A homopolar disc dynamo experiment with liquid metal contacts

We present experimental results of a homopolar disc dynamo constructed at CICATA-Querétaro in Mexico. The device consists of a flat, multi-arm spiral coil which is placed above a fast-spinning metal disc and connected to the latter by sliding liquid-metal electrical contacts. Theoretically, self-excitation of the magnetic field is expected at the critical magnetic Reynolds number Rm ≈ 45, which corresponds to a critical rotation rate of about 10Hz. We measured the magnetic field above the disc and the voltage drop on the coil for the rotation rate up to 14Hz, at which the liquid metal started to leak from the outer sliding contact. Instead of the steady magnetic field predicted by the theory, we detected a strongly fluctuating magnetic field with a strength comparable to that of the Earth's magnetic field, which was accompanied by similar voltage fluctuations in the coil. These fluctuations seem to be caused by the intermittent electrical contact through the liquid metal. The experimental results suggest that the dynamo with the actual electrical resistance of liquid metal contacts could be excited at a rotation rate of around 21Hz provided that the leakage of liquid metal is prevented.