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ArrowBarlow's Wheel

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English mathematician Peter Barlow devised an instrument in 1822 that built on advances from earlier in the century (including the invention of the first battery, the voltaic pile) to create a very early kind of electric motor. It combined the powers of electricity and magnetism to produce continuous motion.



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First, an electric circuit is created with the help of a wet battery (the Grenet cell fueling this applet was invented around 1840). The circuit passes through a metal wheel and continues, when the wheel is in position, through a trough of mercury (which can carry current) located below the wheel. When the wheel is lowered into the mercury using the wheel height slider, the circuit is established. When it is removed from the liquid, the circuit is broken.

But while such a circuit might power a small light bulb, it can’t make the wheel turn. Another force is required: magnetism. A horseshoe magnet positioned around the wheel provides the magnetic field. That field interacts with the magnetic field created around the wire, making the wheel move.

Barlow’s wheel was one of the first machines to harness the interplay between electric and magnetic forces that had been discovered just two years earlier by Hans Christian Ørsted. The machine represents the right-hand rule at work. It shows how an electrical current traveling in a certain direction (in this depiction it is traveling up through the wheel) and generating around it a circular magnetic field can interact with a second magnetic field – moving in our example from the north to the south pole of the horseshoe magnet – to produce the force that makes the wheel move.

As you can see by switching out wheel shapes, it doesn’t matter whether a spike or solid round wheel is used. Although the circuit is broken when a tip of the serrated wheel leaves the mercury, the wheel has enough momentum to keep turning until the next tip dips into the mercury, reestablishing the connection.

Related Electricity & Magnetism Pages


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