We tend to think of electricity as part of the modern world. However, Thales of Mietus recorded information about static electricity around 585 BC. This Greek philosopher found that rubbing amber with fur would cause the amber to attract lightweight objects like feathers. Interestingly enough, a few hundred years later, the aeolipile — a crude steam engine sometimes called Hero’s engine — appeared. If the ancients had put the two ideas together, they could have invented the topic of this post: electrostatic generators. As far as we know, they didn’t.
It would be 1663 before Otto von Guericke experimented with a sulfur globe rubbed by hand. This led to Isaac Newton suggesting glass globes and a host of other improvements from other contributors ranging from a woolen pad to a collector electrode. By 1746, William Watson had a machine consisting of multiple glass globes, a sword, and a gun barrel.
Part of the problem with experimenting with electricity in ancient times is that people didn’t really understand what it was or what you would use it for. There were several devices that appeared around 1600 that made it easier to work on electrostatic projects. Chief among these was the electroscope. William Gilbert’s versorium was the first of these. It was similar to an unmagnetized compass needle and would spin to point at electric charges. You can easily build your own version, by the way, and it looks like it should be highly 3D printable. Gibson’s looked more like a compass needle and less like a propeller, but apparently, either will work. We’ve seen many kinds of stand-in pointers, including straws in the video below (helps if you speak Portuguese, but you’ll probably still get the idea).
Other than using it as a classroom demonstration, the versorium isn’t used much anymore. However, you still see some other types of electroscopes — the name for the class of instruments that detect electric charge. John Canton introduced the pith-ball electroscope in 1754. This is little more than a plastic ball on a thread hanging from a stand. The original used pith as you might have guessed. This is a material that comes from the inside of plants and, yes, it is used in making a real pith helmet. It is also an insulator, which is the key property.
Whatever the material, the idea is simple. A charged object will pull the pith ball towards it and a neutral object will not. The mechanism for this is interesting. The electrons in the pith can’t leave their atoms easily — if they could the material would be conductive. But they can move around some inside their associated atom. A charged object will attract or repel the electrons which are, of course, negatively charged. This will place the attracted particle (either electrons or the nucleus which is positive due to protons) closer to the charged object than the opposite particles. That means the force of attraction will be ever so slightly greater than the force of repulsion. The difference in force is minuscule, but enough to observe.
There is a slight improvement that you can make and that is to use two pith balls. If you touch one ball with the charged object while it touches the other ball you will transfer charge to both balls. Because the balls are charged alike, they will then repel each other. The amount of separation can give an indication of how strong the charge is. Another variation of this design used pieces of straw instead of pith, but the idea was the same.
However, pith balls and straw have a lot of mass, relatively speaking. This led Abraham Bennet in 1787 to develop the gold-leaf electroscope. This was two tiny pieces of gold foil suspended in a glass container to block air currents. At the top of the container was a terminal to accept the charge. That charge passes to the gold leaves and they separate, just like a pair of pith balls but much more sensitive. The well-known YouTube channel Rimstar.org (Hackaday’s own Steven Dufresne!) has a good video about making a foil electroscope that you can see below.
The other useful invention made separately by both Ewald Georg von Kleist and Pieter van Musschenbroek was the Leyden jar which appeared around 1745. This is just a simple capacitor. The jar part is just what it sounds like; a glass jar. The jar has foil on the inside and the outside forming the plates of the capacitor. The glass, of course, is the dielectric. The inner foil has some mechanism for connecting through the lid of the jar. It wasn’t long before people realized they could use a flat glass plate or even omit the glass, even though the glass made the capacitor work better since it is a better dielectric than air.
Actually, the early jars used water as the inner plate and a person’s hand as the outer plate. Obviously, that left something to be desired. A typical one pint Leyden jar has a capacitance of about one nanofarad. Benjamin Franklin, of $100 bill fame, conducted several early experiments with Leyden jars. James Lincoln has a good video, below, that shows the same kind of jar Franklin experimented with that you can take apart.
Another place you might hear about Leyden — other than the beautiful city in the Netherlands — is in Jules Verne’s 20,000 Leagues Under the Sea. A Leyden ball appears in that book as a fictional bullet that holds an electrical charge. Sort of a 19th-century taser or a piezer.
Equipped with a way to measure charge and to store it, more work on generation was inevitable. By 1783 the Dutch had a machine that used glass disks nearly two meters in diameter. Even Georg Ohm — whose name should be familiar — had a similar large machine for experiments.
All the early machines were essentially better versions of rubbing fur on an amber rod. These are called friction machines. In truth, the charge doesn’t occur because of friction, per se. If you place two non-conductive objects together, you will generate charge where they touch. The problem is, everyday objects that may seem flat aren’t that flat. So just putting them in contact leaves lots of little gaps and reduces the amount of charge created. Rubbing, however, causes more direct contact across the material.
