Re: Is toroid a Faraday cage?

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Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>


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From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Tuesday, June 11, 2002 4:59 PM
Subject: Is toroid a Faraday cage?


> Original poster: "Jolyon Vater Cox by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>
>
>
> Apart from role as a capacitor, does the toroid of a TC work along the
> principle of a Faraday cage or Faraday s Ice Pail or -similar to the dome
on a
> Van der Graaf generator- in that the secondary is connected to the inner
> surface where there is no net charge -all charge being collected on the
> exterior?

No... it's a straight capacitor plate with controlled radius of curvature to
raise the breakdown voltage (but not raise it too high..)..

It also has the useful side effect of making the field more uniform at the
top windings of the secondary (which usually has a smaller radius of
curvature(==lower breakdown voltage)) so the sparks come from the toroid,
not the windings.

Raising the topload voltage too high (i.e. big radius of curvature) has a
lot of problems.. breakdowns along the secondary are more likely, for
instance.

A TC that performs very well is a balance between coupling between primary
and secondary, topload size and radius, etc.  You want to be putting in the
energy and feeding the spark at just the right rate to keep it growing,
without increasing losses, flashing over on the secondary and all manner of
other problems.
>
> Also if the inner wall of a Faraday cage has infinite electrical "suction"
ie.
> it can never become charged no matter how much electricity is supplied to
it,
> is the only difficulty in getting charge into the cage due to the
repulsion of
> the electric field on the outside?

Yes... as the top terminal on a Van deGraaff charges, the motor driving the
belt slows down (or, at least is more heavily loaded), because it's having
to push the charge against a higher field, which requires more work (work
being F*d/t and F being proportional to voltage and charge).

There is a limit when charge starts to leak off the outside of the VDG...
Really HV electrostatic generators (like the big 25 MV Pelletrons) have
multiple shields to shape the E field, run in a pressurized tank of SF6, and
have very carefully controlled field distribution along the charging column.