A theoretical calculation is made from which the electrical conductivity or magnetic Reynolds number of a shock‐produced plasma flow can be derived. The results of the analysis predict the effects of the transient magnetic field induced by the interaction between the applied magnetic field and the plasma flow behind a shock wave. The applied magnetic field considered is transversal to the flow. Its intensity is either steady in time and increasing linearly in the flow direction or spatially uniform but increasing linearly with time.If the applied field extends to infinity, the intensity of the induced magnetic field is proportional to the magnetic Reynolds number and to the intensity of the applied magnetic field. With the aid of the present analysis the magnetic Reynolds number can be determined by comparison of the measured field and the calculated magnetic field. This method is applicable even if the magnetic Reynolds number is greater than unity. The effects of the finiteness of the applied field are considered.