Capacitance in electric machines
During the eighteenth century a large variety of electric machines were constructed, mostly consisting of glass globes or plates which were rotated against rubbing pads or even the human hand. These machines were capable of producing a succession of
small sparks, but later it was discovered that much fatter sparks could be obtained if
the charge was collected on a long iron rod placed near the machine. A rod such as this was called a prime conductor. Its function was simply to provide extra capacitance so that a much larger charge could be collected at the same potential before sparking occurred (Fig. 33.7).
In 1746, the Dutch physicist, Pieter van Musschenbrock, was experimenting to see if electricity could be kept in a bottle of water, when he stumbled on a method for storing a large quantity of electricity in a very small space. Briefly, he had a wire hanging from the prime conductor of an electric machine and dipping into a bottle of water held in one hand. After working the machine for a short time he touched the wire with the other hand and immediately received a shock which “shook him like a thunderbolt”.
This bottle later came to be known as the Leydenjar. In its modern form the Leyden
jar is simply a bottle with coatings of metal foil inside and out (Fig. 33.8).Bottles such as this were soon to replace prime conductors on electric machines. Two Leyden jars may be seen in the picture of the Wimshurst machine in Fig. 32.15. The action of the jar will be understood after reading the next section.