Final answer:
To check a pick-up coil with an A/C voltmeter, you would look for voltage fluctuation, which indicates the coil is properly inducing an electromotive force in response to a changing magnetic field.
Step-by-step explanation:
When checking a pick-up coil using an A/C voltmeter, you would look for voltage fluctuation. The movement of a magnet through the coil or the coil through a magnetic field induces a voltage that can be observed as fluctuations on a voltmeter. These voltage changes are due to Faraday's Law of Induction, which states that a changing magnetic field through a coil induces an electromotive force (EMF) in the coil.
This induced voltage will result in an alternating current (AC) if the circuit is closed. When using an A/C voltmeter to measure the coil's voltage, a fluctuation in voltage indicates that the coil is functioning properly, as the motion of the magnetic field relative to the coil is generating a change in magnetic flux, thus inducing an electromotive force across the coil.