What does a voltmeter measure?

A voltmeter is designed specifically to measure voltage, which is the difference in electric potential between two points in an electrical circuit. When connected across a component in the circuit, it provides a reading that indicates how much electric potential is available to drive current through that component. This measurement is crucial for understanding how much voltage is applied in a given situation, which in turn informs how devices will function in their respective circuits.

The other options relate to different electrical measurements. Resistance is measured by an ohmmeter; power is calculated using voltage and current; and current is measured with an ammeter. Each of these tools serves a distinct purpose, which helps to create a comprehensive understanding of an electrical system when used in conjunction with a voltmeter.