Definition
An electronic instrument that measures voltage using vacuum tubes in its input circuit to provide very high input impedance. The high impedance allows the meter to measure voltage in a circuit without drawing significant current from it, which means the meter does not noticeably disturb the circuit being tested. Largely replaced in modern use by solid-state digital multimeters, but historically common in aviation electronics and radio shop work.
Plain English
A voltage-measuring tool that uses vacuum tubes inside it so it can take a reading without changing what's happening in the circuit it's testing.
Context Anchor
Seen in aircraft maintenance and older electronics troubleshooting, especially when checking radios, instruments, or other sensitive electrical circuits.
Derivation
Vacuum tube refers to a sealed glass tube with the air pumped out, used to control electron flow in early electronics. Voltmeter combines volt (the unit of electrical pressure, named after Italian physicist Alessandro Volta) with meter (a measuring device). So the name simply describes a voltmeter built around vacuum-tube circuitry.
Why Pilots Care
Many classic aircraft still in service rely on test procedures written for vacuum-tube voltmeters, and using the wrong modern meter can give inaccurate readings on high-impedance circuits.
Intuition Check
Do not confuse a vacuum-tube voltmeter with an aircraft vacuum-system gauge. Here, “vacuum tube” means an electronic part, not the suction system used to run some flight instruments.
Example Sentence 1
The avionics technician used a vacuum-tube voltmeter to check the signal voltage in the radio without loading down the circuit.
Example Sentence 2
Older service manuals often warn against substituting a digital meter for the specified vacuum-tube voltmeter when measuring grid bias.