What instrument is used to determine whether 25 Hz or 60 Hz is present in an electrical circuit?

The instrument used to determine whether 25 Hz or 60 Hz is present in an electrical circuit is a frequency meter. A frequency meter measures the frequency of an alternating current (AC) signal and can provide an accurate reading of cycles per second, which is essential for identifying the specific frequency being utilized in the circuit. Understanding the frequency is crucial because different frequencies can affect the operation of electrical equipment differently and can be vital for ensuring compatibility with the system in use.

In contrast, a voltmeter measures the voltage level in the circuit, but it does not provide any information about the frequency of the AC signal. An ohmmeter is designed to measure resistance in ohms, while an ammeter measures the current flowing through a circuit but does not indicate frequency. Each of these other instruments serves a distinct purpose, but only a frequency meter is capable of determining the specific frequency present, making it the correct choice for this question.