The magnetic field and the driven gas in a plasma accelerator are separated by a thin, dense current layer, provided that the time scale of the operating impulse is sufficiently short. Theoretical models of current-sheet structure usually involve the assumption that the propellant is a fully ionized plasma; whereas in most real devices, there is no significant preionization. Thus, such formulations are of limited usefulness. Almost invariably, the current sheet itself is the ionizing agent. We have conducted a series of experiments in which the object is the determination of such current sheet structure through detailed mapping of the electromagnetic field structure [E (r, t), B (r, t)] of the system, and mapping of the electron density through a modified form of schlieren photography. Discharge geometries characterized by good optical accessibility and uniform current layers have been used, i.e., a parallelplate accelerator and an inductive "theta pinch." The experimental approach is to relate E, B, and ne through the generalized Ohm's law and thus to draw inferences concerning gross plasma drifts, ion-electron current partitioning, electrical conductivity, and electron temperature. One significant finding is that in a plasma gun running on hydrogen, an important fraction of the total current seems to be carried by ions in a displacement current occurring with the ionization process.