Electrical measuring apparatus employing a plurality of condition responsive devices

Claim 1

Electrical measuring apparatus, comprising: a. a plurality of impedance devices whose impedances vary in response to a condition to be measured, each device having an input terminal and an output terminal; b. a source of high frequency electrical energy having two terminals, one of which is grounded; c. a shielded conductor means connecting the ungrounded terminal of the source to the input terminals of all said impedance devices, to supply to all said input terminals a common input signal of the same amplitude and phase; d. a plurality of low input impedance amplifiers, each having an input terminal maintained near ground potential, and an output terminal; e. unshielded conductors connecting the output terminals of the impedance devices to input terminals of the amplifiers, said amplifiers being substantially unaffected by tray capacitances between the unshielded conductors and ground because of the low potential of the amplifier input terminals; f. a divider having two input terminals and an output terminal; g. means connecting some of the amplifier output terminals to one of the divider input terminals; h. means connecting all the other amplifier output terminals to the other divider input terminal, said divider being effective to produce at its output terminal a signal determined by the impedances of said impedance devices and independent of the frequency and potential of the source and the gain and phase shift in the amplifiers; and i. indicator means responsive to said signal.