What is the Resistance of a Light Bulb?

July 16, 2023
David Sunnyside

A light bulb is a filament inside a glass enclosure. This filament is heated by current to produce the light and heat it is known for. If we were to measure the cold resistance of a 100 watt filament lamp it would be around 9.6 ohms. If we apply 120 volts to this lamp using Ohm’s law the current will be 12.5 amps. The temperature of the filament rises and as this happens the resistance increases to 144 ohms. The power dissipated by the filament is 0.83 amps. This is a non-Ohmic resistance because it doesn’t obey the rules of Ohm’s Law.

This is why we call it a non-ohmic resistor, and one reason you wouldn’t want to use it in place of a traditional ohmic resistor in a circuit. It would be a waste of electricity because the filament will have to work so hard to get its energy from the voltage applied to it.

To check the filament resistance we can use a multimeter set to read ohms. Place one of the multimeter probes on the bottom "button" of the bulb and the other on the side case of the bulb. If the multimeter needle moves at all (preferably from one side of the screen to the other) then it is a good sign that the filament is working properly. If the needle stays at a low reading then the filament may be about to burn out.

David Sunnyside
Co-founder of Urban Splatter • Digital Marketer • Engineer • Meditator
linkedin facebook pinterest youtube rss twitter instagram facebook-blank rss-blank linkedin-blank pinterest youtube twitter instagram