What Is a Leyden Jar?
One of the commonest forms of condenser is the Leyden jar, which is so named because it was invented at Leyden, in Holland. This is a glass jar, upon the outside of which is fastened a coating of tinfoil, that covers the bottom of the jar and extends two-thirds of the way up the sides. Inside the jar there is a similar coating of tinfoil, and through the top of the jar, which is usually made of wood, extends a metal rod. On the upper end of the rod, there is a metal ball, and, at the lower end, is attached a chain which runs down to the bottom of the jar and rests upon the inner tinfoil coating.
In using the Leyden jar, the ball on the metal rod that runs through the top of the jar is connected with an electrical machine, and the jar is supported upon some conducting material, through which electricity may be conveyed from the outer coating of tinfoil to the earth. If the inner coating of tinfoil is now charged with positive electricity, by means of the electrical machine, it induces, upon the outer coating of foil, a charge of negative electricity, which is bound by the attraction of the positive charge on the inside of the jar. At the same time, the positive electricity, on the outer coating of foil, is repelled, through the conducting support, to the earth.
The charge that can be communicated to the coating of the foil, inside the Leyden jar, is greatly increased by the presence of a charge of the opposite kind of electricity, on the coating on the outside of the jar. Each of these charges attracts the other, through the glass of the jar, and serves to bind or hold it. If either coating of foil is removed, the charge on the other coating tends to fly off the tinfoil, and will immediately do so, if a conductor is brought near. It is because the negative effects of the initial charge, inside the jar, and of the induced charge outside the jar, make it possible to communicate, to each coating of foil, a larger charge than it could otherwise be made to receive, that a Leyden jar is called a condenser.
When a Leyden jar is disconnected from the electrical machine, two opposite charges of electricity are present on it, one inside and the other on the outside. If the two coats of tinfoil are now connected, by means of a condenser, they will at once neutralize each other, and the jar will be discharged. A jar may be discharged, by simply taking hold of the tinfoil on the outside of the jar, with one hand, and touching the metal rod, running through the top of the jar, with the other. If you do this, there will be a sudden flow of electricity through your body, your muscles will give a sudden jerk, and you will feel a peculiar tingling sensation. In other words, you will have received a shock.
It is not necessary, for the hand that does not grasp the jar, actually to touch the rod that runs through the top. If the hand is brought toward the rod, rather slowly, you will see a spark leap across the space between the rod and your hand, while your hand is still some distance from the rod. The greater the distance, across which the spark leaps, the brighter will be the spark, and the stronger the shock produced. This distance is sometimes spoken of as the length of the spark, and it indicates the size of the charges on the tinfoil coatings of the jar.