Warp <war### [at] tagpovrayorg> wrote:
> Aydan <hes### [at] hendrik-sachsenet> wrote:
> > >   The most common one being "the voltage is high, but the current is low",
> > > which don't make sense.
>
> > It does make sense if you know that a voltage source always has a series
> > resistance. The series resistance of the high voltage generator is in the kilo
> > ohms, whereas the series resistance of a mains supply is in the single digit
> > ohms or below.
>
>   But the voltage difference is between the wire and the ground, and
> there's nothing else between them than the body of the person touching
> the wire.
>
>   The current *is* very high. But only for a really small amount of time.
> It's not the current that kills you (as the saying goes), but the power
> transfer (the amount of current over time). I think the unit is Watt.
>
> --
>                                                           - Warp

The thing with the series resistance still holds true. The series resistance is
inside the device. If you measure the voltage with a voltmeter, the voltmeter's
resistance is usually several gigaohms, so there will not be a noticeable
voltage drop.
In the special case of the cattle fence there's the question of how the high
voltage is generated. Usually it's done with sort of a transformer. The ratio of
the transformer then is something like 1 : 5000 and the primary is driven by a
12V battery. There's a timer in the device that switches the primary on and off.
The current through the primary will increase "slowly" (kinda) because of the
inductance of the coil. But when you switch the supply off, the current wants to
keep going (magnetic energy stored in the coil) and be discharged over a sparc
gap. The rapid drop in current in the primary will induce a much higher current
in the secondary. The energy is very limited as well as the source impedance
(read resistance) because the secondary coil will have to have a much thinner
and longer wire (higher resistance) because of the many more windings it needs.
Disassemble an ignition coil of a car or motorcycle if you want to know how this
looks like.

As for simplicity of describing something: For some things you just need a basic
understanding of the matter discussed.

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