How do arcs generate radio-frequency

Question

When I have an electric arc, and put an AM radio nearby, I hear a crackle and the station that I previously heard goes away. I am wondering what is going on with the electricity through the air to generate the noise that I heard through the radio.

Answer 1

It's a spark-gap transmitter - here are some words from the internet

"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."

The difference between your welding apparatus and most spar-gap transmitters is that they had tuning coils and capacitors so that the spark energy could be constrained in the electromagnetic spectrum. Having said that they weren't particularly good at that hence they were banned.

Answer 2

The arc results from the air being ionised by a high voltage resulting in a high amplitude pulse, or series of pulses. Such pulses are essentially square waves which generate wide-band RF. Something similar happens with a lightning strike, or a nuclear explosion. The latter produces an intense electro-magnetic pulse (EMP) which generates colossal E and H fields.

Answer 3

The reason is that a spark is a wide band frequency generator. It broadcasts noise over a wide range of frequencies. Some of those frequencies are the same as your radio is tuned to. The way AM modulation works, the radio can't distinguish the what was produced by the radio station and what was produced by the spark over the narrow band it is tuned to.

Answer 4

A circuit with an electric arc in it is actually (well, usually) a 'relaxation oscilator'.

There is a small capacitance between the electrodes of the arc, and both resistance and inductance in series with it.

The voltage rises across the spark gap until it is high enough to form an arc. The arc itself has what is called 'dynamic negative resistance', which means (over a certain current range) the voltage across the arc goes down when the current in the arc increases. Weird, eh?

So, to continue: the voltage across the gap rises until it arcs. The arc discharges the capacitance until it is below a sustaining voltage and the arc goes out. Now the capacitance charges up again until it arcs, and the cycle repeats.

Each discharge in the cycle is very rapid, making broadband RF pulses, as stated in other answers.

When I saw your question, I thought perhaps the negative resistance of the arc is what makes the RF, so I searched on 'negative resistance arc oscillator' and found this link: PoulsonArcOscillator. Turns out negative resistance isn't mostly where the RF comes from, it is the relaxation oscillator, as I described.