Electrostatic motors are now common in MEMS applications, but researchers at the University of Wisconsin and spinoff C-Motive Technologies have brought macroscale electrostatic motors back. [via MSN/WSJ]

While the first real application of an electric motor was Ben Franklin’s electrostatically-driven turkey rotisserie, electromagnetic type motors largely supplanted the technology due to the types of materials available to engineers of the time. Newer dielectric fluids and power electronics now allow electrostatic motors to be better at some applications than their electromagnetic peers.

The main advantage of electrostatic motors is their reduced critical materials use. In particular, electrostatic motors don’t require copper windings or any rare earth magnets which are getting more expensive as demand grows for electrically-powered machines. C-Motive is initially targeting direct drive industrial applications, and the “voltage driven nature of an electrostatic machine” means they require less cooling than an electromagnetic motor. They also don’t use much if any power when stalled.

Would you like a refresher on how to make static electricity or a deeper dive on how these motors work?

Most of the electric motors we see these days are of the electromagnetic variety, and for good reason: they’re powerful. But there’s a type of motor that was invented before the electromagnetic one, and of which there are many variations. Those are motors that run on high voltage, and the attraction and repulsion of charge, commonly known as electrostatic motors.

Ben Franklin — whose electric experiments are most frequently associated with flying a kite in a thunderstorm — built and tested one such high-voltage motor. It wasn’t very powerful, but was good enough for him to envision using it as a rotisserie hack. Food is a powerful motivator.

What follows is a walk through the development of various types of these motors, from the earliest ion propelled ones to the induction motors which most have never heard of before, even an HV hacker such as yours truly.

Franklin’s Electric Wheel

In 1748, Benjamin Franklin invented what he called the electric wheel. This is usually cited as the first electrostatic motor and sometimes as the first electric motor, depending on your definition. The rotor consisted of a wooden hub with 30 glass bars attached to the circumference. These bars were strips cut from window glass. Brass thimbles were attached to the end of each bar. To turn easily, the rotor had a central shaft with an iron point on the bottom and a strong wire at the top, the wire going through a hole to keep it all upright.

The power source consisted of two oppositely charged Leyden jars, the original name given to a cylindrical capacitor made using a jar of some sort. The jar, and therefore the dielectric, was glass in Franklin’s day. A wire extended up from the inner capacitor plate and this wire was placed near where the thimbles would pass by. As a thimble passed, a spark would occur between the thimble and the wire, charging the thimble with the same polarity as the inner plate. Being like-charged, the thimble would be repelled from that wire, turning the wheel. As the thimble reached the wire of the opposite Leyden jar, being of opposite charge, it would be attracted to the jar’s wire. As it passed, a spark would occur between the two, charging the thimble to the same polarity as that wire and causing it to be repelled, turning the wheel some more.

The initial direction of the wheel was set by giving it a push by hand. In a letter (page 29), Franklin wrote that it rotated at 12 to 15 RPM, bearing the weight of one hundred Spanish dollars. He also wrote that:

if a large fowl were spotted on the upright shaft, it would be carried round before a fire with a motion fit for roasting.

Electric Whirl

This motor is a bit older than Franklin’s and dates back to 1745. It was made by Andrew Gordon, a Scottish Benedictine monk. It goes by many names but the original name was the electric whirl. The basic idea is that an electric field at a sharp point will be stronger than at a smooth point. This strong electric field ionizes the air near the point, and depending on the polarity, may even emit electrons to the air. This ionized air is called a corona. In either case, the ionized air around the point has the same polarity as the material making up the sharp point. Since like-charges repel each other, the ionized air repels the sharp point. You’ve of course heard of Newton’s third law which basically states that for every action, there’s an equal and opposite reaction. So the air is repelled in one direction, and the sharp point reacts by being repelled in the other direction. Arrange the point on an arm which is free to move around an axis, and the arm will rotate around that axis. The moving ionized air is often referred to as ion wind and for that reason this is also often called an ion wind rotor, or ion wind spinner or pin wheel.

Usually there are two points, each on their own arms, pointing in opposite directions to make for a more stable system. There can of course be more. The one pictured here is sitting on top of a Van de Graaff generator’s dome, which is at high voltage with respect to ground. The electric whirl is in electrical contact with the dome through the supporting shaft. The other end of the electric field is the surrounding air, the bottom of the Van de Graaff generator and the room.

