The discovery of capacitance.

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Capacitors with perforated plates

Historical notes in electronics series.

Are you interested in how Capacitors were discovered, or possibly invented? Read on....

The Leyden Jar - Condensers.

Section 1. Cuneus and Muschenbrock's Experiments: The discovery of the Leyden Jar.

• "Cuneus, a pupil of Muschenbroeck, a celebrated physicist of the last century, was one day trying to electrify some water in a wide-necked bottle. For this purpose he held the bottle in one hand after having placed in the bottle a metal rod connected to the machine. When he thought the water was sufficiently electrified, he tried to remove the iron rod with one hand, without loosing his hold of the bottle with the other hand. He received a shock that surprised him. Muschenbroeck repeated Cuneus's experiment, but the shock he received in his arms, shoulders, and chest was so great that he lost consciousness, and was so frightened that, in writing to Reaumur about this, then new, discovery, he wrote that for nothing in the world, not even for the crown of France, would he go through it again. But some other physicists were less fearful. Allaman, Lemonnier, Winckler, and the Abbe Nollet varied the experiment in all sorts of ways, and so a new piece of apparatus was added to electrical science: this apparatus, called the Leyden Jar, is named after the place where the experiment was first performed in 1746."

The modern view.

The dielectric constant of water is about 80. This is exceptionally large for a liquid; and results in all kinds of consequences for radio wave and microwave propagation through rain and on to wet surfaces.

In Cuneus's experiment, the iron rod is connected to a source of voltage that is not, in itself, large enough to give a shock to the experimenter. The experimenter disconnects the iron rod and jar from the supply, and then attempts to pull the rod from the jar. The stray capacitance from the rod to the outside of the jar, when removed, is less than 1/100 of the capacitance when it is in the water. Since the charge Q is constant, and Q = CV, it is clear that the voltage rises to 100 times that of the original voltage source. This is sufficient to drive the charge through the arms of the person doing the experiment; hence the shock.

We see here another reason why electricity is dangerous when in conjunction with water and wet objects. Distilled water is a reasonable insulator; it only becomes conducting when there are dissolved salts. But it can hold a large polarisation so capacitors with a water dielectric are efficient storers of energy.