In connection with an investigation of r.f. discharges in magnetrons the measurement of interconductor voltage and lead current in the magnetron output by comparative methods using capacitive and loop probes is discussed. In practice loop-probe measurements are difficult to interpret, and an estimate of the current may not be possible. Some uncertainties may be avoided by representing the magnetron output by an equivalent circuit whose constants can be found from the probe measurements, and from which the current and voltage can be calculated.In the paper equivalent-circuit constants for a magnetron having a coaxial output are derived from ‘cold’ probe voltage measurements carried out using matched, short-circuit, and constant reflection terminations. The voltage and current at certain points in the valve are calculated for the power output and loading conditions under which discharges were observed in oscillation tests.The circuit representation is general, and the methods may be applied in principle to magnetron or cavity outputs different from the one described.