The Leyden Jar – 1745

I have reached the key object in my “History of Electricity in 10 Objects”: the Leyden jar. Philosophically, the Layden jar has promoted more electrical thinking than any other object. It was at the heart of the biggest enquiry into the nature of electricity. It has also been the most fun to research and write.

So far, I have made working models of all of the devices in my list. I have done the same with a Leyden jar, but the results were disappointing. I made a particularly small jar and charged it with my temperamental Lego Guericke device. Despite much anxiety, I was not made insensible. I did not faint or bleed at the nose. I was not confined to my bed for a week. I’m not sure I felt anything at all. My bravery, however, is exhausted, so I will not try again. Here is the text…

A spinning glass sphere can generate thousands of volts, but will provide very little energy – just enough to cause little crackling sparks. Imagine the shock if you accidentally found a way to store large quantities of electricity in a jar, and the surprise when the electricity discharged through you.

I wish to inform you of a new but terrible experiment, which I advise you on no account personally to attempt…. Suddenly I received in my right hand a shock of such violence that my whole body was shaken as by a lightning bolt… the arm and body were affected in a manner more terrible than I can express. In a word, I believed I was done for.

Peter van Musschenbroek January, 1746

Naturally, everyone wanted a go.

The electrical collecting jar was discovered accidentally, twice in the same year. It is a dangerous thing and a miracle no one was killed.

The two discoverers were Ewald Georg von Kleist of Farther Pomerania and then Peter van Musschenbroek of Leyden. The jar became known as the Leyden jar. It was a phenomenon.

…the first time he tried the Leyden experiment, he found great convulsions by it in his body; and that it put his blood into great agitation; so that he was afraid of an ardent fever, and was obliged to use refrigerating medicines. He also felt a heaviness in his head, as if a stone lay upon it. Twice, he says, it gave him a bleeding at his nose, to which he was not inclined; and that his wife (whose curiosity, it seems, was stronger than her fears) received the shock only twice, and found herself so weak, that she could hardly walk.

The History and Present State of Electricity, Joseph Priestley 1767

The Leyden jar was charged with a Guericke sphere. The charge was carried to a pin entering the water of the jar. The outer conductor was your hand. As the sphere turned, the charge slowly built up in the jar. The shocking moment of discovery came when you touched the pin and all of the electricity is discharged.

The Leyden jar became the key to a philosophical question: what is electricity?

At the time, philosophers believed that the electricity was stored in the jar like water, but the ability of electricity to attract and repel was a mystery.

In France, an answer was proposed by Charles François de Cisternay du Fay. Du Fay and his colleagues were convinced that there were two types of electricity: vitreous (obtained by rubbing glass) and resinous (by rubbing resin). Resinous electricity attracts vitreous. Vitreous repels vitreous and resinous repels resinous.

Du Fay explained how the Leyden jar worked using his idea of the two electricities. To give the jar a vitreous charge, the vitreous electricity travels down the chain into the water. Resinous charge in the hand is attracted to the vitreous charge in the water. The resinous charge in the hand attracts more resinous charge down the chain. The jar now contains far more charge than has been possible before.

British philosophers, on the other hand, took sides with an extraordinary gentleman from the colonies, a printer, politician and philosopher: Benjamin Franklin. Franklin preferred simplicity. He proposed a single type of electricity. When an object has too much electricity, it is positive: too little and it is negative.

Franklin explained the properties of the Leyden jar with his single type of electricity. To give the jar a positive charge, positive electricity travels down the chain into the water. The positive electricity in the hand is repelled, leaving the hand negative. The negative hand attracts more of the positive electricity down the chain. The jar now contains far more electricity than has been possible before.

So wonderfully are these two states of Electricity, the plus and the minus, combined and balanced in this miraculous bottle!

Benjamin Franklin, 1747

Franklin enjoyed the Leyden jar enormously. He invented a game called “treason,” which involved an electrified portrait of the king, with a removable gilt crown. Anyone who tried to remove the crown while holding the gilt edge of the picture would be shocked.

His electrical fun nearly ended one Christmas when he used Leyden jars to kill the turkey for the entertainment of his friends:

“I have lately made an experiment in electricity that I desire never to repeat. Two nights ago, being about to kill a turkey by the shock from two large glass jars, containing as much electrical fire as forty common phials, I inadvertently took the whole through my own arms and body, by receiving the fire from the united top wires with one hand, while the other held a chain connected with the outsides of both jars.”

This lesson might have stopped another philosopher from persisting with such dangerous experiments, but not Ben Franklin. In 1752, he carried out the most famous of dangerous experiments: flying a kite in a thunderstorm. He connected the kite string to a Leyden jar to investigate whether the lightning could be stored as electricity. It worked. It is astonishing that Franklin survived long enough to die in his own bed, aged 84.
The Leyden jar did not have any more great secrets to reveal. It took another four decades for the philosophy of electricity to make a significant leap forwards. This time with frogs’ legs and animal electricity.