Apparatus for measuring small capacitance values independent of stray capacitance

Claim 1

CAPACITANCE MEASURING APPARATUS COMPRISING: A DIRECT CURRENT METER FOR INDICATING THE VALUE OF THE CAPACITANCE TO BE MEASURED; A PULSE GENERATOR PROVIDING A CONTINUOUS TRAIN OF PULSES OF AT LEAST ONE GIVEN POLARITY; A MEASURING CIRCUIT COMPRISING: A CAPACITOR CHARGE CIRCUIT COUPLED TO SAID PULSE GENERATOR TO RECEIVE SAID PULSES THEREFROM AND INCLUDING RECTIFIER MEANS FOR BLOCKING CURRENT FLOWING IN A DIRECTION OPPOSITE TO THE CURRENT RESULTING FROM SAID PULSES; THE CAPACITOR CHARGE CIRCUIT INCLUDING TERMINAL MEANS FOR CONNECTING THE CAPACITOR TO BE MEASURED INTO THE CHARGE CIRCUIT, SAID CAPACITOR CHARGE CIRCUIT HAVING A TIME CONSTANT WHICH EFFECTS SUBSTANTIALLY THE FULL CHARGING OF THE LARGEST CAPACITOR TO BE MEASURED WITHIN THE DURATION OF EACH OF SAID PULSES, AND A CAPACITOR DISCHARGE CIRCUIT HAVING A TIME CONSTANT WHICH EFFECTS SUBSTANTIALLY THE FULL DISCHARGING OF SAID CAPACITOR IN SAID MEASURING CIRCUIT IN THE INTERIM BETWEEN THE GENERATION OF SUCCESSIVE ONES OF PULSES AND INCLUDING RECTIFIER MEANS FOR BLOCKING CURRENT FLOW IN A DIRECTION OPPOSITE TO THE FLOW OF THE DISCHARGE CURRENT FROM THE LATTER CAPACITOR; A BALANCING CIRCUIT COMPRISING: A SECOND CAPACITOR CHARGE CIRCUIT COUPLED TO SAID PULSE GENERATOR TO RECEIVE SAID PULSES THEREFROM AND INCLUDING RECTIFIER MEANS FOR BLOCKING CURRENT FLOWING IN A DIRECTION OPPOSITE TO CURRENT RESULTING FROM SAID PULSES AND A VARIABLE BALANCING CAPACITOR TO BE CHARGED, SAID SECOND CAPACITOR CHARGE CIRCUIT HAVING A TIME CONSTANT WHICH EFFECTS SUBSTANTIALLY THE FULL CHARGING OF THE BALANCING CAPACITOR THEREIN WITHIN THE DURATION OF EACH OF SAID PULSES; AND A SECOND CAPACITOR DISCHARGE CIRCUIT FOR THE CAPACITOR IN SAID SECOND CHARGE CIRCUIT WHICH EFFECTS SUBSTANTIALLY THE FULL DISCHARGING OF SAID BALANCING CAPACITOR IN THE INTERIM BETWEEN SUCCESSIVE ONES OF SAID PULSES AND INCLUDING RECTIFIER MEANS FOR BLOCKING CURRENT FLOW IN A DIRECTION OPPOSITE TO THE FLOW OF THE DISCHARGE CURRENT FROM THE LATTER CAPACITOR; AND SAID DIRECT CURRENT METER BEING DIFFERENTIALLY CONNECTED TO SAID MEASURING AND BALANCING CIRCUITS TO RECEIVE CURRENT FLOWING IN OPPOSITE DIRECTIONS RESPECTIVELY FROM SAID MEASURING AND BALANCING CIRCUITS, TO PROVIDE A RESULTANT INDICATION INDICATING THE EXCESS OF THE CAPACITANCE TO BE MEASURED OVER THE STATIC CAPACITANCE IN THE MEASURING CIRCUIT, SAID VARIABLE BALANCING CAPACITOR OF SAID BALANCING CIRCUIT BEING EFFECTIVE FOR ADJUSTING THE READING OF SAID METER TO ZERO WHEN THE CAPACITANCE OF SAID MEASURING CIRCUIT REPRESENTS ONLY SAID STATIC CAPACITANCE.