An apparatus is described which makes possible kinetic studies of phase transformations which occur in metals within times as short as a few milliseconds. The apparatus consists of a capacitor discharge heating unit together with electronic circuitry which permits quantitative kinetic measurements during transformations which produce only small resistivity changes. The small voltage changes produced by the transformations are resolved by differentiating out the dc component of the signal and then integrating the output of the differentiator. Measurements are made on the integrator output, which contains only the ac component of the original signal.