By incorporating a variable position short circuit plunger in one of the side arms, a magic-T can be used to separate the variations in the resistive and reactive parts of a load impedance. The technique is particularly well suited for measuring transient variations of the impedance, and this paper deals with the variation of impedance of a 25 μm wire subjected to 1 μs pulses of current density about 10 KA mm⁻². The results suggest that a longitudinal acoustic wave is excited in the wire.