HERTZIAN WAVE WIRELESS TELEGRAPHY. 365 



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 con- 

 taining 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 con- 

 denser 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.* 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. 



