SIMPLE, SENSITIVE VOLTAGE MOTOR USING 2-LITER SODA BOTTLES

Here’s a simple electrostatic motor that’s based on 2-liter soda bottles and aluminum foil. It’s construction does not require access to a machine shop. It draws a fraction of a microamp during operation, and can run at unexpectedly high speeds (1000 RPM!) It runs on a minimum of 5000 volts DC, which can be had from several different low-current electrostatic energy sources.

Any of the following can power this motor:

* Van de Graaff electrostatic generator (expensive unless home-built)
* Wimshurst electrostatic generator (expensive)
* Negative ion generator, try this one, it runs off a 9v batt.
* Aluminum foil on a TV screen (dangerous?)
* M. Foster’s Cheap High Voltage
* Lenny R’s PVC Pipe generator
* A very large electrophorus (low humidity required)
* “Kelvin’s Thunderstorm” waterdrop machine (very feeble, barely works)
* High-voltage DC supply (dangerous, avoid it unless skilled with HV!)
* Jefimenko-style sky antenna (kite-lifted or balloon-lifted wire with needles at top)
* Or, with some practice, even with a balloon and a piece of fur can sometimes work.
* Batteries won’t work, you need High Voltage

One of these motors is featured in the Electricity exhibit at the Museum of Science in Boston, powered by a hand-cranked Van de Graaff machine.
PARTS:

* three 2-liter pop bottles, at least one with a METAL cap
* roll of aluminum foil
* rubber cement
* silicone caulk
* 13″ metal rod, 1/8″ dia. (could use coathanger)
* Two 8″ pieces of solid copper wire, or coathanger
* wood plank (or metal, or plastic) for the base
* duct tape
* hookup wire for attaching the power supply

The Duluc Dry-Pile (also called the Zamboni Pile) was an “electrostatic battery” permanent power supply used in the early 1800s and constructed from silver foil, zinc foil, and paper. Foil disks of 2cm dia. were stacked up several thousand thick and then either compressed in a glass tube with endcaps and a screw assembly, or stacked between three glass rods with wooden endplates. Of course this is simply a Voltaic Pile, a multi-cell electrochemical battery, albiet one with output potential in the range of kilovolts. Each cell used nearly-dry paper as electrolyte, with zinc foil for one electrode and silver foil as the other.

I have a friend who runs a diecutting printshop who might be able to turn out the disks in large quantities. I suspect that zinc foil is hard to find, but probably is not required. Perhaps aluminized paper and copper foil, or even silver leaf paper can be obtained, then simply punched out and stacked up. Or perhaps carbon paper can act as both electrolyte and electrode, using aluminum foil as the second electrode on each disk.

A 5ft dry pile should raise the hair of anyone who touches the end. Or a shorter one could act as a “magic wand”: hold one end, touch someone’s body with the other to charge them up, then touch their nose with your finger. SNAP!

A book on the history of Perpetual Motion Machines showed photos of “genuine” perpetual motion devices based on the Dry Pile. DuLuc’s version was composed of two series-connected Dry Piles operating a pendulum electrostatic motor of the “Franklin’s Bell” type. The drypile stacks were of the 3-glass-rod variety, and had been insulated by dipping in liquid sulfur (no plastics in 1806!) The device in the book is owned by Dr. A.J. Croft of Oxford’s Clarendon Lab. At the time of publication of the book, this device had been tinkling away for over a century, and the owner of the device mentioned that the clapper-bead was starting to take on a distinct hourglass shape, and may need to be replaced in the next few centuries! A second device by Zamboni was a perpetually rotating “Franklin’s wheel” electrostatic motor powered by two dry piles. Zamboni experimented with drypile-powered clocks in the early 1800s.

Dry piles found commercial use as the power supplies of electrostatic voltmeters (quadrant electrometers), and in infrared converter “night vision” scopes used in World War II.

Here’s another cool thing (although I’ve not yet found a good use for it.)

My DVM (digital voltmeter) has a 200 microamps setting, but some sorts of electrostatic effects deal with lots less than 1uA. My old 20uA panel meter is better for these, but sometimes I want to see things which barely make it budge. I discovered a setting on my DVM meter which is 10,000 times more sensitive! By putting the 20uA panel meter in series with the DVM, then setting the DVM to 200mV (volts, not amps!), a small current which sends the voltmeter to 200mV reading will move the 20uA meter slightly. I estimate that the 20uA meter is indicating about 1/50 of a uA (20 nanoamps), while the voltmeter reads “200″. Aha, the voltmeter has a 10meg input impedance, so if its voltage range is instead used as a current meter, the 200mV range is actually reading 20 nanoamps. So if the DVM reads 1mV, it is actually measuring 100 picoamperes!

Anyone have good ideas for applications for a digital meter with a 20nA full scale range? Maybe use it along with a 100v DC supply to make an ohmmeter with a full scale range of 5 giga-ohms, then use it to measure the resistance of wood, cloth, plastic, etc. Maybe we could detect the current which goes through thin glass. Or the nano-ammeter could be used alone to sense the air ions from a VandeGraaff machine that’s on the far side of the classroom. Or take the meter outside, connect it to a big sheet of foil supported by insulators, and try to detect up the current/m^2 sky current? Oooo, if those Radio Shack DVMs with the RS-232 outputs can do the same thing, then we could GRAPH the sky current during the day, or watch pulses of ions drift across the classroom as the VDG was turned on and off, etc.

