Similarity solutions describing the flow behind a plane hydromagnetic shock propagating with a constant velocity into a uniform ideal gas at rest in the presence of a transverse magnetic field are obtained. The gas is assumed to be infinitely electrically conducting, inviscid and non-heat conducting. The gain in the total energy of the flow between the shock and the inner expanding surface is assumed to be time-dependent. The variations of the percentages of the magnetic, internal and kinetic energies with the strength of the shock are studied. It is shown that there exists two values of the strength of the shock at which equipartition of the internal and kinetic energies of the flow between the shock and the inner expanding surface can occur.