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Author Topic: Information on Robertson Light bulb  (Read 4928 times)

Offline Amptiques

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Information on Robertson Light bulb
« on: July 02, 2004, 06:48:00 am »
Does anyone have any information on a Robertson 230-250V, 250W bulb?. It has CV311 stamped on 1 of the electrodes.

Bulb is tubular, approx 27cm long,6cm in diameter and has a filament shaped WWWWWW thus which is supported midway on a glass standoff.

1 end of the glass tube is black for approx two cm and there appears to be 2 half shaped discs approx 1cm diameter where the filament terminates.

Sorry I am unable to include a picture at this time.

Thanks

Ian

Offline Tim

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Information on Robertson Light bulb
« Reply #1 on: July 02, 2004, 02:22:00 pm »
Hi Ian,

Welcome to the forums.

For what it's worth: I've been told by other collectors that these are carbon filament heater lamps.  I have also heard they were used during WWII in certain radio transmitters as a ballast resistor of some sort...


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Tim
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Mountain Dew Collectibles, Volume I

Offline Chris W. Millinship

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Information on Robertson Light bulb
« Reply #2 on: July 02, 2004, 05:29:00 pm »
I have one of these lamps, it came from another collector in your neck of the woods (Fin Stewart). It was described as a resistance lamp and indeed the construction would lend itsself more to such a use. The hefty carbon filament is definately not made for lighting. However I do believe, as Tim has mentioned, that its primary purpose was a heater bulb. If it were intended as a resistance lamp, it would be logical to me that its resistance be printed on the glass as is the case with all the other resistance lamps I have seen so far. However it has a voltage and wattage rating implying it is meant for direct connection to the mains supply. I havn`t tried this with my lamp as I don`t know what would happen (or have a suitable fixture for it either), but suspect it would burn very hot and rather orangey, like the more common single ended carbon heater lamps.

I`m not exactly sure of the purpose of the darkened end but its construction looks similar to that found in some discharge lamps, with the disc shaped electrodes adjacent to where the filament connects to the lead-in wireat that end. The blackened coating is, I believe, a Rhodium (or similar) getter intended to absorb impuritie, and the lamp would probably have been burned with that end at the top. I suspect this was employed to keep the glass clean and clear during the life, otherwise the characteristic blackening from vaporised carbon would significantly impede the rediated heat as the lamp ages.

I don`t know any of this for sure, but it makes the most sense to me.





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Offline Chris W. Millinship

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Information on Robertson Light bulb
« Reply #3 on: July 17, 2004, 02:50:00 pm »
Just spotted one on Ebay that answers your question:
 http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=4674&item=5709735685&rd=1

Assuming this is your bulb? It`s the style I had in mind when writing my original reply. Apparently these are dummy loads for testing the output of radio transmitters.

The only think I notice about that is the one in the auction photo doesn`t have any blackening at one end as mine does. Presumably this is therefore not a "getter" or other special coating, but instead just builds up over time as the carbon evaporates.





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Offline Amptiques

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Information on Robertson Light bulb
« Reply #4 on: July 17, 2004, 05:38:00 pm »
quote:
Originally posted by Chris Millinship:
Just spotted one on Ebay that answers your question:
 http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=4674&item=5709735685&rd=1

Assuming this is your bulb? It`s the style I had in mind when writing my original reply. Apparently these are dummy loads for testing the output of radio transmitters.

The only think I notice about that is the one in the auction photo doesn`t have any blackening at one end as mine does. Presumably this is therefore not a "getter" or other special coating, but instead just builds up over time as the carbon evaporates.

 





Chris,

well spotted and yes it is the same as the one I have.

I found mine about 10 years ago and unfortunately got to them after they had been unpacked from their wrappings or boxes (but thats another story)

I have 14 of these (2 have broken filiments)if anyone is interested in swapping.

Thanks

Ian Tolhurst

Offline James

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Information on Robertson Light bulb
« Reply #5 on: July 25, 2004, 07:20:00 pm »
Hi Chris,

The dark coating at one end of the lamp is indeed a getter - a film of barium metal.  It serves to maintain high vacuum in the bulb and thus prevent conducted and convected heat losses from its filament.  Without this getter, the filament resistance can vary significantly as a result of changes in temperature caused by the variation in vacuum quality, so a getter was often used in this kind of resistance lamp to keep its electrical characteristics more constant.  The lamp on Ebay has lost is getter coating, thus indicating that the vacuum is no longer of the required quality.

Best regards,

James.