An electrodynamic sensor comprises a high input impedance electrometer adapted to measure small electrical potentials originating from an object under test and having a pair of input probes, characterized in that at least one of said pair of input probes has no direct electrical contact with said object, wherein the circuit arrangement of said electrometer comprises an amplifier which includes a combination of ancillary circuits cumulatively to increase the sensitivity of said electrometer to said small electrical potentials whilst not perturbing the electrical field associated therewith.