What is indicated by a voltmeter reading?

A voltmeter measures the electrical voltage between two points in a circuit, which is crucial for understanding how much electrical potential energy exists to drive current through the circuit. This reading can provide valuable information regarding the performance of electrical devices and help diagnose problems such as insufficient voltage supply or excessive drop across components.

The other choices do not accurately reflect what a voltmeter provides. The length of wires is not measured by voltage; rather, it could influence resistance but not the voltage itself. Total power in a circuit is determined by the product of voltage and current, while the type of components refers to the materials and devices in the circuit, which do not concern voltage directly. Thus, the primary role of a voltmeter is to indicate voltage differences, making the correct answer clear.