Jacob's ladder

A Jacob's ladder is a special arc device with a "V"-shaped electrode arrangement. The gap is smallest at the bottom and becomes larger towards the top end of the "V". When high voltage is applied, an arc forms at the bottom (where the gap is smallest). This arc is then carried upwards by the buoyancy forces of the hot plasma, forcing it to become longer and longer as the electrode distance becomes larger, until it reaches the end and/or quenches because the gap becomes too large. However, as soon as the arc is quenched, a new one will form at the bottom, and the process starts again.

This arc is fed by 5 parallel OBITs, delivering 10kV or 100mA short-circuit current. The electrodes are about 50cm long. The picture is a time exposure, i.e. the shutter was open for several seconds while the arc traveled once from the bottom to the top.

Click on image for larger version.

Snapshots of a jacobs ladder shortly before and just after quenching. The arc is powered by a potential tranformer originally used for converting line voltages of up to 20kV down to 100V.

Click on images for larger versions.

When an arc is fed by AC current, it actually lights up at a frequency of 100 Hz (or twice the mains frequency, in general). This can be seen in a time exposure of a Jacob's ladder: because the arc moves contiuously upwards, it´s brightness varies with the momentary height, causing a horizontal stripe pattern in the photo. This is actually the reason why it's called a "ladder".

This is a magnified detail from the above picture, demonstrating the "ladder" effect.

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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