Jacob's ladder (electricity)

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Symbolic illustration
Jacob's ladder
Arch shortly before going out

The Jacob's ladder has the shape of a horn lightning arrester or horn arrester (electrical spark gap ) and is used in physical show experiments and the like. a. to illustrate how they work.

With the Jacob's ladder, an arc (electrical discharge) ignites at the bottom between two electrodes that diverge at the top due to the smallest distance there. Mainly due to the buoyancy , but also due to magnetic forces, the bow then moves upwards and disappears. Horn arresters for overvoltage protection on insulators work similarly, but at higher currents . B. lightning in overhead lines. Switching contacts at risk of arcing also have this shape. With these, the magnetic force predominates due to the high currents.

Named is Jacob's Ladder to the biblical Jacob's ladder , from Jacob in Genesis 1 is dreaming.

Cause of movement

The arc of a Jacob's ladder moves upwards:

  • because the thermal (updraft) created by the heating of the air pulls it upwards
  • because magnetic forces tend to enlarge the area formed by the arc and electrodes

In the Jakobsladder, the thermal component predominates due to the relatively low current strength.

Horn arresters also cause switches such as contactors or circuit breakers and surge arresters to move and lengthen the arc or switching arc , so that they are extinguished. The extinguishing is supported by spark extinguishing chambers, the movement can be accelerated by side-mounted coils.

Typical parameters

The arrangements shown opposite are fed from a current-limited AC voltage source with an open circuit voltage of 8  kV ( stray field transformer , ballast for fluorescent tubes ). After the arc is ignited, the voltage drops to a few 100 V.

The flow of discharge is only about 0.1  A . The electrodes remain relatively cold, so the typical side effects of an electric arc - the formation of metal vapor due to evaporating electrodes and the glow emission - are not given here. The electrodes must be smooth so that no “ hot spots ” form where the arc would otherwise remain due to glow emissions. The arc always reignites at the bottom when it breaks off at the top and as a result the voltage rises to such an extent that an electrical flashover takes place at the electrodes located close together at the bottom ( video ).

The electrodes in the example are approx. 300 mm long, but longer lengths are possible.

Hazard warning

The experiment is dangerous because of the exposed high voltages. There is a risk of burns and fire due to the high arc temperatures. When operated in air, the very toxic gas nitrogen dioxide is also produced , which can be recognized by its brown color under a transparent dome after just a few minutes of operation.

literature

  • Kronjäger, Jochen: The great high-voltage and high-frequency experimentation manual . Franzis, 2005, ISBN 978-3772359071 , pages 94 and 97

Web links

Commons : Jacob's Ladder  - Collection of images, videos and audio files