Electrical impedance tomography. The provision of an electric field of controllable penetration into a cylinder.

If a cylinder carries on its surface an electric charge of a density which is constant in the axial direction, but which varies sinusoidally with an integral number of wavelengths around the circumference, then inside the cylinder the electric field has its maximum value at points adjacent to the surface, and reduces continuously as the axis is approached. The rate of reduction increases with the number of wavelengths. A rotating cylinder of such charge may be simulated by a number (divisable by three) of equally spaced conducting bars mounted longitudinally on the surface of a stationary cylinder, the bars being connected in sequence to the lines of a three-phase voltage supply. The number of wavelengths of charge around the circumference can be varied by the connection together of a chosen number of adjacent bars effectively to form a single bar, and so fields of varying penetration depths can be produced from a fixed frequency supply. The variation of the resulting currents would be related to the corresponding variation in the complex conductivity of the cylinder material.