The LEYDEN JAR
One of the simplest forms of early condenser is that known as Eligpinus's condenser (shown in figure 1). This consisted of two equal insulated metal plates, supported so that they can be brought into contact with the opposite faces of a plate of glass larger than themselves, or simply placed so as to face each other at a short distance in air. In either case the capacity can be diminished by drawing the plates apart, or increased by moving them nearer together.
Another equivalent arrangement, except that the distance between the conductors cannot be varied, is that known as Franklin's pane, consisting of two equal pieces of tinfoil pasted opposite each other on the two faces of a sheet of glass, large enough to project a good way beyond them on all sides.
When condensers of a very great capacity are required, but are not to be exposed to great differences in potential, they are often made by superimposing alternately sheets of tinfoil and thin plates of mica, or sheets of paraffined paper. The first, third, fifth......sheets of tinfoil are connected together and form one side of the condenser whilst the second, fourth, sixth..... sheets are connected to form the other side.
The Leyden Jar is an another early form of condenser. It consists in general of a glass bottle coated inside and out with tinfoil (figure 2). There is a brass knob projecting from the neck which makes contact with the inner tinfoil coating by means of a chain. The tinfoil may be replaced by conductors of any other material. The reason why tinfoil is commonly used being chiefly that its flexibility allows it to be readily adapted to the surface of the glass.
The Leyden Jar was discovered by accident in the Netherlands in 1746. One day Cuneus, a pupil of Muschenbroek, professor in the University of Leyden, was trying to charge some water in a glass bottle by connecting it with a chain to the sparking knob of an electrical machine. Holding the bottle in one hand, he undid the chain with the other, and received a violent shock which cast the bottle on the floor. Muschenbroeck, eager to verify the phenomenon, repeated the experiment, with a still more lively and convincing result. His nerves were shaken for two days, and he afterwards protested that he would not suffer another shock for the whole kingdom of France.
To obtain sufficient capacity in a Leyden Jar, it is necessary to increase the surface area of the tinfoil and thus use a larger jar. A more practical method is to connect a number of jars in parallel as shown in figure 3. All the inner coatings are connected together by insulated metal rods, and the outer coatings by placing the jars on a tray, or in a box lined with tinfoil. This combination, sometimes called a Leyden Battery, has a capacity equal to the sum of capacities of each separate jar.
At times it is not desirable to apply the full voltage to one condenser as damage to the glass jar may result. In this case it is advantageous to connect two or more jars in series, or in cascade, in a way shown in figure 4.
To discharge a condenser is to bring its two surfaces to the same potential. This may be done suddenly by connecting them by means of a discharger (figure 5), consisting of two metal rods hinged together like a pair of compasses and provided with glass handles, and
terminated in brass knobs. If one knob is put into contact with the outer coating of a charged Leyden Jar and the other is gradually brought near the connection to the inner coating of the jar, a bright loud spark passes between the knobs before they actually contact each other. The jar is afterwards found to be discharged.
"Electricity and Magnetism" by Foster and Atkinson, 1896,
"The Story of Electricity", by John Munro, 1896.