| The Electric Whirl or Electric Fly is made of a number
of brass wires, with rearward-facing sharp points, joined at a hub. This
is pivoted atop an insulated shaft. One terminal of an electrostatic machine
is connected to the whirl and the other is grounded. As the charge builds
up on the metallic parts of the whirl, the equipotential lines are bunched
together at the sharp points, creating a large electric field there. Eventually
the field becomes large enough to ionize the air molecules and create a
space charge that is of the same sign as the point. The mutual repulsion
between the space charge and the point causes the wheel to spin.
The electric whirl at the left in on display at the museum room in the physics department of Washington and Lee University.
The two whirls below are at Kenyon College, and I regularly use them for demonstrations. The top of the left-hand device is probably by Ritchie of Boston, where the complete device with eight points cost 75 cents; with four points it was 50 cents.
REF: Thomas B. Greenslade, Jr., "Electrostatic Toys", Phys. Teach., 20, 552-6 (1982)
| The 1888 Queen catalogue describes this apparatus
as an electrical inclined plane. The apparatus, at the University of Texas
is unmarked, but a good deal of Queen apparatus from the early days of
the university, opened in 1890, is still in existence.
In a visit in January 2003 I suggested that this demonstration be put back into regular use.
A variation on the electric whirl is the electric Orrery . The longer horizontal rod is balanced and rotates because of a sharp point projecting at right angles to it. The shorter rod is electrically connected to long rod, and is again pivoted and spins due to the presence of another rearward-facing point. The apparatus at the left below is at Grinnell College in Iowa, and at the right below is an electrical orrery at Middlebury College in Vermont. Neither piece of apparatus is marked, but a similar device is priced at $1.50 in the 1860 catalogue of Edward S. Ritchie of Boston, which reads "Electrical Seasons Machine, with point and stand; is placed in the centre hole of the prime conductor."
A second method of showing the discharge of electricity from sharp points is the Electric Wind.
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