137. Induction currents in massive conductors. (Foucault's currents)

The occurrence of induction currents in solid conductors can be considered a special case of electromagnetic induction. The resistance of these conductors is low. Therefore, the EMF induction excited in them can generate vortex currents of very high force.

These currents were investigated by the French physicist Foucault. Foucault currents can be used to heat the conductors. This principle is the basis for induction electric ovens. These ovens are used to melt metals in a vacuum when other methods are practically unsuitable.

However, in many electrical devices, the appearance of Foucault currents results in useless energy loss for heat generation. Therefore, the iron cores of transformers, electric motors are not made solid, but are assembled from separate plates, isolated from each other.

Curious phenomena occur when Foucault currents interact with the magnetic field that generated them. Figure 1 shows a massive copper pendulum oscillating between the poles of a strong electromagnet. When the pendulum approaches the electromagnet gap in the pendulum according to the Lenz's law, an induction current occurs in such a direction that the field created by it is directed against the field of the electromagnet. As a result, the pendulum brakes. When the pendulum leaves the electromagnet gap, the magnetic flux permeating the pendulum decreases and (according to the Lenz's law) the pendulum is attracted to the electromagnet and brakes again. As a result, the pendulum stops quickly, but without the electromagnet its oscillations could have continued quite a long time.

This effect is used to quickly calm down the oscillations of the measuring instrument arrows.
If instead of a massive pendulum we take a comb with teeth perpendicular to the field of the electromagnet, a rapid stoping of oscillations will not occur because the vortex currents in the pendulum can no longer develop to large values. Gaps between the teeth prevent this from happening.