While playing with dry ice during the June 1998 Seattle Weird Science meeting, I stumbled across a VERY strange electrostatic effect.

Electrical experiments using plastic tape

There are several things which interfere with our understanding of “Static Electricity.” Most demonstrations incorrectly focus on friction. Also, the nature of matter and the fundamental reasons for charge conservation are usually ignored. And the materials used in demonstrations (silk, fur) are hard to obtain and have a finicky dependence on humidity. The following demonstrations are my attempt to fix these problems.
“STATIC CHARGING” WITHOUT FRICTION

Get a spool of plastic tape. Pull a couple of long strips from the roll, about 20cm each. Hold them up by their ends so they hang downwards, then slowly bring them side by side. Notice that they repel each other? If you try to force the dangling lengths of tape to touch together, they’ll swerve and gyrate to frustrate your efforts. You can stick the strips to a door jamb and on a dry day they will keep repelling each other for several minutes. They will also “attack” anyone who passes through the door. Obviously the tape has become electrically charged. But how? After all, no friction was involved. Something odd is going on.

These demonstrations won’t work when the relative humidity is high. Try the first one above. If the lengths of tape don’t repel each other, then the humidity in the room is probably too high, and none of the other demonstrations will work either. Move yourself into an air-conditioned building, then try again!

Also, 3M SCOTCH Magic(tm) brand tape doesn’t work as well as similar tape from other companies. Perhaps 3M puts “anti-static” chemicals in the adhesive?

Next, pass the entire length of each of the hanging strips lightly between two fingers several times, then hold the two strips near each other again. This time they won’t repel each other. You’ve managed to discharge them by fondling them, and the strips are now nearly neutral. (If your fingers are extremely dry, this might not work. Wet your fingers very slightly, but don’t get the tape wet.)

Next, fold over a couple of cm of the top of the strips. This gives you a non-sticky tab on each strip. (It makes it easy to get the strips apart again in the next part.) Now carefully stick the two strips together so the sticky side of one strip adheres to the “dry” side of the other. To show that friction plays no part in the following, try to avoid rubbing the tape. You should end up with a double-thick layer of tape which is sticky on one side and has two tabs at one end. Grasp those tabs and rapidly pull the strips apart. Hold them distantly separated, then slowly bring them together. You’ll find that this time they attract each other quite strongly. Before they repelled. Now they attract.
REPULSION AS WELL AS ATTRACTION

Next, do the same thing as above, but twice: take four pieces of tape and prepare two *pairs* of tape, each pair having one piece stuck to the back of the other as before. Pull both pairs apart, and either ask a friend for help, or stick a couple of the tapes to the edge of a table so they hang down. As before, you’ll find that the lengths of tape which were stuck together now attract each other. But try holding a strip from one pair near each strip of the other pair. You’ll find that your single strip will attract one of the other strips, but repel the other. When you peeled each pair apart, one the strips took on opposite charge polarities. The “sticky” strip now repels the other “sticky” strip, but it attracts the “dry” strip. When you have four strips, you can demonstrate that opposite charges attract, but also that alike charges repel.

UN-CANCELLED CHARGES
What’s going on here? How did the strips of tape become electrified? There is a simple answer. Contrary to popular belief, “static electricity” is not caused by friction. It’s actually caused by contact between dissimilar insulating materials, and is greatly amplified when those materials are forcibly separated. When you stuck the tape strips together, you instantly caused a separation of charges. When you peeled them apart, you pulled the oppositely-charged areas away from each other, causing “un-cancelling” of charges. Another name for this phenomena is “contact electrification.” A less accurate description is “generate static electricity.”

In explaining everyday electrostatic phenomena, most authors wrongly emphasize the need to rub materials together to generate separations of charge. They often directly state that the friction CREATES the charge separation. This is misleading, since friction really only plays a secondary role in the process. The physics behind “static” electrification usually doesn’t involve friction, it involves chemistry.

When the surfaces of two everyday objects are touched together, they always adhere slightly. Chemical bonds form between the atoms which make up the adjacent surfaces, and this causes the adhesion. If the surfaces are not composed of the same sorts of material, then chances are the chemical bonds will be polar, and the bonding electrons will stay with the atoms of one surface more than with the other. The surfaces become oppositely electrified when they touch, because one surface immediately steals electrons from the other as the chemical bonds form. One surface ends up with more negative electrons than positive protons, and then has an overall negative charge. The other surface has fewer electrons than protons, so it has overall positive charge.

“CREATING” CHARGES?
I must take the opportunity here to point out something that bugs me. Books will often state that charges are “created” or “made” during static electrification. This is extremely misleading. Atoms are composed of positive and negative particles (protons and electrons.) The opposite charges are in intimate proximity so the atoms are normally electrically neutral. We cannot avoid the conclusion that ALL MATTER IS COMPOSED OF CANCELLED ELECTRIC CHARGE. If we define “electricity” to be that quantity carried by electrons, then we could also say that ALL MATTER IS MADE OF NEUTRALIZED ELECTRICITY. Strange, no? But true. Static electrification is a separating, an un-cancelling, of positive and negative particles which were already present in the materials involved. Static electrification is more properly called CHARGE SEPARATION. If you grab an atom by its protons and electrons and separate them far apart from each other, you create “static electricity” or charge separation.

