Copper witness plates were placed in the cavity of the Strathclyde University electron cyclotron maser (ECM). From the resulting damage to the copper surface of the plates it proved possible to measure: the beam diameter, which was 3 mm; the negligible beam divergence as it travelled down the tube:; and its relative position in the centre of the cavity. By approximating the copper plate as a semi-infinite slab it also proved possible to gain an estimate of the beam power flux, approximately=72 MW cm^-2. Hence, knowing the beam accelerating potential (72 kV), the beam current was estimated to be 100 A.