What should NOT be used if an instrument is measuring the wattage of an electric device?

An ohmmeter should not be used for measuring the wattage of an electric device because it is designed specifically for measuring resistance in ohms. Wattage, which is a measure of power, is calculated using both voltage and current (watts = volts x amps). Since an ohmmeter does not measure voltage or current, it does not provide relevant data for determining wattage.

In contrast, a wattmeter directly measures the power consumption of an electric device by providing a direct reading of watts. An ammeter measures the current flowing through the device, while a voltmeter measures the voltage across it. When you have both voltage and current measurements, you can calculate the wattage using the appropriate formula. Thus, the use of an ohmmeter is inappropriate for this specific measurement task.