Final answer:
To measure the resistance of a relay coil accurately, it should be removed from the circuit and measured with the circuit disconnected to prevent damage to the ohmmeter and to avoid including the resistance of other circuit components in the measurement.
Step-by-step explanation:
When measuring the resistance of a relay coil, it should be removed from the circuit and measured with the circuit disconnected. This is because the ohmmeter needs to measure the resistance of the coil alone, without the influence of other components or the power supply.
Attempting to measure resistance with the circuit powered can damage the meter, as ohmmeters are not designed to be connected to a live circuit. Moreover, an accurate measurement cannot be obtained during normal operation or when measured in parallel with other components, as these methods incorporate the resistances of other parts of the circuit.
When using a voltmeter, it should be connected in parallel with the device it is measuring to ensure that it does not significantly alter the current flow through the circuit—in contrast to ammeters, which are connected in series due to their low resistance and purpose to measure current flow. However, to measure resistance accurately with an ohmmeter, the component should be isolated to prevent reading the combined resistance of the whole circuit.