रेडियो आवृत्तियों को मापने के लिए निम्नलिखित में से कौन-सा आवृत्ति मीटर सबसे उपयुक्त है?

Which minimum set of instruments is required to measure the power factor of a balanced three-phase load?

Consider the following statements regarding the electrodynamometer-type instrument: i) It operates on the principle of a DC motor. ii) When used as a wattmeter, the instrument will have a non-uniform scale. iii) It is more expensive compared to a moving-iron instrument. iv) When the instrument is used in an AC circuit, there is no hysteresis loss in its operating torque mechanism. Which of the above statements are correct?