A partial core transformer has been designed, built and tested for its performance in air and while immersed in liquid nitrogen. The transformer was designed as a mock up of a proposed high temperature superconducting transformer, but with aluminium windings. The partial core was a slug of laminated silicon steel. Open circuit and short circuit tests were conducted to determine the performance of the transformer. These yielded discrepancies between the calculated and measured values of many parameters, but did indicate the level of expected standing losses, and showed that the magnetic flux coupling between windings for these transformers is very high and that there is a low percentage of this that is leakage flux. Full load tests conducted on the transformer showed a high level of efficiency and low regulation, even at ambient temperatures.