1) Faraday was not claiming net charges, he was saying that +
and - charges slowly penetrate into the dielectric material from
metallic plates when these are connected to a source of HV. (Today
he would say that some electrons leave the dielectric at the positive
electrode and some enter it at the negative one). Faraday was using
the dissectible capacitor to demonstrate the phenomenon. We started
checking this and unexpectedly found that a net charge is nearly
always present on the single dielectric sheet removed from the
discharged capacitor. The idea of testing Faraday’s penetration
phenomenon (with two layers of dielectric) was put aside to focus on
the net Q on a single sheet.
2) We now have a power supply whose terminals are floating. A
clamp made from plexiglas now holds the dissectible capacitor and
allows us to open it as symmetrically as possible. The reproducibility
is better but far from reasonable 30% or so. And we still observe
occasional reversals of polarity of the net charge. It is likely that
the net charge is an artifact due to the asymmetry in the plate
separation process. (One metallic plate is still very close to the
dielectric slab when another is already further away.) Symmetric
separation is possible only in a gedanken experiment, not in a real
experiment (with a thin dielectric foil between two metallic blocks
attracting it).
3) HERE IS MY QUESTION. Suppose that a net charge is really
present in the dielectric slab of a capacitor connected to HV. How
can this net charge possibly be revealed? A naive approach was
to disconnect C from the power supply, discharge it and stick it
(without dissecting) into the Faraday cup. By doing this we always
find that the net Q is zero. But this refers to the sum of charges
on three objects (the dielectric slab and two electrodes), not to
the Q in the dielectric slab alone. The discharged electrodes are
not necessary neutral, they may have + and - charges induced by
the layers of the remaining bound charges in the dielectric. If so
then the net Q in the electrodes compensates the net Q in the
dielectric.
ASSUME THE NET Q DOES SOMEHOW EXIST IN A DIELECTRIC SLAB OF
A CAPACITOR. HOW CAN ITS PRESENCE BE DEMONSTRATED WITHOUT
REMOVING THE SLAB?