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RADIO BROADCAST
approximately the same. Crystals are not sensitive at all points. The pressure of the contact on the crystal has an important bearing on its behavior. Sensitive points can only
Fig. 3. Modulation of high frequency current. Upper curve represents voice wave; lower curve a high frequency current modulated by this voice current
be found by experiment, and the exact pressure to which the contact should be adjusted by experience.
A valuable aid in adjusting a crystal detector is the use of a test buzzer. A wire is connected from the terminal of the high pitched testing buzzer where contact is made with the vibrating arm. The tiny spark which you can see where the contact is made and broken serves to identify the correct terminal. This contact is connected with the secondary tuning inductance and causes a signal of standard strength to flow through the secondary circuit whenever the buzzer is put in operation. In
this way the most sensitive spot may be found without waiting for signals to come in.
The sensitive mineral should be protected from dirt and grease. An enclosed case is recommended.
All switch contacts should work smoothly and easily. They should occasionally be polished so that they do not become dull from corrosion. Sliders should make firm contact,
Fig. 4. Crystal receiver circuit showing test buzzer connections
yet not too firm to wear out the wire on the inductance.
The care and operation of a crystal set is a simple matter and brings its just reward.
The pleasure you obtain from a crystal receiver hinges largely upon your skill in adjusting the crystal detector. Transoceanic signals have been received on crystal sets, but it is more to the credit of the skill and patience of the operator that to the efficiency of the crystal receiver.