|Jochen's High Voltage Page|
Arcs, in contrast to sparks, are continuous discharges. They are initiated as a spark, but then keep burning. The voltage across a burning arc is much lower than the voltage needed to ignite it, typically only a few hundred or thousend volts. Whether an arc forms or not depends on whether the voltage source can deliver enough current after the initial spark. If some minimal current can not be maintained, the arc is quenched (stops burning).
This arc between wire ends (5cm apart) is fed by two MOTs in series, delivering 4kV at 2kW power. This voltage is only able to jump a few mm in the first place, but the huge power (and short circuit current) makes it possible to pull the arc to a length of several cm once it is ignited. This effect also occurs in high voltage switches, and is a rather unwanted feature there. The inverted "V" shape is produced by the buoyancy of the hot plasma.Larger version
Due to their high power and current capability, MOTs are well suited for arc experiments. However, run time should be limited to a few seconds to prevent overheating of the transformers. To ignite this kind of arc, it is necessary to bring the electrodes so close together that the relatively low voltage (2kV to 4kV) jumps over the gap, and pull them apart afterwards. This can be done by moving the electrodes via a sufficiently long insulated handle. Alternatively, a piece of wire, mounted on an insulated handle, can be used to bridge the gap and ignite the arc.
Here, the electrodes are arranged vertically, so that the buoyancy is even taken advantage of, making the arc even longer (15cm). Doorknobs serve as electrodes. The arc is so bright that it can be photographed without special procedure - it even illuminates the surrounding.Larger version
Flyback transformers are also well suited to make arcs, though by far not as spectacular as MOTs. On the other hand, they are also less dangerous. It is possible to melt and even burn iron and copper wire by using it as one electrode of the arc. The wire first melts into a brightly glowing droplet at the tip, which, when further heated, starts to oxidize (burn), producing an effect similar to a sparkler.
|The voltage sources used for arcs are usually capable of large currents, making them particularly dangerous. Also, electrodes become hot very quickly. Further dangers arise from (intentionally or not) melting and burning electrodes. Safety goggles, fire-proof surrounding and a respectful distance are recommended.|
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