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Re: Danger, and I don't understand why.
Have you ever heard of a magnetic circuit? I have a electromagnet that
I picked up from Edmunds Scientifics that uses a flashlight battery to lift
up to 200 lbs. I have a magnetic keeper and if I turn on the magnet, and
leave the keeper on, then turn off the magnet. There is a residual magnetic
field that is left behind. As long as I leave the keeper on the magnet,
that residual magnetic field will remain. I can hook up an oscilliscope to
that coil, and when I remove the keeper after magnetizing, and removing the
battery. Just removing the keeper induces a relatively high voltage. If
you have ever seen a horseshoe magnet, with a bar across the two poles, that
is a keeper. Usually, very strong permanent magnets come with a keeper to
help keep it magnetized. Now, if the wire in the secondary is resistance
wire as most neon transformers are wound with a current limiting secondary,
and do have several kilo ohms of resistance as a result. And if that
resistance wire is ferro-magnetic, when you break the contact of the shorted
wires, the magnetic circuit is broken within the resistance wire. Because,
those windings can act like a keeper for a bar magnet. When it shorted was
a high voltage, high current moment in the circuits operation. In other
words, it happened when the arc was at a voltage high enough for opposite
charges to actually move the wire, and that also would cause the arc, which
retains the magnetizing current. From there that situation is out of the
circuit, and bypassed due to the mechanical short, and that is where the
magnetic field is retained. Most magnetic circuits are only mechanical
contact points of magnetizable materials, or electrical connections between
them. It does depend upon a few variables, of the situation, like a hall
effect in the resistance wire, which, is typically an alloy, as a result of
the magnetization of that resistance wire. A hall effect is a small current
produced by a metal, or two different kinds of metal similar to the voltage
you see from a solar cell in the light, and it remains as long as the
magnetic field is present. Once you break the circuit, you have weakened
the magnetic field by removing the magnetizing current that would have been
present as a result of the hall effect.
If it is the result of a hall effect, you should be able to tell with an
Ammeter. But, you will need a very strong bar magnet to place on top of
your transformer. As long as the magnet is present, you should see a
measurable current on the ammeter. If all you can get is small bar magnets,
it gets a little more difficult, because you will need north at one end, and
south at the other with a few hundred, or thousand maxwells. You might as
well order a bar magnet from www.edsci-dot-com If you don't see one that
measures up, surplus, or order the free catalog.
-----Original Message-----
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: Thursday, August 12, 1999 4:29 AM
Subject: Danger, and I don't understand why.
>Original Poster: "R.E.Burnett" <R.E.Burnett-at-newcastle.ac.uk>
>
>Hi all,
>
>Today something a little scary and even more strange happened
>during what I thought was a fairly routine test.
>
>I hooked up a simple static gap across a neon transformer, with a
>view to later examining the RF behaviour of the gap with my
>oscilloscope.
>A real simple circuit as show below:
>
> Power switch
>
> /
> O-----o o-----D C----------------!
>240VAC D C O
>Supply D C--- Ground 2mm air gap (bare wire ends)
> D C O
> O--------------D C----------------!
>
>I tried the circuit without the scope connected first. Sure enough a
>tiny flame like arc leapt across the gap between the two pieces of bare
>HV wire. No surprises there.
>
>Now here is the weird bit. After a few seconds of buzzing and fizzing
>one of the wires moved a little and the gap closed up (shorted.) I
>turned off the power switch, unplugged the supply cord, walked over
>to the gap, and pushed the wires apart with a plastic rod. To my
>surprise a 4mm white spark snapped across the gap.
>
>This caught me by surprise. I thought the circuit would be dead once
>the power was switched off, as there are NO CAPACITORs etc. in circuit.
>I could easily have grabbed hold of the wires with my hands, because I
>expected no danger !
>
>I tried to repeat this "occurence" and found that the wires would only
>spark when pulled apart if they had first SHORTED WHILE ARCING with
>the power on, and it was then turned off before pulling the wires apart.
>The spark produced quite a snap but is no where near like a cap
>discharge, more like a spark-plug type click.
>
>I am posting this because there may be a safety implication here, and
>I am keen to know if anyone else has observed this behaviour before.
>Maybe I am being a little slow but I certainly did not expect an arc
>after the power was turned off.
>
> - Richie,
>
> - Re-thinking my safety procedure
> in Sunny Newcastle.
>
>