LESSON THIRTY-THREE. RECEIVING UNDAMPED WAVES. THE TICKER.
A decided difference is encountered between damped and undamped oscillations when receiving signals. The ordinary detector cannot be used for receiving undamped oscillations without first being properly modified.
When telegraphing the dots and dashes of the code by undamped oscillations the change taking place in the detector circuit would merely move the telephone receiver diaphragm from its normal position at the beginning of each dot or dash, causing a click to be heard and nothing more. The telephone receiver diaphragm would remain in a fixed position just as long as the waves from the transmitter kept coming in during each signal. Both dots and dashes would be heard simply as clicks and not appear distinguishable from one another.
The most common and perhaps also the best method is to employ a device called a "ticker" in place of the detector for receiving undamped oscillations.
This arrangement is illustrated in Fig. 63. The left hand part of the illustration is the circuit diagram. A detail of the "ticker" wheel is shown at the right.
The condenser F C is of comparatively large capacity and is fixed. The condenser C is also fixed but is of much smaller capacity. F C is usually a condenser having a capacity of several tenths of a microfarad while C has only a few thousands of a microfarad capacity.
FIG. 63. The Poulsen Ticker for receiving undamped Waves.
T is the ticker wheel and consists of a small brass wheel having a groove in the periphery like a pulley. This wheel is mounted on the shaft of a small motor so that it can be revolved at high speed.
A fine wire is arranged to rub against the groove in the wheel and make contact with the latter.