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Description: | Mullard EN91 2D21 BRITISH MADE (white stencil on the glss envelope). SCC 336616 (at various points on the valve mount. R7J (yellow stencil on the glass envelope) MANU Mullard Radio Valve Company Limited. circa 1955 Mitcham, Surrey, England. DES after HULL,Albert,Wallace. 1927 General Electric Research Laboratory, Schenectady, New York, The United States of America. This is a Mullard EN91, miniature thyratron in the form of a, 7 pin B7G base, xenon filled tetrode with an all glass construction. Its is mounted in a 7 to 9 pin adaptor with five solid state diodes and 2 resistors. The purpose of the components is unknown. The valve's construction suggests that it was not used for starting or stopping heavy currents but may have been used for driving a recording counter, as a relay trigger, or in a grid-controlled rectifier application. It has an indirectly heated cathode, a control grid, an anode, and an additional shield-grid electrode, which almost completely surrounds the other electrodes.
The thyratron was invented by Albert Wallace Hull in 1927.
The D ring getter is mounted on an extension of the plate (anode) .
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.
One feature of the Philips-Mullard 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. Another more serious problem was that for high efficiency at low temperatures the cathodes required an evenly deposited thin layer of oxide on the filament. The azide process was fairly short lived because the barium was unevenly scattered all over the inside of the valve causing a variety of problems.
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. | 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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