LESSON NINE. THE LEYDEN JAR AND CONDENSER.
The Leyden Jar, called after the city of Leyden, Holland, where it was invented, is a form of condenser consisting of a glass jar coated up to a certain height inside and out with tinfoil.
FIG. 24. Leyden Jar.
A Leyden jar may be charged by holding the rod to the prime conductor of an electric machine, the outer coating being held in the hand. If a piece of wire connected to the outer coating is then brought near the rod a brilliant snapping spark will pass across the space.
Any two conductors, separated by an insulating medium termed the dielectric, constitute a condenser and possesses the property of receiving and retaining an electric charge.
If a charged condenser or Leyden jar is discharged slowly by allowing the electricity to pass through a high resistance conductor the flow of current increases in strength at first and then gradually dies away.
If, however, the condenser is discharged through a coil of wire of one or more turns, the discharge consists of a number of excessively rapid oscillations or surgings. The first rush of current serves to more than empty the condenser and charges it the opposite way, then follows a reverse discharge, which also oversteps itself and charges the condenser the same way as the first and so on, each successive oscillation being weaker than the one before until the discharge dies away as in Fig. 36. The discharge of a condenser under such conditions consists of a number of successive sparks in reverse directions.
The ability of a condenser to receive and retain an electrical charge is termed the capacity and is measured by a unit called the farad. The farad is so large a quantity, however, that it is never met in practise and for convenience the micro-farad which is one millionth of a farad has been adopted.
A condenser of one farad capacity is such as would be raised to a potential of one volt by a charge of one coulomb of electricity.
The capacity of the condenser is dependent upon the thickness and nature of the insulating medium or dielectric. The quality of a dielectric which decides the capacity of a condenser in which it may be a part is called its specific inductive capacity. The following table shows the relative specific inductive capacity of several materials, air being the standard:
| Substance. | Constant. |
|---|---|
| Air | 1.00 |
| Paraffin | 1.68—2.47 |
| Petroleum | 2.02—2.19 |
| Gutta Percha | 3.00 |
| Hard Rubber | 2.28 |
| Mica | 6.64 |
| Glass | 6.72—7.38 |