A study has been made of the axial contraction of plasma in a theta pinch, which occurs when the magnetic field trapped by the plasma is reversed in sense relative to the confining field, thus creating a closed-field-line configuration. This contraction takes the form of an apparent area shock—that is, of an increase in area—which propagates from the ends of the coil to its centre. Previous studies have shown that the contraction is of this form and that the velocity is of the order predicted by a simple theory.In the present experiment the area changes have been detected by measurement of the changes in flux distribution arising from them with a balanced magnetic pick-up loop system. The amplitude, velocity and rise time of the area shocks have been measured as functions of the initial trapped field at the start of the pinch. The correlations between these factors have been compared with the theoretical predictions.In particular the velocity of propagation predicted by steady state theory is given by the expression V2 = Cu2αy1(y1 + y0) where V is the velocity of propagation, Cu is a characteristic velocity, α a constant depending on tube and coil geometry and y1 y>0 the ratios of the area behind and ahead of the shock, respectively, to the coil area. The experimental value of Cu in a non-steady-state magnetic field was measured to be 1.05 × 108 cm/sec, compared with the theoretical value of 1.0 × 108 cm/sec calculated for constant magnetic field at its peak value.The area behind the shock has been calculated under the assumption that it is of such a value as to give pressure balance at the piston. The derived expression is y12 = 2z where z is the ratio of the area filled with trapped field to the coil area.Combination of these two equations shows that the energy in the shock comes from the disappearance of the trapped field.Finally the results on the apparent rise time of the shock show that its value depends critically on the trapped flux. It is suggested that this may arise from the effects of reversed field loops which close within the plasma. Consequently a comparison of the data with computer calculations may lead to the derivation of information concerning the gradients of field within the plasma.