Electrometer

**An electrometer is an electrical instrument for measuring electric charge or differences.
Electrometers can be used to make voltage and charge measurements with very low currents.
An electrometer is a sensitive electronic voltmeter whose input is so high that the current flowing into it could be zero. It can measures current, charge, voltage, and resistance. There are two classes of electrometers, mechanical and electronic, but the mechanical instruments have been largely replaced by electronic types.

This picture of electrometers:**

Refrance :
http://en.wikipedia.org/wiki/Electrometer

http://www.testequipmentconnection.net/KEITHLEY/KEITHLEY_610C.jpg