Touch two dissimilar surfaces together and the pos/neg charges in their surfaces become separated. When you pull the surfaces apart again, the chemical bonds rupture, and one surface may end up with more electrons that it started with. The other surface has protons which now lack their nearby cancelling electrons. Oppositely charged particles which had once been adjacent to each other and “cancelled out” within the atoms have now been sorted out and separated by a great distance.

AN ATOM THE SIZE OF YOUR HEAD!
From another viewpoint, peeling the tape causes atoms to become enormously stretched, because the outer electrons of one set of atoms has been pulled far away from their protons. Weird fields of force are still connecting the separated protons and electrons, but these fields had originally existed only down within the microscopic world of the atoms. Stretching out the atoms in this way also “stretches” the tiny atomic force fields. This adds energy to them and causes them to balloon outwards and grow so large that they start to affect us here up in our “macro” world. The invisible attracting/repelling fields which surround electrified objects are the same force-fields normally only found inside of atoms.

So everyday “static electricity” has little to do with rubbing or friction. Instead it involves contact, chemistry, and imbalances in the electrical charges of which matter is made. Electrostatic attraction and repulsion between electrified objects is a feeble residue of the same immense forces which hold solid matter together. Our bodies are held together by “static electricity!” And when a huge crane lifts a steel beam, the immense force within the steel cable is actually an electrostatic force field between the atoms of the cable.

If the surfaces involved in contact electrification are rough or fiberous, then only a tiny part of the surfaces can be touched together at a time. If a balloon is touched against hair, the hair only touches the rubber in tiny spots. The “footprint” of contact area will be a tiny percentage of the total surface. In a situation like this, friction does play a role. If the balloon is DRAGGED across the hair, then the successive areas of contact add up to a much larger percentage. Rubbing a balloon on your head increases the total area of rubber and hair that’s being touched, so it also increases the total amount of separated charges. Friction aids the charging effect, but friction does not create it.

ELECTRIFICATION BY “PEELING”
So why do strips of tape become charged? Adhesive tape is not a single material. The adhesive and the plastic backing are two different insulators. When they are touched together, one surface steals electrons from atoms of the other, and the surfaces become electrified. When they are peeled apart, atoms are torn open and opposite charges are separated. The tape can then attract and repel distant charges.

CONFUSE YOUR VICTIMS
There are other things you can try. Take two lengths of tape, discharge them between fingers so they no longer repel each other, then fold little tabs and stick them so the adhesive sides stick together. Adhesive to adhesive. Now peel them apart, then bring them near again. They will neither repel nor attract. No separation of charge occurred because the materials on both sides were the same. DISSIMILAR materials are required in order to create separated charge. (This trick can be used to fool people. If you stick YOUR tape strips back to front, but tell someone else to stick THEIRS front to front, they won’t notice the difference. When then peeled apart, your strips will attract, but theirs will not! You can then explain your trickery, and teach them a bit of Electrostatic trivia at the same time.)

CHARGED ATTRACTS UNCHARGED
You may have noticed that your charged tape-strips don’t only attract and repel other strips, they also attract everything else! Hold a charged strip near your arm, or the wall, near most any neutral object, etc., and the strip will be attracted. Regardless of whether your tape strip is positive or negative, it will attract a neutral object. A general rule: charged objects always attract uncharged objects. Why? Because the charged object causes the charges inside the uncharged object to separate a bit. If you hold a positively charged strip of tape near the wall, the charge on the tape strip will cause negative charges in the substance of the wall to move a bit toward the tape. At the same time, positive charges in the wall move away from the tape. The tape is then attracted to the negative charges in the wall. This is called “attraction by induction,” since the charged tape “induces” a separation of charge to occur in the wall. Induction works better with conductors, since the charges in a conductor are free to move. If you hold your tape strip near a metal object such as a refridgerator door, it will be pulled a bit more strongly than the wall pulls it. Hold the strip near your arm, and the pull is strong. Your body is salty water, you are a conductor.

WHICH ONE IS WHAT?
How can you tell which tape strip is positively charged, and which is negative? Easy: by comparing them against a known polarity. An expensive and dangerous way to do this is to string 9-volt batteries together until the voltage adds up to several thousand volts. The positive end of the chain will attract negatively charged tape and repel the positive. Don’t touch the battery chain, the high-current capability makes them lethal! A safer, easier way: When you rub a balloon on hair, the balloon’s rubber always becomes negatively charged. To determine the polarity of a tape strip, hold it near a hair-charged balloon. If the strip is negative, the balloon will repel it. If the strip is positive, the balloon will attract it.

