Magnet Revolving on its Own Axis
   This apparatus demonstrates one aspect of Michael Faraday's 1820 discovery of electro-magnetic rotation. The bar magnet is pivoted top and bottom, and can rotate freely about its long axis. An electric current enters the magnet halfway down its length by way of a contact between a projecting wire and an annular trough of mercury, and leaves at the bottom through a similar contact. The current travelling down the entire width of the magnet is thus exposed to the magnetic field lines, and experiences a torque, causing the magnet to rotate. This is an illuminating demonstration for students (and others) who think that the field lines must remain stationary with respect to the current-carrying conductor for a magnetic force to be produced.
    The example at the left is at the museum of St. Patrick's College in Maynooth, County Kildare, Ireland, and was made ca.1875 by Yeates & Son of Dublin. The instrument at the right is at the Virginia Military Institute, and is missing its bar magnet. I am sure that somewhere in the well-stocked VMI demonstration apparatus room there is a bar magnet with pivot holes top and bottom, and projecting contact pins.

How to classify this piece of apparatus? Other examples are shown with the electric motors and others with the apparatus of Daniel Davis, Jr.

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