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.