the navigators call it. But the new compass neither shoots ahead, lags behind nor hunts. It points the plane's direction through a needle on a dial as easy to read as the face on an ordinary alarm clock.

Yet that is not the whole story of the new compass. It has not just one dial but a number — and in the great planes of tomorrow it may have even more! The master indicator is placed on the navigator's plotting table. Repeater dials face the pilot, co-pilot, and bombardier. At all times these dials show exactly the same reading. There is no longer a chance that the pilot, after he gets his course from the navigator, may go off course because he is using a different compass. And the bombardier, as he plots his bombing run, can give the pilot his course to a hair line!

The great advantage of these repeating dials is that the flux gate itself may be placed somewhere in the plane away from the disturbing magnetic fields in the pilot's cockpit. The ferrous metals and the numerous electrical devices that must be in the cockpit play hob with magnetic compasses. The actual working part of this first new magnetic compass since Columbus' time is safely hidden away from these magnetic fields.

With the gyro flux gate compass the navigator no longer uses a correction card. He corrects his compass automatically by turning a couple of knobs on the master dial.

While the flux gate is the heart of the new compass it is really one of the smallest parts of the whole. The compass is actually a partnership of a number of complex units, each of which has its own special job. The flux gate itself is mounted under a gyroscope. Turning at 10,500 revolutions a minute, this tiny wheel holds the flux gate always level with the earth. If the plane banks more than a certain angle the compass must be "stopped" or else it will tip, whereupon it takes the small gyro about 10 minutes to bring it upright again.

The flux gate is not only an intricate magnet but a small transformer which picks up the minute impulses of the earth's magnetic field and changes them into a weak alternating electric current. This current is carried to a vacuum tube amplifier which blows it up into a current of sufficient strength to turn a small motor. It is the shaft of this motor that moves the needle on the navigator's master dial. Actually, it is the relative strength or weakness of the current flowing from the flux gate that determines the needle's position.

That is a simple explanation. To understand exactly what happens in a flux gate compass you would have to be several kinds of an engineer. And there is much about the workings of the flux gate itself that is still secret.

The little plastic triangle that houses the "gate" may not look like much but engineers are sure that the enemy will not be able to obtain the materials for the core or reproduce the gyro flux gate for many years to come.

Seven years of intensive work went into the gyro flux gate compass. It was finally perfected and installed on our bombers shortly after we got into the war.

The beginning was even earlier. Scientists of Bendix Aviation Corporation had been working on the problem for some time, since the gyro flux gate is a kind of great-great-great grandchild to the "earth induction" compass which Pioneer (a division of Bendix) built for Lindberg's transatlantic flight in 1927. Engineer Alfred Stuart developed the principle of the flux gate. Paul Noxson incorporated it into a compass that would work reliably under any conceivable condition. John Emerson figured out a method of manufacturing the compass in quantities large enough to equip a giant air force. All were working under the guidance of W A Reichel, Eclipse-Pioneer's director of engineering.

If the flux gate is the heart of the new compass, then the master indicator on the navigator's desk is its brain. The compass even has its own nervous system. for its various parts, at different locations in the plane, must work perfectly together. The transmitter unit for example — containing the gyro and flux gate — is mounted far out in the plane's wing or tail to keep it away from magnetic disturbances set up by ammunition. armament or other metal. Communication between the various units of the compass is furnished by two other Bendix inventions, the Autosyn and the Magnesyn.

Recently in Philadelphia, the War Department gave permission for half a hundred aviation writers to view this wonder of technical progress, What they marveled at was not so much a remarkable new compass but the amazing engineering skill that had made it possible to produce these complex instruments— more intricately made than the finest watches — at the rate of hundreds a month to match America's huge production of fighting planes. They saw a great military victory being won right on the floor of an American factory. For here American engineers had accomplished what the Nazis — with all their vaunted science — had utterly failed to do. And they knew it was an omen of a bigger victory to come.

This article was originally published in the January, 1944, issue of Flying magazine, vol 34, no 1, pp 69-70.
The PDF of this article includes photos of a model installation, components, and assembly and calibration of the flux-gate compass.
Photos credited to Bendix Aviation Corporation.