Which instrument measures voltage in a circuit?

The correct answer is a voltmeter. This instrument is specifically designed to measure the voltage, or electrical potential difference, between two points in a circuit. It operates by being connected in parallel with the components or voltage source whose voltage is being measured. The voltmeter provides a quantitative reading of voltage, allowing for accurate assessments of circuit performance.

While a multimeter can also measure voltage and a variety of other electrical parameters (such as current and resistance), its designation as a "multi-purpose" tool means that it encompasses broader functionalities beyond just measuring voltage. An ammeter, on the other hand, is intended for measuring current, not voltage. Similarly, a tester may refer to devices that check for voltage presence but may not provide an actual voltage measurement like a voltmeter does. Thus, when focusing solely on an instrument dedicated to measuring voltage, the voltmeter is the clear answer.