# Rotation of aluminum disc

1. Jan 23, 2012

### ajith.mk91

In an induction meter two coils induce two eddy currents in same disc.It is said that the disc rotates due to the torque produced by the interaction of these currents with each other.How is that?I don't know the governing law.Can the torque be derived from the basic maxwell's equation?

2. Jan 24, 2012

### Blibbler

Here's what a famous online encyclopaedia has to say on the matter:

The electromechanical induction meter operates by counting the revolutions of an aluminium disc which is made to rotate at a speed proportional to the power. The number of revolutions is thus proportional to the energy usage. It consumes a small amount of power, typically around 2 watts.
The metallic disc is acted upon by two coils. One coil is connected in such a way that it produces a magnetic flux in proportion to the voltage and the other produces a magnetic flux in proportion to the current. The field of the voltage coil is delayed by 90 degrees using a lag coil.[17] This produces eddy currents in the disc and the effect is such that a force is exerted on the disc in proportion to the product of the instantaneous current and voltage. A permanent magnet exerts an opposing force proportional to the speed of rotation of the disc. The equilibrium between these two opposing forces results in the disc rotating at a speed proportional to the power being used. The disc drives a register mechanism which integrates the speed of the disc over time by counting revolutions, much like the odometer in a car, in order to render a measurement of the total energy used over a period of time.

You might also want to learn about Lenz's Law and the Lorentz force as these will give you greater insight into what is actually happening in the meter.