An Experimenter's Electrometer
Sniffing out subtle electrostatic phenomena
By Richard Hull
The field of Electrostatics concerns itself with charges, potentials, and forces, and is often considered to have first been studied in the late 18th and early 19th centuries. In today's modem world, electrodynamics is the dominant study in electrical engineering. Electrostatics is making somewhat of a come back, and the forces and charges involved can be used for a number of novel purposes in our day-to-day lives. Xerography is a prime example.
The experimenter can arm him or herself with an old classic electroscope and investigate a number of rather large electrostatic effects. In order to gain real insight and do more subtle and difficult experiments, an electrometer is often needed. The electronic electrometer is nothing more than an ordinary voltmeter. The salient difference between your digital VOM and the electrometer is one of input impedance. This relates to the amount of load that the instrument places on the circuit under test. Most good electrometers have input impedance's of 200 Teraohms. This is tens of thousands of times greater than the average electronic VOM(I - 10 Megohm). It will immediately amaze the experimenter just how electrical the world really is, once armed with an electrometer.
Modern electronic electrometers are very expensive. $5,000 will buy a fairly
nice instrument, while $10,000 would be needed for a superlative one. Keithley
Instruments and Victoreen are major suppliers of these specialized instruments.
This article will supply the experimenter with schematics and a broad overview
for assembling a first class electrometer with a very modest outlay of cash.
Actually, this is a unity gain impedance translator or electrometer IX
amplifier, in the strictest sense, as the experimenter will have to supply the
readout system, (usually a VOM or oscilloscope.) It will be assumed that the
experimenter has a modicum of experience in assembling simple electronic
circuits.
DISCUSSION OF THE CIRCUIT REQUIREMENTS As the frequency response of this junior electrometer is limited by the low
pass filter circuitry to around 10Hz, no special PC boards are needed. Point to
point or wire-wrap type connections are all that is required. I have assembled
many of these circuits on the simple pad per hole, 2"x3" circuit boards found in
local Radio Shacks. The key point is shielding and insulation. The front end of
the system utilizes a special precision CMOS FET Integrated Circuit by National
Semiconductor. (LMC 6081) This special IC can be purchased from DigiKey. The
only other IC is an LM-324N and can be found at most Radio Shack stores. I
recommend first quality machined pin IC sockets (DigiKey). Radio Shack also has
aluminum chassis cases which will be needed for shielding. The input connector
must be a Teflon insulated, chassis mount, female, UHF connector type PL-259.
The output connector is unimportant and can be a RCA type phono-jack. I chose
a BNC chassis jack for my output connector. The system is powered by two common
9 volt transistor radio batteries. Parts layout is only critical in that the
part of the PC board containing the input IC (LMC 6081) be located immediately
at the input connector. The critical input pin to this IC (pin #3) is carefully
lifted and not inserted in its socket at all. The two components from this IC
lead to the input connector center terminal are connected floating in mid-air!!
All batteries and wiring should avoid this area and be on the opposite side of
the board and shielded container box. Beyond this, the builder is totally at
liberty to do as he or she pleases. I have a proposed lay out diagram included.
GETTING STARTED Begin by soldering the sockets to the PC board. Next make all connections to
all the various tie points. When the board is wired, add the battery clip leads.
The box should be a fully enclosed all metal item. Place the input connector at
one end and the output connector at the other end of the box. For convenience in
some models, I put the input connector on the top of the box at one end. Holes
must be drilled for standoffs or long machine screws to support the board near
the input connector. The batteries are best placed in the opposite side of the
box near the output connector. Make sure at each step of the drilling process
that all components will fit in the box and not short out or touch one another.
The power switch should also be positioned near the output end of the box. If
you still haven't picked up on it, the input end of the box is special and
PRIVATE! Only the input IC and the ultra short input connection, and air, are
allowed. You are on your own from this point! Good luck. I expect the builder to
rely heavily on the supplied visuals to complete the circuit.
TESTING AND CALIBRATING THE INSTRUMENT With the shielded box still apart. get a 1.5 volt battery and connect it to
the input. Turn the unit on. Measure with a VOM the voltage on the battery.
Write it down. Now go to the output with your VOM and adjust the gain
potentiometer until the reading exactly equals the input voltage of the battery.
