Mounting a condenser in this manner gives it a much better appearance and it occupies less space than otherwise. Fixed condensers are used in two cases only, to shunt the condenser and to prevent the tuning coil from short-circuiting the telephone receivers. In any other position they are worthless.
Variable Condensers.—A simple form of adjustable condenser which may be quickly made for a special experiment is constructed in the following manner. A wooden curtain pole 2 inches in diameter and 18 inches long is covered with a layer of tin foil. The tin foil must be laid on smoothly and cemented with shellac. A layer of paraffined paper is then placed over the tin foil. A piece of sheet copper or tin 6 3/4 inches wide and 18 inches long is rolled up in a cylinder to fit over the rod with its tin foil and paper. The rod is fastened at one end to a base board about 20 inches long and 4 inches wide, by means of a bracket, or it may be mounted on a smaller base in a vertical position.
Connections are made to the tin foil and to the outside metal tube. By sliding the tube back and forth on the rod the condenser may be given a variable capacity.
Fig. 139. Variable Condenser.
The best variable condensers are constructed so that the dielectric between the two plates is formed by air. There are consequently no losses of energy in the condenser, for this mode of construction eliminates all dielectric hysteresis.
Fig. 139 shows such a variable condenser and Fig. 140 the details. It is possible to do very close, selective tuning with this instrument. The efficiency of the receiving circuit may be considerably improved if one is used wherever a condenser is required.
A rectangular box is built up in the manner shown in Fig. 135. The sides, D, are 6 inches long, 4 11/16 inches wide and 1/2 inch thick. The top, A, is 6 inches long, 1 13/16 inches wide and 1/2 inch thick. The bottom, B, has the same width and thickness but is 11 inches long. Eleven grooves are cut in A and B as shown in the cross section. The grooves are 1/16 inch wide, 3/16 inch deep and 1/16 inch apart. They are formed by setting the blade of a circular saw so that it projects 3/16 inch above the table or bed. After cutting one groove, the guide is moved 1/8 inch and another cut made.
Fig. 140. Details of Variable Condenser.