The Keithley model 640 was a vibrating capacitor electrometer.
It was available in the 1970's.
Bruce
>
> On 04 March 2018 at 06:34 george wrote:
>
> Hi all
>
> To the best of my memory Keithley never made vibrating reed
> electrometers, the only one that I am aware of is the Varian Cary 401 which
> did use Sapphire insulators. I was the European product line specialist for
> Varian Cary in the late 1960/1970 era and was involved with the 401.
>
> George G6HIG
>
>
> From: volt-nuts on behalf of
> volt-nuts-requ...@febo.com
> Sent: 03 March 2018 17:00
> To: volt-nuts@febo.com
> Subject: volt-nuts Digest, Vol 103, Issue 3
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> Today's Topics:
>
>1. Re: Precision high resistance measurements / calibration of
> HP 4339B high-resistance meter. (ed breya)
>2. Re: Precision high resistance measurements / calibration of
> HP 4339B high-resistance meter. (Mitch Van Ochten)
>
> --
>
> Message: 1
> Date: Fri, 2 Mar 2018 09:22:32 -0800
> From: ed breya
> To: volt-nuts@febo.com
> Subject: Re: [volt-nuts] Precision high resistance measurements /
> calibration of HP 4339B high-resistance meter.
> Message-ID: <5f6e418c-649b-70e8-8502-facf1176f...@telight.com>
> Content-Type: text/plain; charset=utf-8; format=flowed
>
> Oops - I think I didn't send this message properly yesterday - here goes
> again. Ed
>
> 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
> fingerprint.
>
> 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 semiconduc
