Leyden Jars.—Transmitting condensers in a wireless telegraph station usually take the convenient form of a jar, coated inside and out with tinfoil and known as a Leyden jar.
The jars should be of good Bohemian or Jena hard glass and coated with tinfoil only for about three-quarters of their height, as otherwise the discharge is liable to pass over the top. The tinfoil must be thick to avoid blistering, and is stuck to the glass with shellac varnish. The blistering of Leyden jars is a serious fault, for when this condition exists, the capacity is thereby altered to such an extent that the period of the closed circuit may be sufficiently altered to throw the system out of tune and decrease the radiation of energy.
Considerable expense may be saved if the glass jars are purchased and coated by the amateur. The best jars are those imported from Germany, which have wide mouths so that they may be easily coated inside with tinfoil.
Fig. 54. Leyden Jar.
The jars must be thoroughly cleaned and dried before they are coated. Give the inside a thorough brushing over with shellac varnish, and before it is dry, carefully insert the tinfoil and press it smoothly against the glass. The outside of the jar is treated and coated in the same manner. The inside and outside of the bottom are also coated by cutting the tinfoil in circular pieces and shellacking them on.
Fig. 55. "Aerial Switch."
The whole upper part of the jar is given one or two coats of shellac in order to prevent the collection of moisture and brush discharging. A wooden plug fitted in the top of the jar supports a brass rod, terminating at the lower end in a chain or spiral spring which connects with the inner coating. When trouble is experienced because of an imperfect contact between the coating and the chain or rod, a layer of brass filings an inch or two deep placed in the bottom of the jar will remedy the difficulty. The upper end of the rod usually terminates in a small brass ball or a binding post.