CHALK DUST PHOTOCOPIES
Here’s a way to demonstrate part of the “Xerographic” process used by photocopiers and laser printers. Obtain a flat piece of clear or dark plastic 1/16″ thick or thicker, talcum powder, a rag, and some tape. Peel off a strip of tape, discharge it between fingers, fold a tab at one end, and stick it securely over the surface of the plastic. Put down some newspaper so you don’t get talcum powder all over, then sprinkle talcum powder on the rag and rub it in. Now peel the tape off the plastic, then shake the rag to make a cloud of talcum powder dust in the air near the plastic. The charged area on the plastic surface will attract the powder, and a “charge image” will appear. If your plastic was clear, try holding it against a dark background to make the white powder more visible. (This experiment works best when humidity is fairly low.)

If you can find a big piece of acrylic from a hardware store, try laying several pieces of tape on it to form your initials or to form a simple word. (Always fold little tabs at one end of each strip.) When you peel all the strips of tape and make a dust cloud, you should then be rewarded with a clear example of electric-charge writing.

Another demo: get some wide packaging tape, a marker, and a paperclip (as well as talcum powder, etc.) Stick the tape to the plastic, and unbend the paperclip to give you a sharp pointed tool. Use the marker to outline the tape (and where the charged area will be.) Peel the tape from the plastic, then lightly draw a large invisible “X” on the invisible charged area on the plastic. Flap the talcum-cloth, and you’ll find that the dust cloud is attracted to the charged area as usual, but the “X” will be visible as a dust-free zone. The sharp point of the paperclip wire acted to discharge the plastic. Actually, a tiny corona discharge or “St. Elmo’s Fire” was generated on the sharp wire point. Alike-charged air spewed out of the corona, and the opposite-charge air settled onto the plastic, cancelling out the surface charge. With skill (and a big piece of plastic,) you should be able to write several words on a long tape-charged area. Hint: paint one side of the plastic black for contrast. Another hint: try charging the plastic by rubbing it with fur or wool cloth, then write big invisible letters with the paperclip end. Clouds of talcum dust should make it visible.

In a copier, the talcum powder is replaced by black “toner” powder. The plastic plate is replaced by a light-sensitive coating on a metal drum, which discharges bits of itself wherever light lands on it. The charging device is a long thin wire with high-voltage corona on it that sweeps over the drum. The flapping cloth is replaced by a fuzzy brush made of iron filings stuck to a long magnet, and covered with black toner powder. And finally, the black powder melts when heated, so a red-hot “fuser” bar passes over the black dusty paper and makes the writing permanent.

(The earth-ground is not required.) (The 1-Meg resistor is not required.)

This simple circuit can detect the invisible fields of voltage which surround all electrified objects. It acts as an electronic “electroscope.”Regular foil-leaf electroscopes deal with electrostatic potentials in the range of many hundreds or thousands of volts. This device can detect one volt. Its sensitivity is ridiculously high. Since “static electricity” in our environment is actually a matter of high voltage, this device can sense those high-voltage charged objects at a great distance. On a low-humidity day and with a 1/2 meter antenna wire, its little LED-light will respond strongly when someone combs their hair at a distance of five meters or more. If a metal object is lifted up upon a non-conductive support and touched to the sensor wire, the sensor can detect whether that object has an electrostatic potential of as little as one volt!

* Note: I use the term “electrification” rather than “charging”, in order to avoid confusion between charge and net-charge. Charge is the stuff on the negative electrons and positive protons, while net-charge is the imbalance between positive and negative particles which appears on everyday objects. Realworld objects become “electrified” whenever their pre-existing + and - charges are not equal.

PARTS LIST:

* 1 - Standard 9-volt battery
* 1 - MPF-102 N-channel Field Effect Transistor (FET) Radio Shack #276-2062
* 1 - Red Light Emitting Diode (LED) Radio Shack #276-041
* MISC:
o Battery connector (#270-325)
o Alligator Clip Leads (#278-1156)
o solder, if desired
o 1-meg resistor (not required)
o plastic, fur, foil, comb, tape dispenser, plastic cup

(Tiny version bult atop a 9v battery connector)
Shortcuts:o 1.CONSTRUCTION HINTS
o 2. SENSE E-FIELDS
o 3. SENSE POSITIVE ELECTRIFICATION
o 4. CHARGE IS CONSERVED
o 5. PEELING CAUSES ELECTRIFICATION
o 6. JUMPING ELECTRONS, “VOICE CONTROL”
o 7. VARIABLE GAIN
o 8. FIELD DISTORTIONS
o 9. VANDEGRAAFF SENSING
o 10. HOMEMADE CAPACITORS
o 11. DIPOLE ANTENNA
o 12. THE SKY VOLTAGE
o 13. UNTESTED SUGGESTIONS
o 14. HOW IT WORKS
o 15. WHERE IT CAME FROM
o 15. FET-PANEL MUSEUM EXHIBIT

A simple science project

Iron filings align themselves in strong magnetic fields. This reveals the shape of the field patterns. A similar thing happens with the electric fields created by high voltage and by “static electricity.” If small fibers are exposed to a very strong electric or magnetic field, they will align with the field and make it visible.

