The Unipolar Generator was developed by Michael Faraday (1791-1867) in 1831 as one of his explorations of the phenomena of induced EMF. The experimental situation, familiar to all students who have solved the problem using (a) motional EMF and (b) Faraday's law, has a copper disk revolving in the field of a magnet. The EMF is developed between the axle of the disk and the rim, which originally dipped into a mercury-filled trough. In the two pieces of apparatus illustrated, a falling weight attached to a string wound around the axle and passing over the pulley provided the necessary mechanical motion.

   As is the case with all generators, the apparatus can also be used as a motor. Current furnished by an external source of direct current of EMF passes radially through the disk. The resulting magnetic torque will pull up the weight.

   A good piece of apparatus can also be used in other ways. I use the Kenyon apparatus to demonstrate Eddy Current Damping by placing the poles of a large permanent magnet on either side of the spinning disk, which comes rapidly to a halt.

   The Kenyon apparatus was made by Queen of Philadelphia and was displayed at the International exhibition in Philadelphia in 1884. An 1887 Queen catalogue shows a cut of it and describes it as "Faraday's Apparatus".