Page "Electrometer" Paragraph 19

Solid-state electrometers are often multipurpose devices that can measure voltage, charge, resistance and current.

They measure voltage by means of " voltage balancing ", in which the input voltage is compared with an internal reference voltage source using an electronic circuit with a very high input impedance ( of the order of 10 < sup > 14 </ sup > ohms ).

A similar circuit modified to act as a current-to-voltage converter enables the instrument to measure currents as small as a few femtoamperes.

Combined with an internal voltage source, the current measuring mode can be adapted to measure very high resistances, of the order of 10 < sup > 17 </ sup > ohms.

Finally, by calculation from the known capacitance of the electrometer's input terminal, the instrument can measure very small electric charges, down to a small fraction of a picocoulomb.