How can I measure the wattage of an incandescent light bulb with the marking off? Does the measure of resistance not show this power?
Simply measuring the resistance of an incandescent light bulb with the multimeter does not tell you how powerful it is. What happens is that when the lamp is off and its resistance is measured, the filament is cold and therefore contracted. Its resistance is then many times lower than the resistance that the lamp has in operation. For example, a 100-watt bulb that has approximately 110 ohms of resistance when in operation, if cold tested, will measure less than 40 ohms. Thus, by this indication, it will not be possible to say what its power is. What we can do is compare the resistance of an unmarked lamp to a good one with the mark and see if they are approximately equal. If this happens, we can say that both have the same power. The same is true for heaters and heating elements that operate with heated resistors.
