Ken Shirriff, CuriousMarc Reverse and Repair a Vintage Igniter — and Rediscover Spark-Gap Radio
An unusual box, purchased at auction, turns out to be for lighting xenon bulbs — and doubles as a powerful radio transmitter.
Engineers Ken Shirriff and CuriousMarc have reverse-engineered and repaired a vintage 1960s xenon arc lamp igniter — and in doing so accidentally rediscovered the classic arc-gap radio transmission technology.
"The box below is a 1960s German high voltage unit that CuriousMarc obtained as part of an auction," Shirriff writes. "After some research, we determined that it is an Osram igniter, which generates a 40-kilovolt pulse to ignite a xenon arc lamp.
"The unit didn't work, so I opened it up, figured out its circuitry, and fixed it, so we could generate some sparks. The circuit turned out to be very similar to a Tesla coil, although the sparks are much smaller."
Having discovered a 1964 paper published by the Stanford Research Institute — "just a few miles away," Shirriff notes. "So there's a good chance that Marc obtained the exact unit that was used in this research" — confirming the device's purpose, the pair set about opening it up and investigating the components.
"The most unusual component is the ceramic cylinder in the front," Shirriff explains. "I opened up the cylinder and found a stack of eight metal disks with (maybe) carbon electrodes in the centre. The disks are separated by mica washers to leave 0.33 mm gaps between each pair. This forms a series of 7 tiny spark gaps.
"This type of spark gap is known as a "quenched spark gap." Spark gap transmitters were the first form of radio transmitter, used from 1887 to 1920. They used a spark to transmit Morse code via radio waves (details). The quenched spark gap was one type of spark gap used in these transmitters, as shown in the diagram below. By combining multiple small gaps, the quenched spark gap could cool off efficiently."
Hooked to a 220V power supply, and with a minor fix for its safety interlock system — a failure mode that proved "tricky to find," Shirriff notes, "because when the case was open, the safety interlock was (of course) open" — the pair were able to use the igniter to generate sparks, while CuriousMarc used the hardware as the basis for the embedded video on how spark-gap transmitters came about as a means of transmitting radio signals.
Shirriff's write up, meanwhile, is available on his blog.