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Description: | NR54 (in red paint now barely legible) MANU Marconi?s Wireless Telegraph Company. circa 1940 Chelmsford, Essex, England. DES after The General Electric Company. after 1936 Vacuum Tube Engineering Department, Schenectady, New York, The United States of America. This is an NR54 acorn sharp-cutoff pentode valve with an indirectly heated cathode, which was designed as a detector or H.F. amplifier. It is the Royal Naval equivalent of the RCA 954, Mullard AP4, Philips E1F, and the Marconi ZA2. These valves were based on an American valve designed, in 1933-4, by B. J. Thompson and G. M. Rose of the General Electric Company at the Vacuum Tube Engineering Department of General Electric Company at Schenectady, New York. Acorn pentode valves were sold from 1935 as type 954's under the RCA de Forest brand.
The construction of the valve appears to be like that of the Marconi ZA2, which was first manufactured in 1936 and includes a remote-cutoff grid or variable mu. In this type of grid the helix of the coil has a variable pitch. The magnesium or barium getter is in the upper half of the valve mounted on top of the screen grid with a band of carbon getter painted on the inside of the envelope presumably to mask the getter splash-over. This is a Royal Naval equivalent intended for VHF and UHF use up to 400 MHz. RCA claimed the valve could operate up to 500 MHz.
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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