Electrical tomography apparatus comprises: 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 having a first terminal and a second terminal; and means for generating an alternating current through the first winding so as to generate an alternating voltage between said first terminal and said second terminal. The apparatus further comprises connecting means arranged to connect the first terminal to the first electrode and the second terminal to the second electrode. Generation of the alternating current in the first winding results in generation of an alternating voltage between the first and second electrodes. In certain embodiments, a current sensing transformer provides an indication of current driven through the medium.