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Odd results from an incandescent bulb

  1. Feb 25, 2008 #1
    Hey... After recently doing a practical in an English A2 Physics lesson, my lab partner and I noticed various odd results happening.

    The basic setup was a mains joulemeter (set to flash every 100j) in series with a bulb holder/socket type thingy...
    Next to it, a GLX Explorer (an electronic data logger) with a light intesity measuring attatchment taking readings whenever a key was manually pressed...

    Anyways, we noticed that the values for light intensity appeared to be fluctuating sinusoidally, with both the positive and negative peaks remaining fairly constant but the wavelength varying irregularly.

    According to the teachers, they had not seen this kind of fluctuation occur before and could not explain it - the possibility of natural heating & cooling causing the resistance to be increasing & decreasing was suggested - and the experiment is due to be repeated on Thursday / Friday as part of our school's "Energy Week" but otherwise came up stumps.

    I've since searched Google and have found no reasoning for this or any other examples of this occuring (admittedly, my research has not been particularly thorough) but have been unable to do so, so I'm just wondering whether anybody here would be able to explain this occurence?

    Kwah =]

    P.S. I do not have any data available to hand but will try to get some if possible, and more detailed explanations will be on the way if needed.

    Oh, by the way, apologies if this is in the wrong forum, but since it is not actual homework I figured that there would not have been the best place for this topic ..
    If Moderators / Admin decide it would be better placed elsewhere, feel free to move it (though I'm sure you will do anyway =] lol)
    Last edited: Feb 25, 2008
  2. jcsd
  3. Feb 25, 2008 #2


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    What was the period of the fluctuation?
  4. Feb 25, 2008 #3
    Sorry, I'm not overly familiar with the terminology?

    Are you asking how often did the fluctuations occur? It is better described as a sinusoidal curve, and there was no fixed distance in time before it repeated - in other words, it varied anywhere between 5 and 25 seconds but more commonly approximately 12seconds.

    Again, I feel I must point out that these figures are approximations from memory and I do not have the data to hand to verify them.

    Kwah =]
    Last edited: Feb 26, 2008
  5. Feb 26, 2008 #4


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    Just a guess, but....
    There could be a bit of residual gas in the bulb that periodically gets ionized and then discharges at the glass wall. I believe effects like this were first noticed by Edison, and may be related to the "Edison effect".
  6. Feb 26, 2008 #5

    Andy Resnick

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    There can be a lot of explanations: do I understand your setup correctly? You have a device supplying 100 J of energy to a bulb socket (incandescent? Arc?) periodically, and you are measuring the light output integrated over a single flash.. and then the data shows that the flash-to-flash light output is not a constant?

    What is the flash repetition rate? What kind of bulb are you using? Is it really 100J per flash? How old is the bulb?
  7. Feb 26, 2008 #6
    Sorry, I just noticed a significant problem with my explanation which could lead to confusion - to clarify, electrical current to the bulb was constant.

    The joulemeter itself looks similar to a regular extension cable - it plugs into the mains socket and then the device plugs into it. There is a small red LED on the joulemeter which can be set to flash either every 100j or 1000j using a rocker switch.

    There is no visible flashing from the bulb - to the eye, it remains a constant brightness.

    Using this image to refer to,

    - Wavelengths varied between 5 and 25 seconds but were typically approximately 10seconds.
    - Amplitude remained consistant
    - The wavelengths usually became elongated around the trough.

    As for the ionised gas, I'll look into it but would that not give shorter wavelengths - maybe fractions of a second?

    Kwah =]

    PS, I'm working on getting the actual data.
  8. Feb 26, 2008 #7


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    What is the variation between the peak and trough? Ie, how much brighter and dimmer does it get? Do you have a link to information about this joulmeter - is it just a charged capacitor? Is the source voltage constant?
    Last edited: Feb 26, 2008
  9. Feb 26, 2008 #8


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    Any chance the AC volage itself was fluctuating? I've seen this happen visibly in my house, not a brown out, but a slight dimming of the lights. Sometimes something that draws a lot of power elsewhere in a building is enough to cause this.
  10. Feb 26, 2008 #9
    The very very little data I have to hand is showing that, at approximately 2cm from the edge of the bulb, for an 11W fluorescent bulb it is fluctuating between ~680 and 800 (distance from peak to trough ~ 120)

    A 20W fluorescent bulb showed fluctuations between ~680 and 900
    A 10W pearl bulb, its showing fluctuation between ~660 and 740 (distance from peak to trough ~ 180)

    I assume the units are lumens (though I should have, since the values were odd I didn't record everything for that particular part of the experiment.

    As I stated before, the bulbs did NOT become visibly brighter / dimmer.

    Thanks Jeff but we considered this but decided it was highly unlikely that variation in the power supply would give readings as unusual and erratic as this.
    Personally, I would have checked all of these variables but since I have not been asked to be a part of the repeat test (something I'm actually quite bitter about), I cannot be sure these variables will be taken into account.

    Also, I've been using the term "fluctuations", suggesting that the increase / decrease is rather erratic. Quite the opposite. There is approximately 1 oscillation every 5-10 seconds (~0.05-0.1Hz) when the mains power supply is running at either 50Hz or 60Hz.

    Kwah =]
  11. Feb 26, 2008 #10
    To stop my posts becoming too lengthy, I figure I'd add this to the bottom.

    I forgot to mention that the IR readings for the bulbs remained constant with none of the fluctuations seen in the luminoscity test.

    Here is a copy of the photograph of the experiment.. apologies for the poor quality but I took the image from the support pack we've been given which isn't in huge amounts of detail.
    http://img213.imageshack.us/my.php?image=dsc048722fw4.jpg (low res)
    The red text on top of the image is what I've added in order to help explain what is going on.

    Kwah =]
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