To measure the amount of electrical power used in a balanced three-phase circuit, which instrument should you use?

To measure the amount of electrical power used in a balanced three-phase circuit, a wattmeter is the appropriate instrument to use. A wattmeter is specifically designed to measure electrical power, which is the product of voltage, current, and the cosine of the phase angle (power factor) in an electrical system. In a balanced three-phase circuit, where the loads are equal across all three phases, the wattmeter can provide an accurate reading of the total power consumption by calculating the power in each phase and summing them.

Using a wattmeter allows for a direct measurement of active power (in watts), which is the actual power consumed by the circuit to perform work, contrasting with reactive power and apparent power measurements. This is crucial for understanding the efficiency and performance of electrical systems.

While other instruments listed may serve important functions in electrical work, such as measuring insulation resistance (Megger), visualizing waveforms (oscilloscope), or checking frequency stability (frequency meter), they do not directly measure the power consumption in a circuit. Thus, for the specific goal of measuring electrical power in a balanced three-phase circuit, the wattmeter is the most suitable choice.