What is a common application for a rheostat?

A rheostat is commonly used to adjust the level of current in a circuit. This electrical component operates by varying the resistance within an electrical circuit, which in turn affects how much current flows through that circuit. By increasing or decreasing the resistance, a rheostat allows for precise control over the current, making it useful in applications such as dimming lights or controlling motor speeds.

In contrast to the other options, measuring voltage typically requires a different kind of device, such as a voltmeter. Controlling the frequency of AC signals is accomplished with devices like variable frequency drives, which are specifically designed for that purpose. Storing electrical energy is primarily the function of capacitors or batteries, not rheostats. Therefore, the prime function of a rheostat to adjust current levels in a circuit underscores its widespread application in various electrical systems.