Glass condensers in the form of Leyden jars have been extensively employed, but they have the disadvantage that they are very bulky in proportion to their electrical capacity. The instrument maker's quart Leyden jar has a capacity of about one five hundredth of a microfarad, but it occupies about 150 cubic inches or more. Professor Braun has employed in his transmitting arrangements condensers consisting of small glass tubes like test tubes, lined on the inside and outside with tinfoil, which are more economical in space. The author has found that condensers for this purpose are best made of sheet glass about one eighth or one tenth of an inch in thickness, coated to within one inch of their edge on both sides with tinfoil, and arranged in a vessel containing resin or linseed oil, like the plates of a storage battery. M, d'Arsonval has employed micanite, but although this material has a considerably higher dielectric strength than glass, it is much more expensive to obtain a given capacity by means of micanite than by glass, although the bulk of the condenser for a given capacity is less.
To store up a certain amount of electric energy in a condenser, we require a certain definite volume of dielectric, no matter how we may arrange it, and the volume required per unit of energy is determined by the dielectric strength of the material. Thus, for instance, ordinary sheet glass can not be safely employed with a greater electric force than is represented by 20,000 volts for one tenth of an inch in thickness, or say a potential gradient of 160,000 volts per centimeter. This is equivalent to an electric force of about 500 electrostatic units. This may be called the safe-working force. The electrostatic capacity of a condenser formed of two metal surfaces a foot square separated by glass three millimeters in thickness is between 1/360 and 1/400 of a microfarad. If this condenser is charged to 30,000 volts, we have stored up in it half a joule of electric energy, and the volume of the dielectric is 270 cubic centimeters. Hence to store up in a glass condenser electric energy represented by one joule at a pressure of 20,000 volts, we require 500 cubic centimeters of glass, and it will be found that if we double the pressure and double the thickness of the glass, we still require the same volume.[1] Hence in the construction of high tension condensers to store up a given amount of energy, the economical problem is how to obtain the greatest energy-storing capacity for the least money. Glass fulfils this condition better than any other material. Although some materials may have very high dielectric strength, such as paper saturated with various oils, or resins, yet they can not be used for the purpose of making condensers to yield oscillatory discharges,
- ↑ This energy storage is at the rate of 44 foot-pounds per cubic foot of glass. This figure shows what a relatively small amount of energy is capable of being stored up in the form of electric strain in glass. In the case of an air condenser, it is only stored at the rate of one foot-pound per cubic foot.
