Immagini

DESCRIPTION

If in a conductor there is a magnetic field which varies with time, closed loop electrical currents will be induced according to Maxwell's equations. These currents are called eddy currents because they resemble vortex motions in a fluid. Another name for these currents is Foucault currents.

In this instrument a copper disk is made to rotate between the poles of a permanent magnet through the action of a crank connected to the axle of the copper disk. The disk itself is not attracted to the magnetic poles since copper is diamagnetic. However the charges constituting the eddy currents lead to a difference of potential between the edge and the center of the disk, an example of Lenz's law. This potential difference will produce a current in an ammeter connected between the axle and a contact at the edge of the disk.

The operator of the crank will note a braking reaction that increases with the speed of rotation as well as an increase in the ammeter reading. Besides the copper disk will become warmer due to the resistive contribution of the eddy currents.

For this instrument a current of 10 micro ampere was measured for a rotation of 5 turns per second.

BIBLIOGRAPHY

• [1] E. Amaldi, G. Amaldi, U. Amaldi - La fisica per i licei scientifici - Vol.3 - Zanichelli Ed.

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