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 |
LESSON TEN. THE ETHER AND THE ELECTROMAGNETIC THEORY OF LIGHT.
All space is filled with a weightless, invisible medium called Ether. It is the substance with which the universe is filled, it reaches to the stars and through the very earth itself.
It has been known for some time that light consists of vibrations or motions in the ether. In 1867, Clerk Maxwell offered the theory that these light waves are not merely mechanical motions of the ether, but are electrical undulations. According to this theory, the phenomena of electro-magnetism and the phenomena of light are all due to certain modes of motion in the ether.
Twenty years later, Heinrich Hertz discovered convincing proofs of Maxwell’s theory and succeeded in producing electro-magnetic waves in such a manner that they possessed the same properties, traveled at the same speed, and were capable of being reflected, refracted, polarized, etc.
