Michael Faraday's Homopolar Electric Motor Experiment from 1821

In 1800 Allessandro Volta invented the electric battery - the Voltaic Pile.

In 1820 Hans Christian Oersted found that a magnetic needle (compass) was deflected when placed nearby a wire with a current flowing through it, and the meaning was that an electric current produced a magnetic field. This was the first demonstration of a mechanical movement caused by an electric current.

Michael Faraday was impressed by Oersted's findings and as a result the conversion of electrical energy into mechanical energy by electromagnetic means was first demonstrated by him in 1821. Basically, a free-hanging wire was dipped into a pool of mercury, in which a permanent magnet was placed. When a current flowed through the wire, the wire rotated around the magnet, showing that the current gave rise to a circular magnetic field around the wire that interacted with the magnetic field of the permanent magnet and the resulting force exerted on the wire spun it.

This primitive motor is not of any practical use and serves mainly for demonstration purposes in school physics classes. The toxic mercury is sometimes replaced by brine (salt water). The use of a conductive fluid (mercury, brine) arises from the need to enable the free movement of the wire and to close the electric circuit (the aluminum foil or any stable wire can serve the same purpose).

This simple conversion of electricity into rotation movement can be also demonstrated by a neodymium disk magnet, drywall screw, an alkaline battery cell, a wire, and a battery cell connected in series. The screw and magnet spin.

Simple Homopolar Motor Windell H. Oskay - www.evilmadscientist.com - CC 3.0

Motors that work according to the principles described above are called homopolar motors in contrast to modern DC more efficient motors where a commutator is used to reverse the direction of current flow in order to maintain continuous rotation. A homopolar motor is able to produce continuous rotation without the need for reversal in current. Since the motor requires the same electric polarity for its operation, the Greek homos=same, used in combining with "polarity" creates the term homopolar.

The right side is basically as described above (the free wire circled the fixed magnet). Then Faraday reversed the setup, this time with a fixed wire and a dangling bar magnet which rotated around the fixed wire when current was applied. The principle is again the same - the free part circled around the fixed part. Here the use of mercury enabled, besides conductivity, the magnet to float freely. Take in account that the magnet must be made of a conductive material in order to close the electric circuit.

Faraday's invention, though primitive, was the first step in the development of the electric motor.

The Barlow's Wheel, the earliest kind of a homopolar motor based on the discoveries of Oersted and Faraday, was built by the Englishman Peter Barlow in 1822.

Barlow's wheel - diagram from 1842

An electric current passes through the hub of the wheel which rim is dipped into a small mercury trough. The interaction of the current with the magnetic field of a U-magnet causes the wheel to rotate. The serrated wheel replaces the free wire (the tips) in Faraday's experiment. Though the original wheel, introduced by Barlow, was serrated the wheel will also work with a smooth round metal disk as well, usually made of a conductive material like copper. You can try to compare the efficiency of the two designs.

Whereas the homopolar motor converts electrical energy into mechanical energy, the homopolar generator does the reverse: converts mechanical energy into electricity by reversed action. If in the above mentioned Faraday's electric motor experiments an electric current that passed through the free wire caused it to rotate around the permanent magnet then a moving wire through a magnetic field (perpendicular to it) will produce a voltage on the wire and if the circuit is closed also a current.

In short, in the presence of an electromagnetic field a current can move a wire and a wire movement can generate a current.

Faraday disk generator from 1831

This reversed principle (Faraday's law of induction) was discovered in 1831 by Michael Faraday and as a matter of fact he discovered the operating principle of electromagnetic generators. Faraday built the first electromagnetic generator, called the Faraday disk, a type of homopolar generator, using a copper disc (instead of the wire) rotating between the poles of a horseshoe magnet. When the disc was rotated by a handle the apparatus produced a small DC voltage between its hub and rim.

According to Faraday's law the voltage generated is proportional to the rate of change of the magnetic flux and the practical meaning is that faster you rotate the disc higher will be the voltage generated.
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Advanced project suggestion: demonstrate and explain Faraday’s Paradox:
http://maxwellsociety.net...