Poggendorff/Corona Motor

The 1860s was a time when many were experimenting with influence machines, machines that used electrostatic induction, called influence at the time, to produce high voltages. One such experimenter was Wilhelm Holtz. In 1867, in a stroke of brilliance,  he connected the output of a similar machine to one of his and found it would run as a motor. And so in 1869, J.C. Poggendorff had a simplified version of Holtz’s machine built that had only what it needed to run as a motor.

Poggendorff’s motor consisted of a glass disk with combs facing it on either side. The combs had sharp points on them arranged so that the points would be near the disk but not touching. The combs could be turned so that they were strictly radial with respect to the disk. In this case the disk was not self-starting and, once given a spin by hand, would continue to rotate in that direction. However, the combs could also be turned to an off-radial angle. In this case the disk would self-start and be unidirectional.

One difference between Franklin’s wheel and Poggendorff’s motor is that Franklin’s places charge onto the thimbles using a spark, whereas Poggendorff’s sprays charge onto the disk through ionized air, or a corona. This corona is the same we mentioned when talking about the electric whirl, and is formed in the same way — the sharp points result in a strong electric field which ionizes the air.

Similar to Franklin’s the area of the disk, approaching a comb is charged with the opposite polarity with respect to the comb, and so is attracted to the comb. And the area moving away from the comb is charged with the same polarity, and so is repelled.

Poggendorff’s motor also has some advantages over Franklin’s electric wheel. The combs are always spraying onto the disk, continuously producing rotation, whereas with Franklin’s, this happens only when a thimble passes a Leyden jar. Also, there is always the same amount of disk surface area facing the combs, resulting in a constant torque being produced.

Due to the corona being the mechanism for transferring the charge, these types of motors are generally called corona motors. And in many of them the disk is replaced with a cylinder. The combs then become either just the sharp ends of thin wires or long sharp blades as in the one shown here that was run off of atmospheric electricity.

Capacitor Motors

This next type of motor was invented in 1889 by Karl Zipernowsky, but an easier version to illustrate is a 1904 one by N.G. van Huffel^[1]. In it the rotor and stator are both made up of two partial cylinders, or curved plates. The rotor plates have a slightly smaller diameter to fit inside the stator plates.

The stator plates are charged oppositely and the rotor plates receive a charge from their adjacent stator plates via a sliding contact just as they are rotating away from those plates. That way they repel away from the adjacent stator plate and reach a position where they’re then attracted to the next stator plate.

The big difference between all these motors is in the way the charge is moved around, via sparks as in Franklin’s, via corona, or via sliding contacts. However, since sliding contacts are used here, these motors can run at a lower voltage than the others. These can also run on AC.

Induction Motors

The induction motor is the one that operates on perhaps the most interesting principle. The rotor is not electrically connected to any power supply and is made of a dielectric material. The surrounding stator plates are supplied with AC, and so there is a rotating electric field surrounding the rotor. That electric field induces polarization of the molecules on the surface of the rotor. From the stator’s point of view, this appears as a net charge.

The key is that as the stator plates’ fields change polarity, it takes time for the polarization of the nearby rotor to change in response. That creates a lag between the moving field and the polarized molecules. That lag is what causes the rotor to follow the field.

The illustration shown here is of one made by Rocardo Arno in 1892-1893^[1].

Where Have All The Motors Gone?

As we said, electromagnetic motors are far more powerful. But electrostatic motors do find use in MEMS motors of microscopic dimensions. And of course, they haven’t really gone anywhere if you consider electrostatic tinkerers who love resurrecting these old mechanical marvels. I’ve dabbled in electric whirls and corona motors and have thirty thimbles on order from China for something which the attentive reader can guess at. What electrostatic motors have you toyed with in your home laboratory?

Resources:

[1] Electrostatic Motors: Their History, Types and Principles of Operation by Oleg D. Jefimenko

Electric current comes in many forms: current in a wire, flow of ions between the plates of a battery and between plates during electrolysis, as arcs, sparks, and so on. However, here on Hackaday we mostly deal with the current in a wire. But which way does that current flow in that wire? There are two possibilities depending on whether you’re thinking in terms of electron current or conventional current.

In a circuit connected to a battery, the electrons are the charge carrier and flow from the battery’s negative terminal, around the circuit and back to the positive terminal.

Conventional current takes just the opposite direction, from the positive terminal, around the circuit and back to the negative terminal. In that case there’s no charge carrier moving in that direction. Conventional current is a story we tell ourselves.

