Lessons in Wireless Telegraphy

by Alfred Powell Morgan

FIG. 55.

If an alternating current is applied to the condenser when A receives a positive charge, B becomes negative. When A reverses and becomes negative B becomes positive. This process goes on, the two halves constantly changing their charge with the result that the current continues to flow.

A fixed condenser may occupy one of two places in a receiving circuit, either in series with the tuning coil and detector or directly across the telephone receivers. In the illustration A shows a detector requiring a battery with a fixed condenser in series with it and the coil. The oscillations set up in the circuit by the incoming waves can readily pass through the condenser and effect the detector because they are alternating. If it were not for the condenser the direct battery current would pass through the tuning coil instead of the detector because of the comparatively low resistance of the former.

Crystal detectors do not require a battery and may be connected to a tuning coil with a condenser in series and the telephone receivers either across the terminals of the detector or across the terminals of the condenser. When in the latter position, the proper capacity for the fixed condenser will depend upon the resistance of the telephone receivers, the higher the resistance the less the capacity that will be required and vice versa.

LESSON TWENTY-SIX. VARIABLE CONDENSERS.

The point of sharpest resonance does not always happen to come on a turn of the tuner where it can be reached by the slider. The variable condenser makes it possible to adjust the circuit to the exact point of resonance.

FIG 56. Rotary Variable Condenser.

Variable condensers are of two general types, the Sliding Plate and the Rotary Variable. The rotary variable is the most convenient and easy to manipulate. It consists of a number of fixed semi-circular metal plates between which swings a set of smaller movable semicircular plates. The fixed plates form one half of the condenser and the movable plates the other. In this way the capacity of the condenser is very closely adjustable. The movable plates are provided with a pointer moving over a graduated scale so that the comparative amount of capacity in the circuit is indicated.

The sliding plate type of condenser consists of a number of rectangular fixed plates between which slide a set of movable plates.