Electrometer design? (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Fri, 25 Jun 2004 20:30:52 -0400
From: Bob Paddock <bpaddock@xxxxxxxxxxxx>
To: hvlist@xxxxxxxxxx
Subject: Electrometer design?


I'm interested in designing a differential electrostatic meter simular to the
one shown here:

http://www.amasci.com/freenrg/grado.html

That particular circuit has a few problems.

>From the list archives there was some discussion of other types of
electrometers, such as the vibrating reed.

Can some one point me to an explanation of  how the vibrating reed words as
e-meter?

What is the best you could do with todays op-amps, FETs, etc?

Past list messages:

Date: Tue, 07 Nov 2000 08:29:46 -0500
From: Richard Hull
To: hvlist@xxxxxxxxxxxxxxxxxxxxxxx
Subject: Re: Keithley 619 repair? (fwd)

Electrometers main claim to fame is an ultra high input impedance.  Some of
the best vibrating reed units such as the old Cary and Keithley units could  
detect aggregrates of as few as 100 electrons.
Common input impedances are on the order of  ten petaohms.

This makes them load a circuit under test about 10-100 million time less than
the finest Fluke digital instrument.  The Fluke would look like a dead short
to sources which an electrometer would normally be connected.

The electrometer can also be a good stable coulombmeter for measuring delicate
charges and charge transport mechanisms.

Richard Hull


> ---------- Forwarded message ----------
> Date: Thu, 9 Nov 2000 18:53:23 -0000
> From: Rob Davis
> To: hvlist@xxxxxxxxxxxxxxxxxxxxxxx
> Subject: Re: Keithley 619 repair? (fwd)
>
> Hi Richard,
>
> Ten petaohms!  Thats pretty high in my book, do you by any chance have a
> circuit schematic for a vibrating reed electrometer?
>
> Many thanks, Rob Davis (UK)


---------- Forwarded message ----------
Date: Fri, 10 Nov 2000 01:09:48 -0500
From: Richard Hull
To: hvlist@xxxxxxxxxxxxxxxxxxxxxxx
Subject: Re: Keithley 619 repair? (fwd)

I do have a schematic, of my Cary, but you would be stopped dead in making one
now as the core of the device is a gold plated vibrated reed in a very special
vacuum tube.  All connections must travel through pure sapphire bushings.  
Most of the wiring in these heads are gold, and only after the initial safe
installation of the charge to the first input follower do the wires filter to
a different form to avoid contact potentials and unwanted internal charge
separations.  There are only one or two manufacturers who make separate head,
saphire input electrometers today and none use the vibrating reed.  (To
expensive, to complicated and electromechanical)  To my knowledge these
devices are no longer made.  All of the Cary's and all of the Kethleys are
junk if this device fails!!!  No parts exist to repair these instruments
anymore.

  Certain specially constructed FETs and the 5886 tube can approach  the
petaohm input impedances but not as stabily as the vibrating reeds.

Richared Hull



---------- Forwarded message ----------
Date: Wed, 4 Apr 2001 00:58:39 -0700
From: Richard Hull
To: hvlist@xxxxxxxxxxxxxxxxxxxxxxx
Subject: Boonton voltmeter

That teraohm input Boonton ultra-sensitive voltmeter I obtained at the hamfest
at Timonium works great after replacing a tube and 4 light bulbs.  The tube
was pulled from my tube collection and the pilot lamps were for the huge 6",
zero center, meter and the two dial indicators associated with the range
switch.

This baby is chopper stabilized!  It really behaves well.  voltage ranges are
from 1000 volts to 1microvolt.  Current ranges are from 1 amp to 10pa.  Not
bad for $10.00  

With the positive lead hooked to a 12" toroid on a fiberglass pole.  I was
getting full scale, peg-to- peg,  1000 picoamp excursions while waving a
teflon rod charged from my flannel shirt 6 feet away.

Not quite an electrometer, but very close.  (electrometers have over 500
terohms of input Z)

A piece of old technology, executed with care and consideration, designed to
last a lifetime.  It now enters my collection of rare and seldom encountered
instruments like my Cary vibrating reed electrometer.

Richard Hull

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