But since there is such a variety of forms that current comes in, the charge carrier sometimes does move from the positive to the negative, and sometimes movement is in both directions. When a lead acid battery is in use, positive hydrogen ions move in one direction while negative sulfate ions move in the other. So if the direction doesn’t matter then having a convention that ignores the charge carrier makes life easier.

Saying that we need a convention that’s independent of the charge carrier is all very nice, but that seems to be a side effect rather than the reason we have the convention. The convention was established long before there was a known variety of forms that current comes in — back even before the electron, or even the atom, was discovered. Why do we have the convention? As you’ll read below, it started with Benjamin Franklin.

Franklin’s Experiment

To give you some idea of just how early we’re talking about in the field of electricity’s development, the Leyden jar, the first capacitor, had just been invented in 1745. Word of it, and other discoveries were spreading rapidly through letters and lectures. One such lecturer was Dr. Archibald Spencer. Franklin attended his lectures and even bought Dr. Spencer’s equipment in 1746.

Franklin was a prolific and rigorous experimenter and began writing his own letters about his work and his theories. It’s through those letters that we have the details of the experiment from which we get our direction for conventional current.

In a few letters he described an experiment with persons A, B anc C. Persons A and B stand on wax to insulate them from the ground, whereas C stands directly on the ground. Person A rubs a glass tube against his hand and, as Franklin describes it, “collects the electrical fire from himself into the glass”. B then passes his knuckle near the glass tube and “receives the fire which was collected by the glass from A”. But to C, both A and B appear electrified “for having only the middle quantity of electrical fire, receives a spark upon approaching B,” or “gives one to A, who has an under quantity”. If instead, A touches B then the spark is stronger because the difference between them is greater. If after A and B touch, C touches either of them there is no spark because “the electrical fire in all is reduced to the original equality”.

Franklin’s Explanation

Franklin’s letter then continues by defining some new terminology and establishing the convention that we use today.

“Hence have arisen some new terms among us: we say, B, (and bodies like circumstanced) is electrified positively; A, negatively. Or rather, B is electrified plus; A, minus. … To electrify plus or minus, no more needs to be known than this, that the parts of the tube or sphere that are rubbed, do, in the instant of the friction, attract the electrical fire, and therefore take it from the thing rubbing: the same parts immediately, as the friction upon them ceases, are disposed to give the fire they have received, to any body that has less.”

Thus, Franklin came up with the idea that charge is something that moves from the positive to the negative, or from that which has more to that which has less. That’s the conventional current that was adopted and that we use today.

Note that by rubbing objects together as described in the letters, they’re making use of the triboelectric effect to charge the objects. Just which objects get charged positively, giving up electrons, and which get charged negatively, taking the electrons, is listed in a table called the triboelectric series. From the letters, Franklin correctly deduced which charge different objects will get, glass being charged positively and sulfur negatively, for example.

The problem is that when you get a spark from going near the positively charged glass, Franklin guessed that the electric fluid moved from the positive glass to you, whereas we now know it’s you that give electrons to the glass.

Ebenezer Kinnersley, who was a part of Franklin’s close circle of electrical experimenters is also often credited with this idea so it’s hard to know if only one person came up with it or if it was a result of a collaboration. Franklin seems to hint at the latter when in the letters he writes “And we daily in our experiments electrify bodies plus or minus, as we think proper.”

Faraday’s Current-Direction Dilemma

In the 1800s, Michael Faraday ran into similar problems of having to name charge carriers without having a full understanding. He’d done some experiments with electrolysis and, while working on a paper about them, needed names for what we now call the cathode and the anode.

The two plates of his electro-chemical cell were connected to an electrical circuit and so there was a positive plate and a negative plate. As we saw above, the convention was that in the circuit around the cell, the current left the positive plate and entered the negative plate. After deciding what to call the plates, electrodes, he then needed to distinguish between the two from the point of view of how the ions inside were interacting with them. He also wanted names that were fairly independent of theory.

He looked to an analogy with the Earth’s magnetic field and the direction the current would have to run around the Earth in order to create the fields — that would be the same direction as the sun, east to west, or going up in the east and going down in the west. His friend, William Whewell, suggested kata, Greek for downwards, and odos, Greek for a way, i.e. the way which the sun sets. The result is “cathode”. Similarly using ano, Greek for upwards, resulted in “anode”.

