1. Basic principles

1.1 Elementary principle

Consider the device shown in figure 1 a: a magnet and a disc placed face to face, attached to two shafts arranged in line with each other. The shaft connected to the magnet is driven in a certain direction, while the other is left free of any mechanical constraint.

Let's designate a small area of the plate as M. If the magnet and the plate rotate at two different speeds, M will first face one pole, then the other. The metal masses making up this small zone will therefore be alternately crossed by lines of force, subject to a flux that varies over time; they will be the seat of currents of the same nature as eddy currents.

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