A Tesla transformer similar to that suggested for transformer−type pulsed accelerators is described. The theory is outlined and computational results presented for representative losses under various conditions of tuning and coupling. The behaviour of an experimental transformer housed in an SF6−filled pressure vessel is discussed. The device has a tuning ratio of 1.1, a voltage coefficient of 0.85, and a coupling coefficient of 0.37. The influence of eddy currents induced in the pressure vessel wall on the coupling coefficient and tuning ratio is noted. The particular transformer described can provide a 2.5 MV peak at the third half cycle of the secondary output voltage.