Jacob's Ladder (Climbing Arc) Construction:
------------------------------------------
This is the type of high voltage display seen on many old (and bad) sci-fi
movies. Jacob.s Ladder come in all shapes, styles, and sizes. Here is info
on a common type that is easy to construct with readily available parts.
You will need 12K-15K volts AC (50/60) Hz at say 20-30 mA. A neon sign
(luminous tube) transformer is the usual source though an oil burner ignition
transformer (e.g., 10 KV at 20 mA) will work in a pinch (some say better and
cheaper) or you could build an inverter type power supply.
Luminous tube transformers can be obtained used from sign shops or demolition
companies. Oil burner ignition transformers can be pulled from old oil
burners - probably free for the asking at your local HVAC company.
Take a pair of thick wires - the steel wire from old metal coat hangers works
pretty well - straighten them out and mount them with a gap of about 1/4
inch at the bottom and 1-3 inches at the top. Of course, all on an insulated
non-flammable material! Connect the high tension output of the transformer
to the two wires and you should be all set. Some adjustment of the spacing
at the bottom (to get the arc started) and at the top ((to determine when
the arc is extinguished and how fast it rises) may be required (but do so
only with the power off!). Depending on the voltage and power rating of
your high voltage source, these dimensions may vary considerably.
__ 1-3"
^ \ /
2-3 feet \ /
or more | \ /
v --' '--
1/4"
A Jacob's Ladder works on the principle that the ionized air in the arc
is a lower resistance than the air around it and heated air rises. The
arc strikes at the point of lowest breakdown voltage - the small gap
at the bottom. The heated plasma rises and even when it is an inch or
more in width is an easier path for the current to follow. Eventually,
the gap becomes two wide, the arc extinguishes and is reestablished
at the bottom. For best results, shield the whole thing from drafts but
don't use anything that can catch fire!
SAFETY:
------
Make sure that no one can come in contact with this - particularly curious
onlookers. Separating the potential victims from any possible contact
with the high voltage is really the only foolproof way of protecting
against fools or the unaware - and you from a lawsuit. People not
familiar with high voltage phenomena (or aware only through grade-C
sci-fi movies) can be incredibly naive.
A GFI (ground fault interrupter) is of no use in protecting against HV
contact since the secondary of a neon sign transformer is isolated from the
line but its centertap is usually connected to the case - which should be
grounded. However, a GFI would be a good idea in any case when you are
working with line connected equipment.
12,000 volts will jump approximately 1/2-3/4 inch in dry air - more under
humid conditions. Don't forget that 12,000 VAC is approximately 17,000 V
peak. Neon sign transformers have current limited outputs - 30 mA is
typical - but that is still highly dangerous - lethal under the wrong
conditions.
You can build a small Jacob's Ladder using a high voltage transformer
of lower capacity or a DC-AC inverter using a TV flyback transformer.
While these would be less dangerous, there is little room for carelessness
when working with any type of high voltage device. Even if there is no
resistive path, the stray capacitance can permit enough AC current to flow
to give you a painful experience!
* Do not work alone.
* Always keep one hand in your pocket when anywhere around a powered
line-connected or high voltage system.
* Wear rubber bottom shoes or sneakers.
* Don't wear any jewelry or other articles that could accidentally contact
circuitry and conduct current.
* Use an isolation transformer if there is any chance of contacting line
connected circuits.
* Don't make adjustments with power on.
* Finally, never assume anything without checking it out for yourself!
Don't take shortcuts!
Date: 31 May 1996 01:58:05 GMT
Original Subject: Re: old-SF-movie device with climbing spark??