>That's a good trick. But you would agree with me that you are measuring

It's not a trick, you can buy these things, they are used in research.

>resistance at a current > 0A? One of the questions debated was whether
>there is a resistance (or whether it makes sense to speak of one) if
>there is no current flowing. In your experiment, current is flowing,
>forth and back, if I understand it correctly.

Consider whence the energy comes. A current means by definition moving
charge. The the moving is done by the probe charge so the current *is* the
charge and *it* moves. The field stays put and minds its own business.
It (its poles) oppose the motion of the charge with a force (electrostatic
attraction/repulsion). As far as it is concerned the probe charge does not
exist (or is too small and too far away to notice). A small perturbation
occurs because the probe impedance is not infinite so it couples a little
ac into the field.  Actually it is more complicated because these devices
can measure the charge of a single electron. Millikan's famous experiment
is a version of this method (used as a null instrument).

With superconductors and Josephson loops you have current flowing in a
circuit with zero resistance. There are superconducting laboratory magnets
in common use which can do this for months. You just supply LHe. If the
idea of current in a circuit of zero resistance bothers you imagine the
whole thing is a permanent magnet and it's field comes from the spin
orientation of the atoms in it. No current flowing, right ? Now think of
it again, but this time the atom is the size of the device and the
spin-analog which is current is 'travelling' in it, through the whole
device which is made 'transparent' by the superconducting materials. Still
no voltage. The current is real enough to cause the device to
self-destruct if it comes out of the superconducting state before being
discharged. Search Google for 'quenching superconducting laboratory
magnet'. I do not use such devices but they are real enough.

Peter

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