Owing to the combination of high current density and low forward voltage drop in the rectifying junction of a semi-conductor rectifier, the physical size and thermal capacity for a given rating are low, so that the current on overload can be much greater than for other types of rectifier. Consequently, the junction-temperature rise on overload is extremely rapid, and in some cases considerable temperature fluctuations occur at mains frequency. It is therefore necessary to be able to monitor the junction temperature dynamically (i.e. under working conditions at mains frequency) and to observe the rise of temperature with time. A further necessity for dynamic testing occurs since the measurement of the forward characteristic at high current by d.c. methods can result in excessive heating and possible failure. In the apparatus to be described the separation of forward and reverse half-cycles is secured by means of a synchronous commutator, permitting independent measurement of forward and reverse current and voltage at mains frequency, and hence continuous indication of junction temperature. Operation of the rectifier at full rating without expenditure of the full power necessary to accomplish this by direct methods is also possible. This is particularly useful in life testing large numbers of semi-conductor rectifiers. Limited experience indicates that this method of testing provides operating conditions which are in all ways identical to those which a device experiences under fullload conditions