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Description: | MADE IN GREAT BRITAIN FIL. VOLTS 4 D138 Registered Trade Mark MULLARD RADIO VALVE DU/2 (silver stencil on the glass envelope). A (denoting the anode terminal moulded in the bakelite base MANU Mullard Radio Valve Company Limited. circa 1927 Nightingale Lane, Balham, London, S.W.12., England, This is a Mullard DU/2 4.0 volt 1.1 amp full wave azide rectifier. Although the valve has its original box and accompanying leaflet it is not listed in the equivealents manuals but can be replaced by a Cossor 412BU or a Marconi Osram U9. The carton is stored separately in a cardboard box on top of SCI:1
Sir John Ambrose Fleming (1849-1945) investigated into the cause of the blackening of the insides of lamps due to the evaporation of the filament. He also looked at the shadows in the deposits on the lamp envelope cast by the filament supports. These effects were caused by the fact that at about 2,500 C a tungsten filament in a vacuum begins to slowly evaporate. When used as a hot cathode in a thermionic valve it limits the life of 'bright' emitter filament. However, lower temperatures dramatically lower the thermionic emissions of the cathode greatly reducing its efficiency.
Malleable tungsten is produced by a powder metallurgy method invented by William David Coolidge (1873-1975) in 1903. Coolidge found that the addition of a little thorium oxide made the tungsten even more ductile. Irving Langmuir (1881-1957) found that filaments made in this way also produced greater electron emissions than pure tungsten and could work at much lower temperatures. The original "dull" emitters exploited this phenomenon and were also known as thoriated tungsten filaments. They operated at about 1,800 C.
In 1903 Arthur Rudolph Berthold Wehnelt (1871-1944) had found that grease contaminating a platinium filament produced enhanced emission and that oxides of alkaline earth metals such as calcium and barium etc. produced emission equal to pure tungsten but at much lower temperatures.
The oxide-coating process is often confused with the azide process, which was developed at Eindhoven by Philips in about 1924 and introduced to Britain when Philips acquired a half share in Mullard's in 1925. Among the Philips-Mullard; PM series of azide valves were the PM3 and PM4, however, there were some technical problems at the Mullard works probably due to the instability of the azide compounds, which were explosive if not handled correctly. Alledgedly a lot of the early valves were imported from the Philips factory at Eindhoven in Holland and relabelled as Mullard valves.
The first British-made azide type to reach the market was the PM3 in late 1925 or early 1926. It was quickly followed by the PM4 which had a higher specification but was harder to make. Mullards had so many thousands of below- specification valves that they bought the Six-Sixty Valve Company in order to provide a cheap market for these defective valves.
In the azide process the surface of the cathode filament was oxidised. The anode was coated with barium azide and on completion of the assembly of the valve components the anode was heated decomposing the highly unstable azide to form barium metal and nitrogen. The barium would react with the oxide on the filament to form a barium oxide layer on the tungsten. However, the barium also deposited itself over most of the inside of the valve especially noticeable on the inner surface of the envelope. When the envelope was sealed the barium metal acted as "getter" to absorb the final gas molecules after the valve was sealed.
The process "chemical gettering" was first used by Sir William Crookes (1832-1919) in 1876 for regenerating the hardness, or chemically pumping down, the vacuum in discharge tubes. One feature of the early Mullard form of the azide process was the internal blackening of the glass envelope. For a more cosmetic effect magnesium was often deposited before the azide was decomposed in order to make the glass look silvery. For high efficiency, cathodes required an evenly deposited thin layer of oxide on the filament. The azide process was fairly short lived because it was indiscriminate so that the barium was unevenly spread all over the inside of the valve causing a variety of problems. | License: | http://www.hmag.gla.ac.uk/spirit/rights/ | Publisher: | Hunterian Museum and Art Gallery, University of Glasgow | Rights holder: | Hunterian Museum and Art Gallery, University of Glasgow | Subjects: | SCIENTIFIC COLLECTION : | Source: | Hunterian Museum | Creator: | Hunterian Museum and Art Gallery, University of Glasgow | Identifier: | http://www.huntsearch.gla.ac.uk/cgi-bin/... | Language: | en-GB | Go to resource |
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