Interestingly, in the same paper, after offering these names, he shows his concern in naming things while it was still early days in their understanding. He writes “and whatever changes may take place in our views of the nature of electricity and electrical action … there seems no reason to expect that they will lead to confusion, or tend in any way to support false views.” Sure enough, due to the discovery of the electron and the fact that the moving charge carrier’s direction is actually the opposite, it’s been suggested that kata odos, the way down, can now be interpreted as the way down into the cell, i.e. where the electrons enter the cell.

Thomson’s Discovery Of The Electron

The discovery of the real charge carrier in a wire, the electron, started out with research into cathode rays. Cathode rays were first observed as a glow emitted from the cathode in a rarefied gas. In the 1870s, Sir William Crookes produced the first cathode rays in a high vacuum and showed that they moved from cathode to anode. He also used a magnetic field to deflect them and realized that they were negatively charged.

But it was J.J.Thomson in 1897 who realized that the rays were actually unique particles and made good estimates for the particle’s charge and mass. He called them ‘corpuscles’ but their name was later changed to ‘electron’. Thomson also found that they are what are being given off by incandescent light and by the photoelectric effect and it wasn’t long after that they were found be the charge carrier for electricity in wires.

Does It Matter?

It turns out that whether you use conventional current or electron current doesn’t matter, as long as you’re consistent in your use. Kirchhoff current law, for example, says that the sum of the current going into a junction (node) in a circuit is the same as the sum of the current going out of the junction. It doesn’t care which directions are in and out, as long as you keep track of the signs.

However, conventional current is represented in the shapes of various components in schematics. The ‘arrowhead’ shape of a diode points in the direction of conventional current, as do the ‘arrowhead’s in transistors. But it’s easy to remember that electrons flow against the arrows. The right-hand rule also uses conventional current when figuring out the direction of the Lorentz force or the direction of the magnetic field around a current carrying wire. So it seems you do at least have to be familiar with conventional current.

The Winner’s Circle

Which did you first learn? Which do you prefer? Do you use conventional current for some things and electron current for others? In my experience producing corona discharges across air gaps, it matters whether or not the sharp electrode is providing the electrons since the resulting coronas are produced differently. Share your experience and opinions in the comments below.

[Colin] tells us it all started with [Benjamin Franklin]’s battery of capacitors. It was a bunch of leyden jars hooked together in series and there wasn’t even chemistry involved yet, but the nomenclature stuck and it wasn’t long before it evolved into the word we use today.

For the word to change, things got chemical. [Alessandro Volta] introduces his voltaic pile. Once scientists latched onto the idea of a stable reaction giving a steady stream of magic pixies for them to play with, it wasn’t long before the great minds were turning their attention to improving this new technology.

In the classic game of one-upmanship loved by technical people all over, we quickly skip forward to the modern era. An era where no man is unburdened with the full weight of constant communication. It’s all thanks to a technology that’s theoretically unchanged from that first pile. Video after the break.

The history of capacitors starts in the pioneering days of electricity. I liken it to the pioneering days of aviation when you made your own planes out of wood and canvas and struggled to leap into the air, not understanding enough about aerodynamics to know how to stay there. Electricity had a similar period. At the time of the discovery of the capacitor our understanding was so primitive that electricity was thought to be a fluid and that it came in two forms, vitreous electricity and resinous electricity. As you’ll see below, it was during the capacitor’s early years that all this changed.

The history starts in 1745. At the time, one way of generating electricity was to use a friction machine. This consisted of a glass globe rotated at a few hundred RPM while you stroked it with the palms of your hands. This generated electricity on the glass which could then be discharged. Today we call the effect taking place the triboelectric effect, which you can see demonstrated here powering an LCD screen.

In 1745 Ewald Georg von Kleist in Pomerania, Germany tried to store electricity in alcohol thinking that he could lead the electricity along a wire from the friction machine to alcohol in a glass medicine bottle. Since electricity was considered a fluid it was a reasonable approach. He reasoned that the glass would act as an obstacle to the escape of the electrical “fluid” from the alcohol. He did this similarly to how it’s shown in the illustration, by putting a nail through a cork and into the alcohol and while holding the glass bottle in one hand. He wasn’t aware at the time of the important part played by his hand. Von Kleist found that he would get a spark if he touched the wire, a more powerful spark than he’d normally get from the friction machine alone.

