Triggered Spark Gap

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Found in a box of 807s and other large valves at Holsworthy amateur radio rally, this thing measures 6" high by 1 1/4" in diameter. It has a standard ES Edison screw base with contacts for the two outer electrodes and a 2 BA terminal at the other end for the 'trigger?' electrode.

There is no visible manufacturers marking, the only writing being "20 AMP." on the ES base.

This has been tested with a 'Megger' at 500 and 1000 volts and no apparent discharge was visible, either between the two base electrodes or between either of the base electrodes and the cap electrode. I have yet to construct a high voltage trigger circuit to fully test this. Many of the commercial examples have hold off voltages (the voltage which may be applied to the base electrodes without breakdown of the gap) in excess of 20 kV. Trigger voltages in excess of 10 kV are frequently required.

Triggered spark gaps are high voltage, high current switches. A large potential can be applied between the two base electrodes without breakdown of the gap. If a pulse of very high voltage is applied to the trigger electrode an arc is initiated and the conduction path between the other two electrodes is established. It can have a resistance of a few milliohms.

Modern uses include xenon flash quench circuits, explosive detonation, crowbar protection, electrically pumped gas lasers, medical lithotripters, rocket engine ignition, missile stage separation, and other high-energy applications.

References:

Here is a picture of a somewhat similar but more modern looking device ( second row, Ferranti CC11)

http://homepage.ntlworld.com/electricstuff/sparkgaps.html

Here are two explanations of the function and use of triggered spark gaps

http://www.fortunecity.com/greenfield/bp/16/hvtrigsg.htm

http://www.ee.ualberta.ca/~schmaus/elect/pas9.html

Here is a homemade example

http://home.golden.net/~kpwillia/sgap.html

Suppliers

These devices are still in use. Here are some suppliers of modern equivalents, They all seem to have ceramic, rather than glass, bodies.

http://optoelectronics.perkinelmer.com/

http://www.erico.com/products/TSGDiverter.asp

http://www.polytec-pi.fr/EGG/Eclateurs/Triggered%20Spark%20Gaps.htm

http://www.eepn.com/Locator/Products/ArticleID/15137/Action/Issue/15137.html