URM-25x models also contain an internal VTVM ( vacuum tube voltmeter ) and crystal calibration.
2.
Such amplified multimeters are called VTVMs ( vacuum tube voltmeters ), TVMs ( transistor volt meters ), FET-VOMs, and similar names.
3.
The vacuum tube voltmeter ( VTVM ) uses the high input impedance of a valve to buffer the circuit being measured from the load of the ammeter.
4.
In a potentiometer bridge circuit, this could be reduced to a fraction of a microampere . Vacuum tube voltmeters from the 1940's on had this high or higher input impedances.
5.
"Vacuum Tube Voltmeters " or valve voltmeters ( VTVM, VVM ) were used for voltage measurements in electronic circuits where high cathode follower input circuit, and thus did not significantly load the circuit being tested.
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