Electrical tomography apparatus comprising: a first electrode; a second electrode; and current driving means for driving an electrical current between the first electrode and the second electrode through a medium, wherein the current driving means comprises: a first transformer having a first winding and a second winding, the second winding 10 having a first terminal and a second terminal; and means for generating an alternating current (221, 222) through the first winding so as to generate an alternating voltage between said first terminal and said second terminal, the apparatus further comprising: connecting means arranged to connect the first terminal to the first electrode and the second terminal to the second electrode, whereby generation of the alternating current in the first winding results in generation of an alternating voltage between the first and second electrodes (11a, 11b); and current sensing means arranged to provide a current signal indicative of current flowing through said second winding, characterized in that the current sensing means comprises: a second transformer having a primary winding and a secondary winding arranged such that an alternating current flowing in the primary winding generates a corresponding alternating voltage across the secondary winding, the primary winding being connected in series with the second winding of the first transformer such that the same current flows through the second and primary windings (212, 411) and a voltage developed across the secondary winding providing said current signal. a first electrode; a second electrode; and current driving means for driving an electrical current between the first electrode and the second electrode through a medium, wherein the current driving means comprises: a first transformer having a first winding and a second winding, the second winding 10 having a first terminal and a second terminal; and means for generating an alternating current (221, 222) through the first winding so as to generate an alternating voltage between said first terminal and said second terminal, a first transformer having a first winding and a second winding, the second winding 10 having a first terminal and a second terminal; and means for generating an alternating current (221, 222) through the first winding so as to generate an alternating voltage between said first terminal and said second terminal, the apparatus further comprising: connecting means arranged to connect the first terminal to the first electrode and the second terminal to the second electrode, whereby generation of the alternating current in the first winding results in generation of an alternating voltage between the first and second electrodes (11a, 11b); and current sensing means arranged to provide a current signal indicative of current flowing through said second winding, characterized in that the current sensing means comprises: a second transformer having a primary winding and a secondary winding arranged such that an alternating current flowing in the primary winding generates a corresponding alternating voltage across the secondary winding, the primary winding being connected in series with the second winding of the first transformer such that the same current flows through the second and primary windings (212, 411) and a voltage developed across the secondary winding providing said current signal. connecting means arranged to connect the first terminal to the first electrode and the second terminal to the second electrode, whereby generation of the alternating current in the first winding results in generation of an alternating voltage between the first and second electrodes (11a, 11b); and current sensing means arranged to provide a current signal indicative of current flowing through said second winding, characterized in that the current sensing means comprises: a second transformer having a primary winding and a secondary winding arranged such that an alternating current flowing in the primary winding generates a corresponding alternating voltage across the secondary winding, the primary winding being connected in series with the second winding of the first transformer such that the same current flows through the second and primary windings (212, 411) and a voltage developed across the secondary winding providing said current signal.