Jochen's High Voltage Page

Tesla discharges

Discharges of a tesla coil are strange in so far as they originate from only one electrode and end in free air. This kind of discharge is usually termed "streamers".

Streamer from my tesla coil, approx. 50cm long.

Larger version

The reason for this effect is the high frequency (several 100kHz) of the voltage produced by the tesla coil. The ionized air of the streamers acts as a capacitance against the surrounding walls etc. (ground), which at this frequency has a low enough impedance to allow a quite noticable current, which then sustains the once formed streamers. In more elementary terms, the frequency is so high that charge can be pumped into and out of the streamers in rapid alternation, without need for a second connection at the other end.

When a tesla coil is disruptively driven, i.e. using a spark gap (such as the one in the above picture), one often observes an effect similar to the Jacob's ladder. In every half mains period, the tesla coil is switched on once, producing a spark. Now once a streamer is formed, it leaves a track of ionized air, which may then be re-used by the next discharge. However, through slight movements of the air e.g. due to buoyancy, the ionized air track slowly drifts around, lighting up with every discharge at a slightly shifted position. This can be seen in a time exposure.

Magnified detail of a tesla coil streamer. The Jacob's ladder effect is clearly visible.

The length of streamers depends not only on the output voltage of the coil, but also strongly on the available power, similar to an arc.

Of course, one may also let the streamers hit a second electrode - this results in regular sparks or arcs, depending on power, which are much brighter than "open ended" streamers.

Being hit by a tesla coil streamer is not harmless. The high voltage output is not pure high frequency, but contains a 100Hz component that may still kill you. Also, skin effekt in human tissue is a myth, as its conductivity is by far too low. Arcs between secondary and primary or to the mains wiring may put you in direct connection to primary or mains voltage, with the same result as touching primary or mains voltage.

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