Fixed condensers consist of a few sheets of tinfoil interposed between sheets of paraffined paper or in some cases mica. The condenser is inclosed in a suitable case, usually a hollow molded block of insulating composition, and is provided with suitable terminals to facilitate connection.
FIG. 81.—Rotary variable condenser.
When a conductor is charged with electricity it has the power of exerting an opposite charge in any adjacent conductors. The two halves of a condenser constitute adjacent conductors, the separating medium in between being called the dielectric. An alternating current will pass through a condenser because the charge on the plates keeps changing from negative to positive and back from positive to negative again. A direct current will not pass through a condenser.
These facts are utilized to considerable advantage in the receptor of a wireless station. As has already been explained, the high frequency oscillatory currents will not readily pass through the coils of the telephone receivers, but a path is provided through the condenser. The detector rectifies the alternating current into a direct current which the condenser opposes and forces to pass through the telephone receiver and produce sounds.
When a battery is used in connection with a detector a condenser is also necessary to oppose the direct current of the battery and prevent it from flowing around through the tuning coil instead of through the detector. The capacity of the condenser may be smaller if the resistance of the telephone receiver is very great for the reason that as the wire grows smaller it offers greater impedance to the current. The opposite also holds true and condensers of large capacity are better fitted for use with telephone receivers of low resistance.
FIG. 82.—Interior of rotary variable condenser showing construction.
Variable condensers are divided into two general types, the "rotary" and the "sliding" plate, accordingly as the plates forming the condenser are adjusted with a rotary or a sliding motion. The rotary type consists of a number of movable semi-circular aluminum plates which swing between a series of fixed semi-circular plates of a slightly larger diameter. The plates must not touch one another and move back and forth with perfect freedom. The dielectric is formed by the air spacing between the plates.