`This sets the instrument for unity gain. You are calibrated! Next, we have to
check out the finished system. Assemble the box and case for final use. I prefer
to use a banana plug with a stiff 24 gauge rod soldered inside the plug to make
a vertical whip antenna if the input connector is on top of the instrument. If
not, just use a short flexible wire soldered to a banana plug and connect it to
a stainless steel salad bowl which is setting on a clean drinking glass. It is
crucial that no path of any resistance exist to ground from the isolated and
insulated metal object. Ground the chassis of the instrument (a good solid earth
ground is needed.) Connect the output to an Oscope or VOM. Set the scope's sweep
for a long period sweep of 10 seconds across the screen or more. Set your meter
for the 10 volt range if you have no scope. Turn on the instrument. Now move
about a bit and notice the varying voltage you are impressing on the bowl or
antenna! The impedance is so high (teraohms) that the instrument can resolve
voltages that would be overloaded and swamped back to zero by other meters of
lower impedance. If you have encountered any problems in getting the system to
work, you have made a simple mistake somewhere. Check your batteries. Are they
good? Are they hooked up right? Are the ICs in their sockets correctly? Recheck
all wiring. You can blow the input IC very easily by touching the input lead
while your body is electrostatically charged. I hope you purchased a spare!
PROTECTING YOUR INVESTMENT It seems bizarre that we use static sensitive components in an instrument and
then use them in static experiments. The instrument gets its ultrahigh impedance
from the use of feedback and the delicate micro thin layer of insulation
separating the FET gate lead from the main substrate in the device. This is a
layer which is easily punctured by static. Needless to say, such puncture means
the destruction of the device and IC. The wise application of the electrometer
device is the best precautionary measure one can take. Never leave anything
hooked to the input connector when the instrument is not in immediate use! The
size of the isolated capacity (item hung up as an antenna and hooked to the
input) relates to the amount of charge collected, and thus the voltage
collected. This instrument will respond to voltages in the range of +/- 7 volts
or so. Large collectors around high voltage equipment such as Van DeGraf
Generators, Wimhurst machines, Tesla Coils, etc., will collect hundreds of volts
and thus destroy the input IC. The voltage collected also relates to proximity
of the source of charge for any given collector size. If in doubt, use small
collectors or a whip antenna hooked to the input. Never walk across the room and
just touch the input collector or connector. You may carry thousands of volts on
your body! Always ground yourself on the grounded instrument box. Remember, the
box must always have a solid connection to a real earth ground to function
properly.
SUMMARY Your meter can be used to determine the polarity of various insulators.
Plastics are all negative. Polished, clean glass is always positive. Tapping
one's toe in time with a piece of music while sitting on a modern carpet can
induce +/-10 volt potential change on a can of Spam 5 feet away! The study of
atmospheric electricity can be investigated. A people proximity detector could
be devised. No matter what your use, you will see in short order, that every
object is continuously exchanging charges with other objects in our everyday
world. We exist in a sea of charge. For more information on electrostatics, an
excellent book by A.D. Moore entitled Electrostatics is available again at a
very modest cost and many experiments are suggested to introduce the amateur
scientist to the fascinating world of electrostatics.
PARTS LIST AND SOURCES FOR THE EXPERIMENTER'S ELECTROMETER
- Local Radio Shack stores and outlets.
- Digikey Electronics, DigiKey 701 Brooks Avenue South, Thief River Falls, MN 56701-0677 1-800-344-4539 Digikey order line.
Fig 2. Basic Electrometer Circuit
GROUNDING?
A true and properly driven electrical ground just for an electrometer would be a costly and time consuming business. A good alternative would be an AC plug which has only a single wire connection. A modern three wire grounded plug can be purchased from a hardware store and used to supply an electrical ground for the instrument in this article. I would recommend a 10 piece of common lamp cord in which both of the two 18 gauge wire leads are connected to the ground lug of the plug. Please be sure that the connection is made only to the ground of the plug!!! The hot AC connection must never come into contact with the wire and is left unconnected as is the neutral connection. I use a hot glue gun to cover and insulate the hot and neutral connections inside the plug in order that no accident can occur. The other end of the cord can be hooked to an alligator clip, spade lug or other convenient method of attachment to the case of the electrometer. Needless to say, never use a two to three prong adapter. If you use this type of plug it would defeat the whole concept. If you have only two terminal outlets in your home, you must secure a good ground elsewhere. This may mean driving a 6 or 8 foot ground rod just outside a window. These copper clad steel rods are available at most home builder supply stores. You should then run a wire lead inside your house through a window. Both the noise immunity of the instrument and reliable results rely heavily on the excellence of the ground connection.