FET and vacuum-tube voltmeters:

It known that good voltmeter will disturb the circuit under test as less as possible, and this requires that the meter posses a high internal resistance. Besides the electrostatic type voltmeter, there is other way to obtain an extremely high internal resistance. When the miniscule amount of current is drawn from the circuit, the equivalent resistance is extremely high.

The most effective technique to accomplish the amplification, while making sure that the current drawn is less, is to use either a vacuum tube or the field-effect transistor. You are not to worry about how such amplifiers work right now. A voltmeter which uses a vacuum tube amplifier to minimize the current drain is called as a vacuum tube voltmeter (VTVM). If an FET is used, meter is called as FET voltmeter (FETVM). Either of these devices gives an extremely high input resistance along with the good sensitivity and amplification. And they allow the measurement of lower voltages, generally than electrostatic voltmeters.