______
||    ||
||____||
___—–    —–___
/               |    \
|         _      \   |
|    –__   \        |
|              \  \    |
|     -              \ |
|      __       —___ |         _______________
|                  __  |       /  MAGNET        \
|   ___–       —    |       \________________/
|        __–       /  |
|   _           /      |
|  /               /   |
|       /             |
|      |         |   |  THE FLOATING FIBERS MAKE THE
|            |       |  MAGNETIC FIELD VISIBLE
\__________________/
BOTTLE

3D MAGNETIC AND ELECTRIC FIELD VIEWING BOTTLE
MATERIALS:

* Magnet
* Extra-fine steel wool (type 000 or 0000, hardware store)
* Plastic bottle full of baby oil with paper label
* Scissors
* OPTIONAL:
o rubbing alcohol to remove label
o pan or shallow dish for the alcohol
o White spray-paint, if desired

Links to magnetism sites

Other build-it projects

REMOVE THE LABEL

Make sure to buy baby oil with a removable paper label, NOT the kind with a permanent, painted-on label. Even better, try to find a bottle that has a label only on one side.

If your bottle has labels on both sides, peel the label from one side of the oil bottle. You can do this by picking at the paper label with fingernails while running warm water on it. An easier way is to soak one side of the bottle in a shallow dish of rubbing alcohol for about 10 or 15 minutes. Peel off the gooey label. Use a bit of alcohol and a paper towel to clean off the remaining glue. (It really is easier to find a bottle at the store that only has a label on one side!)

MAKE THE STEEL FIBERS
Obtain extra fine steel wool. This is the kind that looks like a rolled-up wad of grey hair, NOT the kind that looks like a coppery coarse net used for scrubbing dishes. Any grade of wool will work, but extra-fine wool will settle more slowly, so you don’t have to shake the bottle so often.

Find the end of the roll, and unroll the steel wool part way. We will use the scissors to make cuts ACROSS the wool. First trim the wool straight across to remove the frayed fibers, then repeatedly cut across the wool to make many very narrow strips, narrower than 1/8 inch. Try to cut them 1/16 inch if you can. This will give you thousands of short steel fibers. Cut up about a heaping teaspoon of fibers, or about one square inch of unrolled steel wool. Don’t pack them down too much if you can help it. If you use a really tiny bottle of baby oil, use less than a teaspoon of fibers. If you use too large an amount of fibers, the fibers will clump and settle to the bottom of the bottle too fast. If you use too small an amount, the fibers will be hard to see.

MIX THE FIBERS INTO THE OIL
Gently wad up the fibers, drop the wad into the bottle of baby oil, cap it securely and shake the bottle. Shake until the wad of fibers is spread evenly throughout the oil. It helps to shake the bottle with a violent rotating wrist motion rather than just shaking straight up and down. Up-and-down shaking only works if there is a large bubble, rotation- shaking works good even with no bubble at all

If you have difficulty finding a magnet, try Radio Shack stores. They sell small disk magnets which can be stacked up to form bar magnets. A few dollars worth of their 1-inch “donut” disk magnets will be enough to make several big stacks. These can be used in other future science projects. You’ll also wind up with a lifetime supply of refrigerator magnets! Don’t forget to keep magnets away from credit cards, computer disks and video tapes, magnets can erase these. Keep them away from color TV and computer screens, since they can create permanent color blotches which can only be removed with a TV repair “degausser” coil or a cassette tape bulk eraser. (Hint: wave your magnets around an old Black&White TV screen, see what happens to the picture!)

While the fibers are still mixed into the oil, hold a magnet near one side of the bottle and watch the tiny fibers. They will all align themselves and reveal the three-dimensional magnetic field pattern. It helps to have the bottle in bright light so you can see the tiny fibers against the white label on the back of the clear bottle. The fibers start to settle to the bottom in 10 or 15 seconds, so you’ll have to shake the bottle every so often if you want to keep experimenting.

When the fibers settle out, shake them up again. If the fibers clump against the magnet, then you are holding the magnet too close to the bottle. Hold it about 1/2 inch or 1 inch away. Try holding the magnet sideways as shown above. Also try it up and down. Also try holding one magnet pole near each side of the bottle. Try an “N” pole with an “N” pole (the poles which repel each other.) Also try an “N” pole facing an “S” pole (the poles which attract.)

______
||    ||
||____||
___—–    —–___
/                    \
|                    |
|                    |
|                      |
|                      |           /  \
|                      |          |    |
|    ? What will you   |          |    |
|         see?         |          |    |
|                      |          |    |
|                      |          |    |
|                      |          |    |
|                      |          |    |
|                      |           \__/
|                     |           MAGNET
|                    |
|                    |
\__________________/
BOTTLE

______
||    ||
||____||
___—–    —–___
/                    \
|                    |
|                    |
|                      |
|                      |
S              N    |                      |   N            S
___________       |     WHAT WILL        |     __________
/            \     |     YOU SEE?         |    /           \
\____________/     |                      |    \___________/
MAGNET         |                      |        MAGNET
|                      |
|                      |
|                    |
|                    |
\__________________/
BOTTLE

You can improve your bottle by removing all labels and painting one side of the bottle with white spray paint. This gives you a smooth white background against which to view the floating dark fibers.

