resistence of 60W glow lamp

[zooeb]

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.

 

[Larry Brasfield]

"zooeb" <zooeb@libero.it> wrote in message news:62b9dcf3.0501141130.80194a0@posting.google.com...

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.

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
Above views may belong only to me.

 

[Anthony Fremont]

"zooeb" <zooeb@libero.it> wrote

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.

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).

 

[JeffM]

took a glow lamp (classic home lamp 230 V, 60W)...
resistence was 64 ohms.
zooeb

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

 

zooeb wrote:

the value displayed was 64 ohms ...
the correct resistence value must be ... 880 ohms

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

 

[John Popelish]

zooeb wrote:

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.

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