Nernst Filaments NFT2 and NFT4 - UK Vintage Radio Repair and Restoration Discussion Forum

Gridiron · 2nd Jun 2018, 10:46 am

I found three of these in a box of valves and bulbs I bought at a meeting.(2 NFT 4, 1 NFT2 ) apparently new in plastic tubes dating from the 1980's, made by the English Electric Valve Co. Looking on the net they were used in "glowers" as sources of infra-red radiation. Does anyone have any more information about them?
Mike.

TonyDuell · 2nd Jun 2018, 11:51 am

I don't know about those particular filaments, but Nernst glowers were used as the infrared light source for spectrometers used for chemical analysis. The great advantage over a normal metal filament is that you can run them in air, without a glass bulb around them which might absorb some wavelengths of infrared light (In such spectrometers, reflection optics -- mirrors and reflection gratings are used wherever possible, the transmission compoennts (windows, the odd lens) on the ones I worked on were made from sodium chloride (yes, common salt) and had to be kept dry for obvious reasons).

In the instrument I remember most clearly, the Nernst glower was connected to the mains with a ballast resistor in series. This was originally 3 0.3A Barretters, with switches to select 1, 2, or 3 in parallel. When those failed, normal 60W bulbs were used, which seemed to work just as well. The glower has a negative temperature coefficient of resistnce, the resistance reduces as it gets hot, hence the need for the ballast. And also the resistance at room temperature is so high the thing will not start by itself. Posh units had a nichrome (or similar) heating element wound round the glower. But the instrument I worked on had to be started by hand. A screwdriver with a cotton wool pad on the end. Soak that in alcohol, light it, and stick it under the glower (remember, connected to the mains). After a few seconds the glower would light up (as did the ballast lamps), you took the flaming screwdriver out and put the covers on.

Gridiron · 2nd Jun 2018, 6:48 pm

Thanks for the information, I don't think I will try them, the mode of operation sounds a bit scary!
Mike.

emeritus · 2nd Jun 2018, 7:11 pm

Nernst lamps were produced for domestic lighting in the Edwardian era as they were more efficient than Carbon filament lamps, but were superseded by Tungsten filament lamps. Interesting to know that they continued to be manufactured into the 1980's for specialist applications.

2nd Jun 2018, 10:46 am	#1
Gridiron Heptode Join Date: Apr 2010 Location: West Midlands, UK. Posts: 662	Nernst Filaments NFT2 and NFT4 I found three of these in a box of valves and bulbs I bought at a meeting.(2 NFT 4, 1 NFT2 ) apparently new in plastic tubes dating from the 1980's, made by the English Electric Valve Co. Looking on the net they were used in "glowers" as sources of infra-red radiation. Does anyone have any more information about them? Mike.

2nd Jun 2018, 11:51 am	#2
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,208	Re: Nernst Filaments NFT2 and NFT4 I don't know about those particular filaments, but Nernst glowers were used as the infrared light source for spectrometers used for chemical analysis. The great advantage over a normal metal filament is that you can run them in air, without a glass bulb around them which might absorb some wavelengths of infrared light (In such spectrometers, reflection optics -- mirrors and reflection gratings are used wherever possible, the transmission compoennts (windows, the odd lens) on the ones I worked on were made from sodium chloride (yes, common salt) and had to be kept dry for obvious reasons). In the instrument I remember most clearly, the Nernst glower was connected to the mains with a ballast resistor in series. This was originally 3 0.3A Barretters, with switches to select 1, 2, or 3 in parallel. When those failed, normal 60W bulbs were used, which seemed to work just as well. The glower has a negative temperature coefficient of resistnce, the resistance reduces as it gets hot, hence the need for the ballast. And also the resistance at room temperature is so high the thing will not start by itself. Posh units had a nichrome (or similar) heating element wound round the glower. But the instrument I worked on had to be started by hand. A screwdriver with a cotton wool pad on the end. Soak that in alcohol, light it, and stick it under the glower (remember, connected to the mains). After a few seconds the glower would light up (as did the ballast lamps), you took the flaming screwdriver out and put the covers on.

2nd Jun 2018, 6:48 pm	#3
Gridiron Heptode Join Date: Apr 2010 Location: West Midlands, UK. Posts: 662	Re: Nernst Filaments NFT2 and NFT4 Thanks for the information, I don't think I will try them, the mode of operation sounds a bit scary! Mike.

2nd Jun 2018, 7:11 pm	#4
emeritus Dekatron Join Date: Jan 2012 Location: Brentwood, Essex, UK. Posts: 5,337	Re: Nernst Filaments NFT2 and NFT4 Nernst lamps were produced for domestic lighting in the Edwardian era as they were more efficient than Carbon filament lamps, but were superseded by Tungsten filament lamps. Interesting to know that they continued to be manufactured into the 1980's for specialist applications.