He communicated his discovery to a group of German scientists in late 1745 and the news made its way to Leyden University in the Netherlands, but in a confused form. In 1746 Pieter van Musschenbroek and his student Andreas Cunaeus at Leyden University succeeded in doing the same experiment but with water. Musschenbroek then informed the wider French scientific community of the experiment. It’s considered that von Kleist and Musschenbroek independently discovered it. But as you can see below, this was only the beginning.

Abbé Nollet, a French experimenter, gave the jar its name, Leyden jar, and sold it as a special type of flask to scientifically curious, wealthy men.

It was realized also at Leyden University that it worked only if the glass container was held in your hand and not if it was supported by an insulating material.

Today we realize that the alcohol or water in contact with the glass was acting as one plate of the capacitor and the hand was acting as the other while the glass was the dielectric. The high voltage source was the friction machine and the hand and body provided a ground.

Daniel Gralath, a physicist and the mayor of Danzig, Poland was the first to connect multiple jars in parallel to increase the quantity of stored charge. In the 1740s and 1750s Benjamin Franklin, in what was to later become the United States of America, also experimented with Leyden jars and called this collection of multiple Leyden jars a battery, due to its similarity with a battery of cannon.

Franklin did a lot of experiments with both water filled Leyden jars and foil lined Leyden jars and concluded that the charge was stored on the glass and not in the volume of water. He did this by working with dissectible Leyden jars (see the photos above), ones where the outer and inner foils could be removed from the glass. This was later proven to be incorrect. Franklin worked with soda glass which is hygroscopic. As the foils were removed from the glass, charge was transferred via corona to moisture on the glass. When a jar of paraffin wax or baked glass is used instead, the charge remains on the metal plates. There is another weaker effect called dielectric absorption which involves the dipoles within the glass, or dielectric, and allows capacitors to retain some of their charge after the plates are shorted.

Franklin subsequently worked with flat glass plates with foil on either side, described connected in series in one letter.

It was around this same time that Franklin, in experiments not involving capacitors, showed that electricity had just one charge carrier, though he considered it a ‘subtle fluid’, the discovery of the electron having to wait until the late 1800s. He found that a charged object either had an excess of this fluid or a deficiency. This disproved the idea of the two types of electricity, vitreous electricity and resinous electricity.

In 1776 Alessandro Volta, working with different methods to measure electrical potential (voltage, V) and charge (Q) discovered that for a given object, V and Q are proportional, i.e. the law of capacitance, though it was not called that at the time. It was for this work that the unit volt was named after him.

The term ‘capacitor’ didn’t start being used until sometime in the 1920s. For a long time they were referred to as condensers and still are for some applications and in some countries. The term ‘condenser’ was first coined by Volta in 1782, deriving it from the Italian condensatore, due to its ability to store a higher density of charge than an isolated conductor.

In the 1830s Michael Faraday did experiments which determined that the material in between the capacitor’s plates had an effect on the quantity of charge on the capacitor’s plates. He did these experiments with spherical capacitors, basically two concentric metal spheres in between which he could have air, glass, wax, shellac or other materials. Using a Coulomb’s torsion-balance he effectively measured the charge on the capacitor when the gap between the spheres was filled with air. Keeping the potential difference constant he then measured the charge when the gap was filled with other materials. He found that the charge was greater with the other materials than it was with air. He called it the specific inductive capacity and it was for this work that the unit for capacitance is called the farad.

The term ‘dielectric’ was first used in a letter from William Whewell to Faraday where he speculated that Faraday had coined the term dimagnetic in analogy to dielectric and that perhaps Faraday should have used diamagnetic but that it wouldn’t work as well for diaelectric, given that the two vowels are together.

Leyden jars and capacitors made of flat glass plates with foil remained in use for spark gap transmitters and medical electrotherapy equipment until the late 1800s. With the invention of wireless (radio) capacitors began to take their modern form, partly due to the need for lower inductance to work with higher frequencies. Smaller capacitors were made using flexible dielectric sheets, such as oiled paper, often rolled with foil on either side. But the history of modern capacitors is a large topic for another post.

One fun thing about the early history of capacitors is that they have a very DIY feel to them, many having been homemade. In fact, Leyden jars are still used today by high-voltage hackers, as in this 3D printed Wimshurst machine and for pure fun as in this Leyden jar of doom. Do you make Leyden jars or any other types of capacitors for any things you build? Also, are there instances where you use, or see used, the term condenser instead of capacitor? We’d love to know about it. Let us know in the comments below.