INTERACTIVE EXPERIMENTS AND EXHIBITS
ritratto di Foucault

Jean Leon Foucault (1819-1868), France. A physicist of great stature, known for his invention of a method for the measurement of the speed of light, his famous “Pendulum” and the discovery of “parasite inducted currents".


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What happens when a massive conductor moves in a magnetic field or when a magnet moves adhering to a massive conductor?

To see, click on the photo
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These are Foucault, or “parasite”, currents. They are harmful when they circulate where they are not useful, as for example in the nuclei of the soft iron of transformers, which would overheat due to the Joule effect. For this reason, where necessary massive conductors are composed of piles of thin laminas isolated from each other.


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But can we “see” Foucault currents? We have connected a measuring device to the two ends of a strip of aluminium. Let’s see what happens.

To see, click on the photo
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The intense current induced in the thickness of the aluminium which circulates through two closed passages which are equivalent to two short-circuited coils, determines a slight measurable difference in potential due to the not null resistivity of the conductor. As the magnet slides, in the aluminium it “tows” two opposite and contiguous magnetic poles, the sign of which is such as to restrain the motion in the aluminium.

 

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The existence of Foucault currents makes it possible for some kinds of motors to work. An example is the rotating magnetic field motor, a model of which is shown here.
To see it work, click on the photo

 

The history of it is given on the page of Rotating Magnetic Field.. A technical application of parasite currents is in the electromagnetic brakes used in measuring the power of motors, performed by measuring the amount of Joule heat caused by the Foucault current in the massive metal disk that rotates in the magnetic field.

 

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