The glass plates may be secured by removing the emulsion from old 8 x 10 inch photograph plates. Hot water will soften the gelatin on the plates so that it may be very easily scraped off. Twenty-four plates of this size are required. The tinfoil is cut 8 x 8 inches, so that an inch margin is left on all sides. The alternate sheets are connected together by heavy tinfoil or thin copper foil strips. The condenser should be placed in a convenient sized wooden box and poured full of paraffin.
The plate condenser shown in Fig. 57 is of .02 microfarad capacity. The condenser is mounted in a plain wooden box with several binding posts brought out, so that the capacity may be varied by connecting in various sections. The condenser is manufactured and designed by the Clapp-Eastham Company for use with the transformer illustrated in Fig. 51.
It is very necessary to have the transmitting condenser adjustable so that its capacity may be varied, for the proper value depends upon the wave length, spark frequency, power and persistency of the wave train.
When the condenser capacity is too small the spark will be somewhat flaming like an arc, and the potential to which the aerial is charged will be low. If too much capacity is used the spark will be very irregular and intermittent.
Fig. 58. Methods of Varying Capacity.
Fig. 58 shows condensers connected in series and in parallel and a combination of the two. Two condensers of equal capacity connected in parallel have twice the capacity of one, while in series they will have only one-half the capacity of either. This may be otherwise stated as the capacity in series is equal to the reciprocal of the sum of the reciprocals of their capacities separately. By this means of connecting either in series or in parallel almost any desired adjustment of capacity may be brought about.
Oftentimes a high voltage may be divided between two condensers by placing them in series and thus using them safely on a voltage which would rupture either one alone. For example, two condensers built for 20,000 volts and to be used on 30,000 volts could be made to perform this duty safely and only undergo a potential of 15,000 volts, which is a large margin of safety.
It is obvious that if the capacity of the circuit were to remain constant, four condensers connected up in series-parallel would be necessary.
In case of several Ley den jars or condensers connected up in a transmitting circuit, the leads or conducting wires connecting the various units should all be of as nearly the same length as it is possible to have them.