Benjamin Franklin’s Electric Motor

by / Wednesday, 04 October 2017 / Published in Electrical Engineering

Benjamin Franklin (1706-1790) was not only a scientist, but also an engineer. More than a decade before James Watt invented his improved steam engine and launched the industrial revolution in England, Benjamin Franklin devised a working electric motor.

Electrical technology in Franklin’s day consisted mainly of scientific instruments. By 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 employed these new devices to explore how electricity could be generated, stored and transmitted, but made little practical application of their knowledge.

Franklin developed 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. However, he also sought more productive use of his findings.

Franklin’s Electric Motor

In 1748, he invented 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.

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