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.

Reference: Electricity and Magnetism, by A Guillemin, published in 1891 by Macmillan, London and New York. Chapter 5 page 231.

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.

10 March 1997