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Your Engineering Heritage
Benjamin
Franklin and His Electric Motor
by
David L. Morton, Ph.D.
On this 250th
anniversary of Benjamin Franklin's famous kite experiment, it is
worth recalling that Franklin was not only a scientist, but also an
engineer. In fact, he invented one of the first electric motors,
and thus was one of the first to put electricity to work — sort
of.
More than a
decade before James Watt invented his improved steam engine and
launched the industrial revolution in England, Benjamin Franklin
had devised a working electric motor. Electrical technology in
Franklin's day consisted mainly of scientific instruments. By
about 1745, electrical scientists exploring the nature of the
"sublime fluid" had developed crude electrostatic
generators and an early form of the capacitor, which they called
the "Leyden jar." Experimenters found interesting uses
for these new devices. In addition to showing how electricity
could be generated, stored and transmitted, they gleefully shocked
themselves and their colleagues or used static electricity to
stick bits of wool together. Franklin imagined that electricity
could do something a little more useful.
Franklin took an
interest in electricity in 1747 after receiving an
"electric tube" from a friend. Over the next few years,
he conducted experiments sporadically and collected examples of
the latest electrical instruments. Like other scientists, he
explored the ability of various materials to accumulate charges
and the curious attractions or repulsions these charged bodies had
for each other.
By 1748,
however, he had come up with an invention of his own. Calling it
the "electric wheel," Franklin's machine consisted of a
vertical shaft that was free to rotate, from which several glass
bars extended like spokes. Each bar was tipped with a brass
thimble. Placing the terminal of a "negatively charged"
(as Franklin understood it) Leyden jar near the wheel allowed the
thimble/glass assemblies to act as capacitors; as each assembly
charged up, it tended to be repelled from the Leyden jar. A
second, "positively charged" jar set nearby had the
opposite effect, pulling the spokes toward it. The result was that
the wheel would begin to rotate, and it would remain in motion
until the charges on the Leyden jars dissipated.
The motor was
only strong enough to continue turning at 12 to 15 rpm when loaded
with 100 Spanish dollars. Franklin probably suspected that even a
full-scale version of his electrostatic motor could not compete
with steam engines or water wheels as a practical source of power
for industry. However, he suggested that if a turkey were mounted
on the shaft and the whole thing placed before a fire, the wheel
could be used as a sort of automatic spit. Unfortunately, he never
reduced this proto-rotisserie idea to practice.
A few others did
try to put the wheel to work, including an unknown toymaker who
constructed a small, motorized carousel in about 1830 (an example
of which resides in the Bakken Museum and Library, www.thebakken.org).
Published in Franklin's well-received 1751 book on electricity,
the wheel was overshadowed by his other findings, including his
famous proposal to use lighting rods to protect houses from
damage.
Later
practitioners found little use for the electrostatic devices
invented in the late 18th century. According to historian Michael
Schiffer, the invention of the voltaic pile (an early form of
battery) in 1800 dampened enthusiasm for electrostatic generators,
while the development of the electromagnetic motor by Michael
Faraday and others a few years later permanently turned the course
of electric motor development away from machines like Franklin's.
So while Franklin is remembered as one of the greatest electrical
experimenters of his day, his electric wheel has faded into
obscurity.
David
Morton is a research historian at the IEEE History Center at
Rutgers University. Visit the IEEE History Center's Web page at: www.ieee.org/organizations/history_center/.
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