1746: Ben Franklin and the Leyden Jar – SPARK!

The first in the SPARK! series, discussing the history of electricity and electrical technology in modern society.

In modern times, we take energy storage for granted. Every portable device we cling to requires us to store charge. Whilst we’ve understood fundamentals of electricity since history began, the concept of storing it eluded us- until the eighteenth century and the ‘Age of Enlightenment’.

Benjamin Franklin was a very interesting guy. Founding father, philosopher, physicist and the US’s 6th President, he wore so many hats it’s hard to keep track. He’s pretty infamous, though, for his kite experiment- one of the first to prove that electricity existed past your standard static electricity.

Sparks are something that probably fascinated you when you were a child; we’ve all received an electric shock from a door handle. Humans have known about static electricity existed since the Ancient Greeks observed amber attracting small particles much like when you get a shock- and ‘amber’ is even where we get the term ‘electricity’ from.

Those particles are attracted to the amber by static electricity, which occurs when the charges of two surfaces don’t align. Those electrons don’t want to be on the surface that’s crowded with their friends, so they jump to your lower charged skin- giving you a little shock in the process.

Before Franklin could dream up the use of electricity, it had to be generated. In the mid-17th century, before Franklin was born, Otto van Guereke created one of the first electrostatic generators: a globe of sulphur which, when rubbed, could attract small particles like the Ancient Greek amber. He even reported seeing “flickers of light” coming from the globe in the dark. Interestingly, he didn’t consider this as any form of electricity; to him, this was a demonstration of the Earth’s gravity.

The Kite Experiment is the stuff of legend in modern times. Convinced that lightning was made of the same stuff that comes out of these static generators, Franklin flew a silk kite in a thunderstorm. Water wet the kite and it’s string, so when he flew it within the thunderclouds charge could flow through the string and into a key tied about halfway up. If your hand then approached the key, ‘electric fire’ would spark from the key to your knuckle!

Sparks would just fly from the key until Franklin introduced an innovative kind of energy storage. They were called Leyden jars, discovered in 1746- a mere five years before Franklin’s experiment was reported- that were simply a jar with a nail through the lid, filled with water. This ‘electric fire’ could be stored inside of those jars, from which ‘spirits may be kindled, and all the other electrical experiments [can] be performed’- proving that lightning was the same electricity being tinkered with by his peers.

The Leyden jar, being used to store charge coming from a static generator. The shock Cuneas, Muschenbroek’s laboratory assistant, received when pulling the metal from the jar put him out of action for a full two days. Source.

We know the jars now as capacitors, able to store less charge (at a lower current, with fewer electrons) than batteries but at a far higher energy (or voltage). Capacitors are essential for our personal electronics to function and form a core part of energy storage worldwide. But how does the jar work? Again, this was one of Franklin’s big experiments.

Leyden jars are the perfect prototypical example of dielectric capacitors. It turns out that the water itself doesn’t act as charge storage; instead, it’s the glass of the flask that holds the electricity, as discovered by Franklin’s experimentation. Charge simply passes through the nail hammered through the jar’s lid and through the water, before coming to rest on the glass- which is insulating. To discharge the jar (and get a shock in the process), you ground yourself and bring your finger close to the nail through the lid.

It didn’t take long for this discovery to spread and improvements to be made. They first eliminated the use of water inside the glass, before adding a tinfoil coating both inside and outside of the jar. These act as plates, conducting the positive and negative charges associated with electricity far better than water ever could.

The only problem is, Franklin was wrong. In his quest to explain the Leyden jar he built a ‘dissectable’ version, with the metal coating and glass jar as removable layers. If you charge the jar and then take it apart you can get a spark from the glass and not the metal, implying the charge is stored on the glass. However, this isn’t entirely accurate. 

In actuality, the charge would only be stored on the conducting layers. The problem with the dissectable jar is the type of glass used to make the cup. The surface of soda glass is hygroscopic, so loves to absorb water- and this builds up a conducting layer on the surface, able to do the same job as the metal plates. When Addenbrooke added a paraffin wax coating to the glass in 1922, the charge stayed on the metal- exactly as we see in modern dielectric capacitors.

Franklin’s dissectable Leyden jar. His experiments on this device led him to (falsely) believe that charge was stored on the glass; in fact, the glass was absorbing water and letting him down. Source.

Any experiment in electricity during the 18th century was driven by the Leyden jar. Even Volta’s work into Voltaic Piles, the earliest form of modern chemical batteries, would’ve stalled without access to the portable charge you can get from a Leyden jar.

Electricity was far from practical in the 18th century, a hot topic used for jazzy demonstrations world-wide. Franklin in particular enjoyed the ‘electric kiss’, in which a woman sat on an insulating stool with her hand on a charged Leyden jar. If any audience member dared to kiss her, they’d be shocked by a spark closing the gap between the pair. 

Little did they know that this phenomena would lead to a technological revolution only 200 years later.

With thanks to Micheal B. Schiffer, and his book ‘Draw the Lightning Down: Benjamin Franklin and electrical technology in the age of Enlightenment’.

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