The wooden plug or cover is dried in an oven to expel all moisture and then boiled in paraffin.
Small Leyden jars may be very conveniently made from six-inch test tubes and mounted in a rack so that the capacity of the condenser will be adjustable by removing one or more of the tubes. An ordinary test tube rack such as is used in chemical laboratories serves very well for this purpose. The tubes should be connected in parallel, that is, all the outside coatings together and all the inside coatings together.
Figs. 55 and 56 illustrate condensers of this type which are on the market. The tubes are all separately removable so that the capacity may be adjusted.
Fig. 56. Amco Oscillation Condenser.
Glass Plate Condensers.—Glass plate condensers offer several advantages over Leyden jars and are coming into wide use. They are not so bulky or expensive and, above all, do not blister.
Plate condensers are often placed in a rack and made adjustable by means of movable contacts. Much the better plan is to place the plates in oil, as this eliminates all corona or brush discharges and much sharper tuning is rendered possible. The container is usually a tight wooden box filled with oil or paraffin after the plates are in place.
It is impossible to state the size of condenser suitable for induction coils of a given power or spark length, because many factors such as inductance, length of aerial, etc., which differ in various stations, influence the capacity. A condenser of convenient size suitable for coils or small transformers consuming from 250 to 300 watts is that described below. It is about the proper size for the small open core and 1/4-K.W. closed core transformers, described in the last chapter.
Fig. 57. Clapp-Eastham Oscillation Condenser.