A better approach is the so-called influence machine. These machines use electrostatic induction, a principle discovered in 1753 by John Canton and again in 1762 by Johan Carl Wilcke. This induction uses a similar mechanism as the one at work in the electroscope. A nearby charged object causes the electrons to either attract to or repel from the charge. In an insulator, the electrons can only go a little bit inside their atoms, but in a conductor, some of the electrons can move freely through the object. The net charge remains neutral, but the distribution of charge across the object will change.
The difference with induction charging is that once the object we want to charge has redistributed its charge, it is grounded. Ground, for this purpose, has a practically infinite number of positive and negative charges. The nearby external charge will attract the opposite charge from ground which will cause that charge to remain in the object we want to charge. So if a positive charge is nearby, negative charge will flow into the object leaving it negatively charged. Another way to think of it is that grounding the object removes the charges that are repelled from the external charged object.
Around and Around
Volta built a type of capacitor that used induction to create an imbalance of charge called the electrophorus. The electrophorus had a dielectric plate and a metal plate with an insulating handle. You would rub the dielectric plate to charge it and then place the metal plate atop it. Grounding the plate temporarily will cause it to change to the opposite charge. The charge on the dielectric doesn’t change so you can use the metal plate to transfer the charge and then go back and get some more. At least until the charge naturally bleeds off. Of course, the fresh charge is actually coming from ground, so you aren’t violating any conservation laws.
These are not hard to make. You can see Thomas Kim’s video that uses some pie tins and other household items below. Note when he touches the plate he is using his body to ground it. In addition, you can see him multiply a charge by repeatedly transferring charge between two plates.
Georg Christoph Lichtenberg made a six-foot electrophorus with pulleys that could produce a fifteen-inch spark and this led to Lichtenberg figures. These machines were the precursor of all induction or influence machines that would later dominate electrostatic generation.
Influence machines use multi-stage electrophorous devices to amplify charge. The biggest innovation was the move from a linear motion to put the objects in proximity, to a rotary motion that was easier to mechanize.
By the mid-1800s, there were a dizzying array of rotating influence machines such as the Holtz machine and the Schwedoff machine. There were also a few mixed machines that used friction and influence such as the Kundt machine. You can find a very large list of machines that Antônio Carlos M. de Queirozif has built. He also links to other similar machines, but the number he has built personally is very impressive.
In 1878, James Wimshurst set out to improve the Holtz machine and his design became very popular after it was published in 1883. Apparently, Holtz and Masaeus had built similar machines but didn’t get the popular vote. The machine is a common fixture in movie laboratories and features counter-rotating glass disks with metal plates attached. As the disks spin, inducted charge is built up and collected, usually into a Leyden jar. You can find many plans and even buy a kit, should you want to have your own to adorn your lab. We’ve also seen them powering lasers, so there are some practical uses, even now. If you want to go cheap, grab some CDs and drink bottles and you are in business (see below).
Van de Graaff
There are literally dozens upon dozens of different types of electrostatic machines that were built during this time period. If you see one today, though, there’s a good bet it will either be a Wimshurst machine or a Van de Graaff generator.
This type of generator has a metal globe at the top and an insulating belt running from the base of the machine, through a tube, and into the inside of the metal globe. The belt becomes charged in the base and a metal comb transfers the charge from the belt to the globe.
An MIT physicist named Robert J. Van de Graaff developed this setup in 1929 for use as a particle accelerator. There was also a famous Scientific American Amateur Scientist column that described how to make such a particle accelerator. The Van de Graaff generator’s globe made contact with a similar looking apparatus that didn’t have a belt, but instead had a source of electrons (a filament) in a vacuum. The positive charge on the globe would accelerate the electrons towards the globe and anything in the tube between the base and the globe would be bombarded by fast electrons.
By 1931, these machines could produce a million volts and with the use of an insulating gas, 25 megavolts was possible. However, the cyclotron eventually replaced the Van de Graaff as the machine of choice for accelerating particles and would lead to things like modern-day supercolliders.
Rimstar.org has a good video that shows one of these generators and includes a lot of information about how they work, that you can see below.
A type of electrostatic generator that uses charged water droplets and the wind has seen some use in the Netherlands. Electrostatics are used in photocopiers, powder coating, paint spraying, dust precipitators, and more. However, these days, the electrostatic field is more likely to come from an electronic device like a corona wire instead of a traditional generator.
You do, however, still see these old generators in science museums raising hair and creating large sparks. These machines call back to an earlier time and understanding them can help you think better about the true physical nature of electricity.