Dielectric change sensing device

Claim 1

A sensing device for use in the sensing of impedance changes, which sensing device comprises a sensing head and an output circuit connected thereto, wherein the sensing head comprises a bridge circuit and a conducting screen in the form of an open-ended box within which the bridge circuit is rigidly mounted, said bridge circuit comprising: a. a first diode and first resistor connected in series; b. a second diode and a second resistor connected in series; the first and second diodes being connected to a first junction which is adapted to be connected to a modulated 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 being connected to a second junction; c. a first capacitor, the plates of which are connected respectively to earth and to a junction which is between the first diode and the first resistor; and d. a second capacitor, the plates of which are connected respectively to earth and to a junction which is between the second diode and the second resistor; the conducting screen having an aperture therein adjacent to the first capacitor; and said output circuit comprising: i. a direct current blocking capacitor, one plate of which is connected to said second junction, and the other plate of which provides the output from the device; and ii. a load resistor connected between said second junction and earth; wherein, in use, a change in the dielectric properties in a body adjacent to the first capacitor produces a change in the pulsed output signal from said output provided by said direct current blocking capacitor.