Oops - I think I didn't send this message properly yesterday - here goes
Yes, David, unless you go to very extreme measures, you won't see real R
values that have any practical meaning beyond E12 ohms or so. Most
practical insulation Rs may be around E12-E14 tops, unless you go to
sapphire. Up in that region, the R may be all within a material, or
include surface components like a film of dirt or moisture, or a
E11 resistors can be made to fairly high precision, and maybe E12
nowadays. In the old days, higher values were made by stacking E11s -
like ten in series to get E12 with decent precision. The glass packaging
also limits how high it can go, due to leakage within and on the
surface. I once used a glass reed relay capsule as an ultra-high
resistance in a circuit. There was no precision or stability at all, but
it made a nice high resistor (probably E14-ish dry) even though there
was no element in there, and the circuit didn't care, as long as it was
very high, but not infinite.
The specs on this HP unit are likely just the most extreme capability
taking maximum voltage over minimum current resolution, but any
measurements would tend to be very noisy and unstable anyway. Also,
testing at the extreme 1 kV makes the numbers seem more impressive, but
the voltage coefficient of resistance will pretty much be unpredictable.
If this is a digital meter, then the other spec trick that tends to
obscure the real performance limit is that the ultimate resolution and
noise is that last digit - or even last two or three - that may may be
pretty jumpy, unless very long averaging time is used.
There may be newer, fancier electrometers nowadays, but Keithley used to
be the standard for these in the old days, before several digits of DVM
resolution complicated the specs. They had a vibrating capacitor
electrometer with all-sapphire input structure back in the 1970s/80s I
think, that was the epitome of electrometers. I forget the model number,
but vaguely recall that it could reach the aA region full scale - not
that last digit of resolution thing. It's long obsolete, and I don't
think they ever made anything actually better - only added DVM digits to
less capable, conventional semiconductor amplifier techniques. If you
can find info on it, it's an interesting read. I found a pdf of the
manual years ago, but have no idea where it is now, or what info may
still be around.
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