Introduction
Electrostatic speakers are the just about the lowest distortion drivers that can be made. But you already know about their wonderful attributes or you wouldn’t be interested in making them, so I’ll dispense with the BS. I present here a simple process for making ESLs. I have not included anything about crossovers or cabinets. This is strictly a “how to make the drivers” article.

Warnings:
Before we go any further, I want to warn you about a couple things you may not be aware of. Electrostatic loudspeakers use high voltages to operate. They need a DC bias of up to 5000 V and use AC voltages up to 5000 V. The DC bias is usually supplied by a power supply running off 120VAC electrical circuits which can be dangerous. The AC voltages used to drive the ESL are usually produced by connecting your stereo amp to a vacuum tube amplifier type output transformer. The voltages produced by the transformer are dangerous! Don’t screw around! If you have little kids in the house or if anyone might for any reason touch the speakers while they are operating, design your speakers so that it is not possible to come into contact with the drivers. If you don’t know how to handle high voltage circuits, enlist the help of someone who does, or buy one of the commercially available ESLs.

Section I: Making the speakers
Building ESLs involves the use of tools and materials that if handled improperly can be hazardous. Please make sure you know how to use these things before you begin. By all means, use safety glasses at all times. If would be foolish to trade your vision for the pursuit of audio ecstasy!

What you need:
1) Transformers, one or two per speaker - use tube amp output transformers, 4 ohm:8K -20K ohm. I have used Tango CRD-8 ( 4:8KCT) transformers that I bought in Japan. You can use transformers by Triad, Stancor, etc. Just get units that are good for about 15-20 W at 30 Hz and give a large impedance (i.e. voltage) transformation. Expect to pay about $50 each for transformers. Tube amp output transformers are available from Antique Electronic Supply, 602-820-5411, and other sources.

2) Plastic film for speaker diaphragms- Mylar or other polyester, thin (5-6 microns), and large enough to make the size of driver you want to build. This can be obtained from companies that make plastics for industry- this film is commonly used to make capacitors (don’t get metalized film!). I bought a roll that is 1200 m long by 1 m wide for about $85 in Japan a few years ago. I have used about 15 m of it so far. I have heard of people using Saran-wrap, but I have never heard a driver built using it. If you’re making small drivers, or experimenting, try it! It certainly won’t cost much…

3) Powdered graphite, dish soap, or antistatic solution to coat diaphragm. Powdered graphite is available from K-mart or your local hardware store for lubricating locks. It will cost no more than $2 for enough to make about 50 speakers. Graphite has to be rubbed into the film using cotton balls. Dish detergent and antistatic solution will work also, and are easier to apply, but may not be “permanent”. I use graphite. Someone in Australia suggested that drafting ink formulated for drawing on “film” (the draftsman’s name for polyester) will make a good, easy to apply, high resitvity diaphragm coating. I haven’t tried it yet, but applying a colored liquid ought to be easy and make it easy to verify that it only went where you wanted it.

4) Perforated aluminum or steel- You need a piece for the front and the back of the driver. It should be flat and have about 60% or more open area (holes). Hole size? The stuff I use has holes that are about 3 or 4 mm diameter. The “rules of thumb” say don’t use holes larger than about 1/4″. Check your local Yellow Pages phone book for listings under Perforators, or Sheet Metal. Your local hardware store may have some available also. Aluminum is much easier to cut than steel, and it is much lighter weight, but may cost a little more than steel. If you buy from a perforator you can get them to cut the metal to size and roll it flat for you.

5) Acrylic or fiberglass PC board stock for driver frame. Fiberglass is hard to cut (you need a carbide blade), and the dust from sawing is a health hazard, but epoxy will bond to it. Acrylic or other plastics are easier to work with, but epoxy may not form much of a bond to them (contact cement will probably work just fine). I have used both acrylics and PC board and for all it’s trouble, I prefer the PC board material. You can get fiberglass from a PC board company- try to raid their scrap pile- and get them to cut the pieces to size for you. We’ll talk about thickness later.

6) Glue - Previously I recommended epoxy to hold the ESL together. Epoxy works fine for attaching the perforated metal to the insulator frame. The problem with epoxy is that it doesn’t really bond to the mylar film. A little mechanical stress can break the very weak bond and allow the film to peel away. This can be an advantage. If you find that a driver doesn’t work, if you assembled it with epoxy it will be easy to rip apart and rebuild.

I have done some additional research and found a contact cement manufactured by 3M that works for attaching the film to the insulating frame. Scotchgrip #4693 is the stuff to use. You put a little on one or both surfaces to be glued and let dry for 10-20 minutes. Then you put the two surfaces together and Voila!, instant bond. The bond is so good that the film will tear long before the glue lets go. Other contact cements may work well also. The only disadvantage is that once you’ve assembled the driver using contact cement, you have to live with it. If the driver doesn’t work, you’ll have to build another because you won’t be able to tear the old one apart.

7) High voltage DC bias supply (1000-5000VDC, almost no current.) This can be made as a voltage multiplier that works off the power lines. You’ll need high voltage diodes and capacitors, a few resistors, a circuit board and a line cord. You can get away with one supply, but one for each speaker is easier to deal with- you won’t have to run high voltage wires all over your listening room. See the Bias Supply section near the end of this document.

