Editing Multimeter (section)

=== Vacuum tube voltmeters ===
Vacuum tube voltmeters or valve [[voltmeters]] (VTVM, VVM) were used for voltage measurements in electronic circuits where high [[Electrical impedance|input impedance]] was necessary. The VTVM had a fixed input impedance of typically 1&nbsp;MΩ or more, usually through use of a [[Electronic amplifier#Common terminal|cathode follower]] input circuit, and thus did not significantly load the circuit being tested. VTVMs were used before the introduction of electronic high-impedance analog [[transistor]] and [[field effect transistor]] voltmeters (FETVOMs). Modern digital meters (DVMs) and some modern analog meters also use electronic input circuitry to achieve high input impedance—their voltage ranges are functionally equivalent to VTVMs. The input impedance of some poorly designed DVMs (especially some early designs) would vary over the course of a [[sample-and-hold]] internal measurement cycle, causing disturbances to some sensitive circuits under test.