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Langmuir Sheath Demo

Langmuir Sheath Demo

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These lamps are made up for training purposes, to illustrate the effect of the Langmuir sheath and the reason why GLS filaments are coiled. The filament consists of a 230/60 coiled-coil, part of which is stretched out to a single coil, and another part to a straight wire. It is all made from the same piece of wire.

The Langmuir sheath is a layer of stationary gas, usually about one sixth of an inch thick, which exists around nearly all heated filaments. Heat from the filament is conducted through this stationary gas sheath, and lost to the rest of the gas filling which is moving due to convection currents. Since the diameter of the sheath is always fairly constant, it is desirable to reduce its length and thus minimise the area through which heat may be lost from the filament. One way to reduce the length of the sheath is of course to coil the filament and shorten it.

When energised the straight wire does not light up at all. Its surface area to volume ratio is so large that it loses too much heat to the gas filling.

In the single coil, gas flow is prevented between individual turns by the Langmuir sheath, since the spacing between each adjacent turn is much less than the sheath thickness. Thus the effective surface area to volume ratio available for cooling is reduced dramatically, and the filament runs hotter.

In the coiled coil the situation is further improved and this reaches the highest temperature of all.

This simple demonstration clearly shows that the more tightly coiled filament will lose less heat to the gas filling, and is therefore the most efficient.

A triple coil filament can theoretically be made, but its efficacy increase is not so significant as for single and double coiling - delivering only about 1% extra efficacy. The cost and complexity of making a triple coil filament is not justifiable by the small efficacy increase it delivers.

Incidentally when this same demo lamp is made up with a vacuum bulb, all three filament sections of course attain substantially equal temperature, since there is no loss of heat to the surroundings by conduction or convection effects.

TH_DJ_QIR30A55CL-230_E27_Philips.jpg Incandescent_-_Photo_-_Sealed_Beam_-_Sylvania_DNF_Side.jpg Langmuir Sheath Demo.jpg GLS Arcing.jpg

Light Information

Light Information

Filament/Radiator Type:S / C / CC-9
Factory Location:Lyon, France
Fabrication Date:July 2005

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Date added:Nov 21, 2005
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1kW Narva Blue

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Nov 21, 2005 at 06:30 PM Author: pslawinski
Neat lamp, I don't think sylvania will be marketing them anytime soon

Phillip Slawinski

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Nov 22, 2005 at 01:41 PM Author:
Just imagine if Sylvania made these for general use it would be neat to see peoples reactions to this type of lamp that they have never seen before

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Aug 15, 2014 at 06:12 AM Author: Lampman
Nice picture.
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Mainly the electrical side of things

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Aug 21, 2017 at 01:27 PM Author: sox35
Fascinating, would love to get hold of one..!

Ria in Aberdeen
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