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Re: Ions

William Beaty asked:

...... Shouldn't electrets "discharge" over time as they
attract opposite charge onto their surfaces (on humid days,
for example?) But then, how do electret microphones continue
to operate for years? Even if they are inside an airtight case,
wouldn't the ions from background radiation supply a source
of leakage? .....

If the charge is not on the surface, and if the insulator is nearly
ideal (a non-conductor), then there is no electric field outside to
attract ions. Except for the fringe effect, which can be minimized,
the electric field between the parallel sheets of +Q and -Q is
confined to the dielectric material.

This should be contrasted with a piece of dielectric containing
a cloud of NET charge. The electric field produced by the cloud
is not confined and ions of opposite sign are attracted toward
it, till E becomes zero.

I recall using an electrostatic pocket dosimeter which was not
self discharged significantly in one or two months. It was
essentially a "parallel plate air capacitor" discharging mostly
when the air BETWEEN the plates is ionized by radiation.
The wire which can be seen against the background scale
(calibrated in mrads or mrems) is part of a "voltmeter".
|V|=|Q|/C

Ludwik Kowalski