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Spark gap transmitters are the oldest type of radio transmitter made by man. They were first used around 1888 and remained legal until the 1920s when their use became greatly restricted. World War II delayed their complete ban outside of emergency communications for a few years. Now the only way to use them legally is inside a faraday cage. They operate as jammers for the same reason they were banned, they take up a lot of the radio spectrum.
A spark gap transmitter is fairly simple. Send a high voltage current through an air gap, when the resistance of the air breaks down a spark will cross the gap. When this happens electromagnetic radiation is emitted. You can test this in your house fairly easily. Turn on some speakers so they are powered but nothing is coming out of them. Computer speakers that are on will work as will a stereo set to CD or tape with no CD or tape playing. Flip your room lights on and off in rapid succession, you should hear a clicking from the speakers. The clicking is RF energy that is being picked up from the light switch.