Optional:
Plastic coating for the perforated metal. I’ve heard that latex house paint works fine…

Figure 1. Exploded view of a basic electrostatic driver.
Making the drivers:
Step 1. Design your drivers.
Decide on the size and make the frames for the drivers. It is generally easier to make small drivers than to make big ones, but with small drivers you will need a lot of them so mounting them can be a pain. You need one insulator for the front and one for the rear of each driver. Ideally, the insulator frames should be cut from a single piece of insulating material. But they don’t absolutely have to be made from a single piece. Be sure to plan and leave room for electrical connections (3 wires per driver) and mechanical mounting. I have built many drivers using different geometries and found that the following thicknesses and bias voltages will result in drivers that closely match the sensitivity of conventional boxed bass drivers without the addition of a lot of attenuation in the low frequency section of your crossover:ESL use total ESL area DC bias insulator thicknessmid/tweet >2 ft2 1500 V 1/16″

full range >4 ft2 3-5000 V 1/8″-1/4″

The insulator thickness to use is a function of many variables. If you want to reproduce low frequencies (down to 100 Hz or lower) you need to have room for the diaphragm to move. That means thick insulators. You will also need to use high bias voltage and high driving voltages (two transformers) to get reasonable sensitivity.

The mechanical force on the diaphragm varies as the square of the distance from the stator plates. That means that if you double the thickness of the insulators, you need to use four times the voltage for equivalent acoustic output. It isn’t easy to make full range ESLs, and they almost never deliver enough bass. You need really huge surface areas to get bass, but that increases the capacitance of the driver and can limit high frequency response. You can improve the bass by using electronic equalization and mounting the drivers in the corners of a room. There is plenty of room for experimentation.

For midrange/tweeter drivers to be used in a hybrid system, there is considerable flexibility in the insulator thicknesses and spacing, bias voltage, and driving voltages. 1/16″ PC board material is extremely common and low cost so it is almost ideal (except for the difficulty in cutting it) for this application. * 1/16″ is easily enough room for the diaphragm to produce ear splitting volumes at frequencies down to 300 Hz or so, using a single transformer to drive each speaker.

Another benefit to using PC stock is that it is usually metalized on one or both sides, a feature that can be very useful when making electrical connections to the drivers. It will be best to have one insulator frame metalized on both sides, and the other metalized on one side, but we can make due with any material, even unmetallized.

There is a “rule of thumb” about the dimensions of an ESL that relate to the insulator thickness. The rule is that the diaphragm should be supported at least every 100X units, where X is the thickness of the insulator pieces. ‘Supported’ means that you should put insulating strips in the driver to support the diaphragm in at least one direction. ‘One direction’ means that long narrow drivers are OK. If you use insulators that are made from 1/16″ PC board stock, the diaphragm should be supported every 4-6 inches. If you look at Martin- Logan ESLs you’ll see they have support insulators every 4-6 inches and that they are unevenly spaced, presumably to move resonances of each section to different frequencies.

Figure 2. One way to make the ESLs showing the use of PC board stock.Drawing not to scale. Electrical connections are soldered to the copper pads labeled “A”, “B” and “C”. Be sure to leave room for hardware to mount the driver to some sort of frame.Step 2. Electrical connections
You will need to make an electrical connection to the diaphragm. This can be done in any number of ways, but remember that you must maintain a high voltage potential between the metal plates and the diaphragm. That’s why we were careful to vacuum up the graphite powder. You may want to clean the insulators with alcohol and a very clean cloth before proceeding.

The electrical connection is made by physical contact between a metal strip and the graphite coated surface of the diaphragm. The metal strip may be the copper on a piece of PC board stock used for the insulator (very rugged and solderable), or it can be a piece of aluminum foil, or Radio Shack burglar alarm foil tape (both somewhat delicate and not solderable). Just remember that you have to be able to connect a wire from the HV bias supply to the metal. Also, epoxy is generally not electrically conductive (there are conductive epoxies available, but they are usually quite expensive), so don’t completely cover the metal with epoxy.

Here is a tip to help insure long life for your ESLs. When you connect DC bias to the diaphragm, connect the minus side of the bias supply to the driver and the plus side to the center tap of the driver transformer. If you connect it the other way around, you’ll find that over time the metal electrode that connects to the diaphragm will corrode like the plus battery contact in your car.

Step 3. Stretch, coat, and attach the diaphragm to the insulators.
Stretching the diaphragm can be accomplished in two relatively easy ways. One way is to use a heat gun to shrink the diaphragm after it has been attached to the insulators. People have reported good results using this technique, but I haven’t tried it.

I use a stretcher table of the type shown in figure 3. The table allows you to coat the diaphragm under full tension and allows you to make multiple drivers with nearly identical resonances (by inflating the tube to the same air pressure for each driver). To use it you lay the film on the table and use double sticky tape to attach the edges of the film to the underside of the table. You then pump a few strokes of air into the inner tube and watch as the wrinkles in the diaphragm disappear. You can put extreme amounts of tension on the film using this table, so be careful. Make sure you put a small hole through the table top surface to allow air trapped under the diaphragm to escape when you start pumping!

