1. A method for measuring the value of an unknown voltage which comprises: applying a time-varying current from a reference voltage source through a first resistor to a capacitor thereby to increase the voltage across said capacitor as a function of time; measuring the time Tr required for the voltage across said capacitor to reach a selected voltage level when current from the reference voltage source is applied to said capacitor; restoring the voltage across said capacitor to its initial state; applying a time-varying current from an unknown voltage source through a second resistor, whose resistance is equal to that of said first resistor, to said capacitor thereby to increase the voltage across said capacitor as a function of time; measuring the time Tb required for the voltage across said capacitor to reach said selected voltage level when current from the unknown voltage source is applied to said capacitor; and calculating the unknown voltage from the times Tr and Tb, wherein said first resistor connected between said reference voltage source and said capacitor and said second resistor connected between said unknown voltage source and said capacitor comprise two distinct resistors.