Resonance Effects: Tesla Bifilar Coil Alternating Field Rotating a Magnetic Compass

Here’s a little illustration of an interesting phenomenon. A small magnetic compass is placed onto a Tesla Bifilar pancake coil, and the coil is driven by an alternating square wave at a frequency that corresponds to a natural mechanical resonant mode of the compass. When the tuning is just right, the compass starts rotating steadily (more or less) in one direction. Once the rotation is synched with the FG’s output, a gradual increase in the frequency causes the rotation of the compass to accelerate a bit.

This is NOT an illustration of a rotating magnetic field! The field, which circulates radially through the coil, is just switching directions, and as the compass needle tries to follow it, it overshoots, and the timing is such that the overshoot continues around and around as the coil’s field switches direction with every half-cycle of the square wave driving it.

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