What is the primary use of a voltmeter in electrical circuits?

A voltmeter is specifically designed to measure the electrical potential difference, or voltage, between two points in an electrical circuit. This measurement is essential for understanding how much electrical energy is available to drive current through a circuit, which is crucial for diagnosing issues, evaluating system performance, and ensuring safe operating conditions. When connected across two points, the voltmeter displays the potential difference without significantly affecting the circuit's operation, as it has a very high resistance. This characteristic allows it to take accurate readings without drawing substantial current, which could alter the behavior of the circuit being measured.

The other options focus on different aspects of electrical measurement—current flow, resistance, and frequency—each requiring specialized instruments like ammeters for current, ohmmeters for resistance, and frequency counters for measuring frequency, highlighting the importance of using the right tool for specific electrical parameters.