Abstract: An electric charge is created on a moving reproduction carrier by each of a plurality of electrodes extending in a line perpendicular to the direction of transport of the reproduction carrier. Each of the signals applied to the electrodes has an amplitude corresponding to the density of the corresponding point of the original. The charge created by the electrode has undesirable variations resulting from differences in the electrode characteristics and from differences in the amplifiers amplifying the signal before application to each electrode. After each charge distribution from a copy has been created, a test signal is supplied to the inputs of all amplifiers. This creates a test charge distribution on the reproduction carrier. After developing of the reproduction carrier the test line created by developing of the test charge distribution is scanned photoelectrically.