Abstract: A sensing head for use in the sensing of impedance changes includes a bridge circuit, a first diode and first resistor connected in series and a second diode and a second resistor connected in series. The first and second diodes are connected to a first junction which is adapted to be connected to a high frequency alternating voltage supply such that, in use, the diodes will pass electrical signals of opposite sign from the junction. The first and second resistors are connected to a second junction which is adapted to be connected in use to an output circuit. There are also provided a first capacitor, a plate of which is connected to a junction which is between the first diode and the first resistor, and a second capacitor, a plate of which is connected to a junction which is between the second diode and the second resistor. When in use, a change in the dielectric properties in a body adjacent to the first capacitor produces a change in the output signal across the output circuit.