| This replica of Faraday's original demonstration
of electromagnetic rotation is at the National Museum of American History
at the Smithsonian Institution in Washington, D.C. A vertical bar magnet
of circular cross section is half-submerged in a pool of mercury. The end
of the flexibly-supported conductor dips into the pool of mercury and carries
a current. The interaction of the current with the fringing field of the
magnet produces a transverse force, and the tip of the wire travels in a
Oersted's 1820 demonstration of the deflection of a compass needle led to efforts by William Hyde Wollaston in 1821 to produced a continuous rotation of the needle. This failed, but on Christmas Day, 1821, Michael Faraday (1791-1867) showed that the pole of a bar magnet would rotate around a current-carrying wire. The inverse effect (a current-carrying wire rotating around a magnetic pole) was also demonstrated by Faraday.
The design, with its large exposed surface area of mercury, was soon discarded, but the principle remains in the design of the electric motor.
| The apparatus at the left, in the Greenslade
collection, shows the magnet pole rotating around the current-carrying wire.
The description in 1916 catalogue of the L.E. Knott Apparatus Co. of Boston reads: "Magnetic Pole Rotation Apparatus for demonstrating the tendency of a current and a magnet pole to rotate about each other. The apparatus consists of a jewel-mounted magnet supported so that the current passes along half the length of the magnet and is carried away at the middle. The magnet is supported at its center on a fine point on top of a post. The current is brought to a mercury cup fitted to the magnet, and a bent copper wire attached to this cup dips into an annular mercury cup, whence it is carried by the upright support to which the cup is attached. When a strong current is passing (3½ amperes at 7 volts pressure, at least) one pole will rotate steadily. By reversing the direction of the current, the direction of the rotation is reversed ... $4.35"
| This apparatus at Middlebury College in Vermont
is shown in the 1925 catalogue of E. Leybold's Nachfolger (E. Leybold's Successors)
of Cologne at $24.50.
With the aid of attachments not included in the picture, it can be used for showing "1. Rotation of a current around a magnet; 2. Rotation of a magnet around a fixed current; 3. Rotation of a magnet about its own axis under the influence of a current; 4. Rotation of a current by another current." It is set up for the second demonstration.
| Here are two examples of a magnetic
rotating about its own axis. The current passes down the magnet
from the top to the point half-way down, where it is picked off by
a pointed wire dipped into a pool of mercury in an ivory or iron annular
The apparatus at the far left is at Colby College in Waterville, Maine, while the similar apparatus at the near left is at Wesleyan University in Middletown, Connecticut.
| The magnet revolving about its own axis at
the near right is in the collection at the National Museum of American History
at the Smithsonian Institution, and was made by J. Newman of London. Its
price in the 1837 catalogue was in the range from £1 10s to £2
The instrument at the far right is at Transylvania
University in Lexington, Kentucky. It was purchased from E. M. Clark of London