An 1842 diagram of Barlow's wheel Barlow's wheel was an early demonstration of a

homopolar motor A homopolar motor is a direct current electric motor with two magnetic poles, the conductors of which always cut unidirectional lines of magnetic flux by rotating a conductor around a fixed axis so that the conductor is at right angles to a s ...

, designed and built by

English English usually refers to: * English language * English people English may also refer to: Peoples, culture, and language * ''English'', an adjective for something of, from, or related to England ** English national ...

mathematician A mathematician is someone who uses an extensive knowledge of mathematics in their work, typically to solve mathematical problems. Mathematicians are concerned with numbers, data, quantity, structure, space, models, and change. History On ...

and

physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate cau ...

, Peter Barlow in 1822. It consists of a star-shaped wheel free to turn suspended over a trough of the liquid metal mercury, with the points dipping into the mercury, between the poles of a

horseshoe magnet A horseshoe magnet is a magnet made in the shape of a horseshoe or a U-shape and has become the most widely recognized symbol for magnets. It was invented by William Sturgeon in 1825. This type of magnet can be either a permanent magnet or ...

. A DC

electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The movi ...

passes from the hub of the wheel, through the wheel into the mercury and out through an electrical contact dipping into the mercury. The

Lorentz force In physics (specifically in electromagnetism) the Lorentz force (or electromagnetic force) is the combination of electric and magnetic force on a point charge due to electromagnetic fields. A particle of charge moving with a velocity in an elect ...

of the

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

on the moving charges in the wheel causes the wheel to rotate. The presence of serrations on the wheel is unnecessary and the apparatus will work with a round metal disk, usually made of

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...

. :"The points of the wheel, R, dip into mercury contained in a groove hollowed in the stand. A more rapid revolution will be obtained if a small electro-magnet be substituted for a steel magnet, as is shown in the cut. The electro-magnet is fixed to the stand, and included in the circuit with the spur-wheel, so that the current flows through them in succession. Hence the direction of the rotation will not be changed by reversing that of the current; since the polarity of the electromagnet will also be reversed." :(''Excerpt taken from the 1842 edition of the Manual of Magnetism, page 94'') It is used as a demonstration of electromagnetism in physics education. Because mercury is toxic,

brine Brine is a high-concentration solution of salt (NaCl) in water (H2O). In diverse contexts, ''brine'' may refer to the salt solutions ranging from about 3.5% (a typical concentration of seawater, on the lower end of that of solutions used for ...

is sometimes used in place of mercury in modern recreations of the experiment.

How it works

Action of a magnet on current and how rotational motion is produced due to this can be shown by Barlow's wheel experiment. The apparatus consists of a star shaped copper wheel capable of rotating freely in a vertical plane about a horizontal axis. The point of each spoke of the star just dips into a pool of mercury kept in a small groove on the wooden base of the apparatus. The pool of mercury is kept in between the two opposite poles of a strong magnet. The wheel rotates with its plane being perpendicular to the direction of the magnetic field and during its rotation only one point of the star dips into the pool of mercury at a time. When the axis of the wheel and the mercury are connected to electric cell, the circuit is completed through the axis of the wheel (when a point dips in the mercury) and the mercury. On passing current through the circuit the wheel will begin to rotate due to the action of the magnet on the current. The direction of rotation of the wheel can be determined by applying Fleming's left hand rule. While rotating and when a spoke of the wheel just leaves the mercury the circuit breaks but due to inertia of motion the wheel continues its motion and brings the next spoke in contact with the mercury thereby restoring the electrical contact. In this way rotation of the wheel continues. On reversing the direction of the current or that of the magnetic field the wheel rotates in the opposite direction. The speed of rotation depends upon the strength of the magnetic field and the strength of the current. Here mechanical energy is obtained from electrical energy.

References

External links

Barlow's wheel
designed by Eckling 1840 (German page)
Barlow's Wheel - Interactive Java Tutorial
National High Magnetic Field Laboratory

{{DEFAULTSORT:Barlow's Wheel Electric motors