What is the function of a voltmeter?

A voltmeter is specifically designed to measure the electrical potential difference, or voltage, between two points in a circuit. This tool is essential for assessing how much electrical energy is available to push electric charges through the circuit, which is critical for understanding the performance and operation of electrical systems. By providing a direct reading of voltage, a voltmeter helps electricians and engineers diagnose electrical faults, ensure components are operating within their intended voltage range, and verify the proper functioning of power supplies.

Other options describe functions associated with different types of measuring instruments. For instance, measuring resistance requires an ohmmeter, while current flow is measured with an ammeter. Power usage measurement typically involves a wattmeter, as it combines both voltage and current measurements to provide power consumption data. Thus, the voltmeter's unique capability to measure voltage levels is what makes it the correct answer in this context.