Figure 3. View of the underside of the diaphragm stretcher table.The film is laid on the top side of the table and the edges are folded to the underside and secured with double-sticky tape attached to the inside of the table edge. Inflating the tube stretches the film tight. A rectangular table works just as well as a round one and is probably easier to make.How much tension is enough? That’s a difficult question. The tension you use is a balancing act. It depends on the bias voltage you will use, the thickness and spacing of your insulators, and on the frequency range over which you intend to operate the driver. Usually you will want to operate the driver above its fundamental resonant frequency. If you want full range operation, that means you want the resonant frequency to be below 100 Hz or so. That requires low diaphragm tension but low diaphragm tension means you may have to use a reduced bias voltage or you may have the driver break into a low frequency oscillation where it pulls to one side, sticks until the diaphragm is discharged, then returns to the center until the diaphragm charges up again, etc., etc.

In reality the amount of tension you use isn’t critical. Rectangular drivers have multiple resonances and you will always have some of them in your pass band. I have never been able to identify any of them by the sound of the driver when running test tones through it, and certainly never when listening to music. It may be possible in an anechoic chamber or by using a FFT analysis of impulse response, but in your listening room there will always be room mode resonances and multipath effects that will dwarf the driver resonances. If the tension proves too low you can always reduce the bias voltage.

OK, so you have the diaphragm under tension on the table. Now what? Time to put the resistive coating on the diaphragm. First put the insulators in another room. Then place a little (very little!) graphite on the film and grab a clean cotton ball and start rubbing the graphite into the film. Rub it in hard. Add more graphite as needed. You really don’t need to use much. You want the film to be coated with the stuff so that it has very high resistivity. It’s really not critical. After you have rubbed the graphite in, grab some clean cotton balls and rub some more. You can measure the resistance of the film by dropping a couple pennies on it a few inches apart and checking the resistance between the pennies with a DMM. You want a high but measurable resistance. Move the pennies around and check a few places. If you get resistances on the order of 100K or more, you’ve done a good job. If you measure lower resistances, rub with clean cotton balls some more. Get out your vacuum cleaner, put a brush attachment on it, and vacuum the entire surface of film that has been coated and the area where you were using the graphite. Now wash your hands very thoroughly! Then wipe the insulators with alcohol and a very clean rag to make sure they are absolutely clean before proceeding.

Why is the resistance important? Sooner or later, a bug will get into your speakers, or you will crank the volume a bit too high and your speakers will arc. If you use a metalized diaphragm (low resistance) there is a good chance that the entire diaphragm will flame out and you’ll have to rebuild the speaker (but it’ll impress your friends!). If you use a high resistance coating, the amount of current available to the arc is very small, resulting in a low temperature arc that will at worst put a pin hole in the diaphragm. High resistance coatings that I’ve tried do not cause the normally self-extinguishing polyester diaphragm to become inflammable. This is another reason for using a very large resistance between the diaphragm and the bias supply.

If you feel that you really need extremely high resistance for your speakers, try using dish detergent or antistatic solution to coat the diaphragm. I have built drivers using all three coatings and find no audible differences between them (but maybe your ears are better than mine).

Attaching the diaphragm is easy. You simply put glue (Scotchgrip #4693) on one of the insulators (again- don’t completely cover the metal) and place it, glue side down, on the coated film. The bond forms instantly, so make sure you set the frame down on the diaphragm exactly where you want it. Once the glue has set (after about 10 microseconds), let the air out of the tube and cut the film away from the table along the edge of the insulator. Now turn over the insulator/film assembly and set it back down on the table, diaphragm up. Coat one side of the other insulator with glue, wait about 10-20 minutes, then set it glue side down on the insulator/film assembly. Be sure to align the two parts carefully before pressing them together- you don’t get a second chance. You might consider building some sort of fixture to ensure accurate alignment.

Now you can epoxy the perforated metal sheets to the insulator assembly. The perforated sheets are made by running a roller with metal pins over the sheet metal. That leaves the edges of the holes on one side rounded and the edges on the other side sharp. Put the rounded edge side toward the diaphragm. Epoxy the stators one at a time and be sure the epoxy has time to set before you pick up the assembled driver.

I have done some experiments aimed at rounding the sharp edges of the holes. One of the things I recalled from high school chemistry experiments is that corrosion of metals occurs fastest at points of stress and sharp edges. I tried using ferric chloride PC board etching solution from Radio Shack. Since aluminum is more ‘reactive’ than copper I had to dilute the solution by cutting it with water at about 1 part FeCl to 4 parts water. This kept the speed of the reaction slow enough to allow me to observe progress of the reaction and remove the aluminum when the edges were rounded. If you try this, be sure you dilute the FeCl and then put a small scrap of aluminum into the solution to test it before you put in the pieces you will use for your speakers. If you don’t dilute the solution you’ll end up with a bad smelling, boiling mess!

Step 4. Testing
Stand the driver up using styrofoam blocks to insulate it or hang it from a frame using nylon cord. Connect the transformer(s) to the driver per figure 4. Next, connect the bias supply wires to the transformer and the driver. Power on! If all is well you should hear a very quiet click or nothing at all.