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Description: | none MANU Metropolitan Vickers Company Limited circa 1950 Trafford Park, Manchester, !7, England. DES after ACHESON,Edward,Goodrich. circa 1890 Monongahela, Maryland, The United states of America. This is one of a pair of radiator fins for a metrosil disc made by the Metropolitan Vickers Electrical Company. The fin is in the form of a nickel plated disc with a hole in the centre. Its purpose was to conduct the heat energy, created within the metrosil disc by the passage of high energy electrical currents, to the surrounding atmosphere.
A metrosil disc, also known as a variac or varistor, is a composite of alpha silicon carbide, in the form of small hexagonal crystals and a ceramic supporting material. These components were developed, by Metrovick, as anti-lightning or surge protection devices. The material is a semi-conductor that looses electrical resistance when the voltage rises to its designed level, in this case 100 volts DC, whereupon it becomes an efficient conductor. This disc is in effect a non-ohmic or non-linear resistor designed to operate as a resistance up to 100 volts in a DC, direct current, circuit. The disc has a hole at the centre for ease of mounting because the discs could be used in series as stacks separated by themal radiator fins. Each disc was capable of withstanding energy inputs of up to 50 K Joules per second and could operate at temperatures of up to 600 degrees Celsius without failing.
It is therefore sobering to see that the metrosil disc and its sister radiator fins have been pierced by a high energy electric spark. The damage consists of discolouration and a fulgurite in the form of a 1.1mm diammeter hole in the edge of the disc. There is corresponding 3mm notch cut in the rim of this radiator fin. A fulgurite is a fossil lightning strike, from the Latin word "fulgur" meaning lightning bolt. The reason that this has occured at the rim is due to the fact that the fins have conducted the high voltage spark, at this point, due to the Faraday effect directing the current to the edge of the conducting material.
It would appear that this damage has been caused by a high energy event such as a lightning strike or discharege from the field coil of a heavy duty transformer. As the discharge passed through the three discs the metal of the first radiator fin melted to form a jet of super-heated liquid or gas (plasma) that then cut a 1.1mm hole through the metrosil disc.This material was then joined by the silicon carbide and its ceramic support from the metrosil disc, which cut 3mm diammeter notch in the second radiator fin. It is not known in what orientation the assembly was placed so whether the damage was caused by the initial positively charged streamer or the negatively charged main surge cannot be deduced.
An electrical discharge, across an air gap, such as a spark or lightning bolt forms when a negatively charged area on one side of the gap creates a long step-leader of negative ions. The leader forms a channel toward a short streamer of positive ions on the other side of the gap, which has been induced by the negative charge. When the streamer and step-leader meet a discharge occurs forming a superheated channel of ions known as a plasma, at supersonic speed, creating the charcteristic crackle or bang associated with an electrick a spark or the thunder associated with lightning. The characteristic violet glow of the plasma is due to the ionisation of the nitrogen gas in the air.
Impure silicon carbide melts at temperatures above 2000 degress up to a maximum, for absolutely pure SiC, of 2700 degrees Celcius. Brass, which is a variable alloy of copper and zinc typically melts at a temperature between 900 and 1000 degrees celsius. This explains why the notches in the fins have a wider diameter than the hole in the metrosil disc. Metrosil discs have found a variety of uses in the electrical industry as relay, transformer and generator field-coil protectrors. The material, shaped into rods, was used in creating the hight tension anode voltages, typically 15,000 volts, required by black and white, 405 line, televisin 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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