# A homopolar disc dynamo experiment with liquid metal contacts

## TL;DR

This paper reports on an experimental homopolar disc dynamo with liquid metal contacts, observing magnetic field fluctuations and proposing conditions for self-excitation at higher rotation rates.

## Contribution

First experimental demonstration of a homopolar disc dynamo with liquid metal contacts, analyzing magnetic fluctuations and identifying conditions for self-excitation.

## Key findings

- Detected fluctuating magnetic fields comparable to Earth's magnetic field.
- Observed liquid metal leakage at 14 Hz, hindering steady magnetic field formation.
- Suggested dynamo excitation at 21 Hz if liquid metal leakage is prevented.

## Abstract

We present experimental results of a homopolar disc dynamo constructed at CICATA-Quer\'etaro 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 10 Hz. We measured the magnetic field above the disc and the voltage drop on the coil for the rotation rate up to 14 Hz, 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 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 the rotation rate of around 21 Hz provided that the leakage of liquid metal is prevented.