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A really cool demonstration of "stepped
up" high voltage is the "Jacob's Ladder".
You take two straight wires and mount them vertically
in a slight "V" shape, with the narrow part
at the bottom. Don't let the wires touch! Keep the gap
at the bottom about 1/2 cm. Place a spring at the top
mounting to keep the wires taut. Place the wires on their
mount inside a glass tube. Connect each wire to a 5000
volt supply and watch the spark climb upwards. |
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Why the glass tube? This is to protect the spark from
air currents in the room, and to create a chimney effect.
Why?
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The spark is very hot. It warms the air around it, with
the air above being warmer than below (cooler air is more
dense and sinks - hot air is less dense and rises). Warm
air has less resistance to a spark jumping the gap than
cool air, so the spark will rise to where the resistance
is less......hence the spark climbs the "ladder".
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| The glass tube also creates convection currents
which gently "blow" the spark higher. Once the
spark is ignited, its electrical resistance is less than
no spark at all, so the spark climbs upward, getting wider
and wider until finally its resistance becomes greater
than the smaller gap at the bottom. Then the spark quits
at the top and restarts at the bottom. Cool, huh? It's
best seen in the dark. Easy to build, just don't electrocute
yourself ! |
