this article is very interesting and descriptive:
New York Times (February 28, 1928):
"Explains Magnet In Fuelless Motor"
Hendershot Says Shifting Its Field To east And West Causes Rotary Motion ~ Winding Of Magnet Secret ~
Inventor Asserts Engine Weighs But 4 Ounces Per Horsepower
Mildly indignant because the manner in which his fuelless motor gains its power had been misrepresented in dispatches from Detroit and Washington, Lester J. Hendershot today stated there was nothing mysterious about his motor, that the force that energizes it is the "same force that pulls the needle of the compass, and there is nothing mysterious about that".
The fuelless motor was not his objective, he explained, at the time he began his experiments some three years ago, when he first became interested in aviation.
"I soon learned that the ultimate development of aviation depended upon the discovery or invention of an absolutely true and reliable compass", he explained. "The ordinary magnetic compass does not point to the true north -- it points to the magnetic north, and varies from the true north to a different extent at almost every point on the earth’s surface.
"There is another compass, the magnetic induction compass, that indicates true north. But it must be set before each flight, and is not always reliable.
"I found that with a pre-magnetized core I could set up a magnetic field that would indicate true north, but I didn’t know just how to utilize that in the compass I set out to find.
"In continuing my experiments, I learned that by cutting the same line of magnetic force north and south, I had an indicator of the true north, and that by cutting the magnetic field east and west, I could develop a rotary motion.
I now have a motor built on that principle that will rotate at a constant speed, a speed predetermined when the motor is built. It can be built for any desired speed, and a reliable constant speed motor is one of the greatest needs of aviation.
The main secret of Mr. Hendershot’s invention, his Friend Barr Peat declares, is the method of winding a magnet in the motor so that it will rotate in the opposite direction than the earth revolves. He says there is no heat, because magnetic forces are cold and the motor is stopped only by breaking the magnetic field in the windings. The magnet in the motor, he thinks, probably would have to be recharged after about 2000 hours of operation.
Mr. Hendershot declares that one of his motors, complete and ready to be installed in an airplane would weigh little more than four ounces for every horsepower it developed, while the best of the gas engines now built weighs about two pounds per horsepower.
Mr. Hendershot says that altitude would not affect the efficient operation of his motor, for the magnetic influence of the earth has been found to remain the same as high as man has ever reached.
He said that the same principle which made his original model operate only when it was placed in one direction, north and south, will be developed so that it will provide a compass that will always indicate true north.
Google and read about the induction compass, very interesting!