resistence of 60W glow lamp

• posted

Today I experienced a strange fact. I took a glow lamp (classic home lamp 230 V, 60W) and I tested the resistence with my digital tester: the value displaied was 64 ohms. But this value is wrong (in theory), because 230*230/64=827W, while the correct resistence value must be:

230*230/60=880 ohms. Now, I would like to know if someone of you experienced the same thing by himself (and the motivation of a similar value) or if my tester is broken. Thank you.
• posted

If you measure it with the filament at the same temperature that it operates at, you should get the value you expected. What you overlook is that the resistance depends strongly on temperature. In fact, this effect is why lamps are used as crude current sources.

```--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com```
• posted

When I think *glow lamp", I think *neon*. An ohmmeter will give a reading of infinite. The word you're looking for is *incandescent*.

• posted

Metals, in general tend to be ohmic (current proportional to voltage) only at a fixed temperature. Those rare alloys that hold a very nearly constant resistance (volts per ampere for a given shaped chunk) are highly prized ot make resistor elements. Tungsten is not such a material. It has a very strong positive resistive coefficient of temperature. This means that you can use a filament lamp as a resistive temperature sensor element (nice and stable, because the metal is sealed in a near vacuum, so no corrosion), or as a simple current regulator. Or you can break the glass and use it as a self heated (just warm) vacuum sensor since it will get warmer and drop more voltage in a better vacuum, with a fixed current heating it. Just don't expect it to behave as a fixed resistor.

```--
John Popelish```
• posted

"zooeb" wrote

Your tester is not broken, the resistance of the filament goes up with temperature. A cold filament has a very low resistance compared to the heated filament. A 100W bulb has less than 10ohms of resistance when cold in the US (110V).

• posted

Sounds about right to me. There is about a factor of 15 change in the resistance of tungsten between room temperature and operating temperature (in the neighborhood of 2800 K).

Mark

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