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Old 19th Apr 2021, 7:00 am   #10
broadgage
Nonode
 
Join Date: Jan 2010
Location: North Somerset, UK.
Posts: 2,129
Default Re: Crompton 60W bulb

Quote:
Originally Posted by emeritus View Post
I don't have any vacuum lamps to try myself, but an old school physics book describes an experiment with two bulbs of the same wattage, one vacuum and one gas-filled, where the glass of the vacuum lamp was said to remain cold to the touch in operation. It was said that, for the vacuum bulb, radiant heat would pass through glass without appreciable absorbtion, whereas the gas-filled bulb's glass would get heated by convection. I believe this was why bare vacuum bulbs could be used outdoors without their glass being damaged by falling rain.
Only pertaily true IMHO.
A vaccuum bulb of 60 watts will produce about the same heat as a gasfilled bulb of 60 watts.
The gas filled lamp will get very hot in that part of the glass directly over the filament, due to convection, and if rain drops strike this part then the glass may shatter.

The vacuum bulb will heat the glass by radiation, the glass below the filament will be warmed to a similar degree as that above.
Therefore the whole bulb becomes moderatly warm and unless of unusually high wattage, should survive contact with rain.

Vacuum lamps tend to be of lower power, so not only is the heat better distributed, but there is less heat in total.

15 watt and 25 watt mains voltage lamps tend to be vacuum types, with 60 watt and up being gas filled. 40 watt may be either.
Vacuum is preffered for very low current lamps, it is the operating CURRENT not the wattage that is relevant. A 21 watt vehicle bulb will be gas filled, but as above a mains voltage 25 watt lamp be vacuum.


Returning to the red 60 watt lamp in the O/P, this was probably for decorative use, indoor festive lighting, or possibly industrial warning lamps that need to be seen from a distance.

Probably not for fuel effect electric heater, as has been said these tended to be clear, and also amber not deep red.

Not suitable for photographic dark room use as the red does not extend over the entire bulb, observe a little white light escaping near the cap.

Also unlikely to be for red warning lights atop tall structures, these tended to use clear lamps in a red enclosure.
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