448
��Popular Science Monthly
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��another part of tre nouse, there will always be the temptation to call from one "sta- tion" to the other and to make corrections in that way instead of depending strictly upon the Morse signals. Only by relying upon the line and Morse signals exclusively can the best practice be had.
An excellent system of connections for a practice line is shown in Fig. 5. For each station there are needed a buzzer, dry cells, telegraph key, and telephones. Almost any sort of buzzer or telephones will do, though it is better to get a buzzer which gives a high musical tone and to use tele- phones of the usual wireless type. The small buzzers which can be purchased from any electrical sup- ply store have fairly high tones and are very good. There are special "wire- less" test buzzers made, which have three connection points and which are designed to give a very high and clear tone of about
the same sound as that produced by a modern radio-telegraph station. This is the best type to buy. The preferred tele- phones are of high resistance (about 2000 ohms total) and consist of two "watch- case" earpieces mounted upon a headband which automatically holds them in place and so leaves both hands of the operator free for sending, writing out the message being received, or adjusting his apparatus. An advantage of buying both the buzzer and the telephone of the sorts used in regular wireless telegraph work is that both may be used later on when radio-telegraphy itself is taken up.
Wiring the Two Stations
The diagram, Fig. 5, shows how the two buzzer telegraph stations are to be con- nected. First the dry cells and the key are connected in series across the two ordinary terminals of the buzzer, marked I and 2 in the figure. Then the binding post I is con- nected to earth, and a wire is attached to the contact post near the buzzer armature, marked 3 in the figure. This wire is con- nected with one terminal of the telephones. To make sure that everything is in good shape at either station, the remaining terminal of the telephone may be con- nected with the ground, so placing the
��In answering a call at the other station ac- knowledgment of the call is sent in return
��receivers in series between terminals I and 3 of the buzzer. If the adjustments and wiring are correct the buzzer will operate whenever the key is pressed, and the tone of the buzzer will be reproduced loudly in the telephone receivers. If the tone is so loud that it becomes annoying, a resistance of several hundred ohms may be connected across the telephones as shown in the left hand station of Fig. 5. If this resistance is variable, as it is in a potentiometer used with radio receivers, the loudness of the sound in the telephone may be made any- thing desired by adjusting the resistance so that any chosen portion of the line current will be shunted around the winding in the "by-pass" circuit. Assuming that both stations have been set up and adjusted to operate properly, it is only necessary to con- nect the outer ter- minals of the two telephones by means of a line wire. This line wire may be of almost any kind since it carries very little current. Any copper wire may be strung from one station to the other; it need not be larger than No. 30, and does not require careful insulation unless the distance is large. When the two stations are set up completely, as in Fig. 5, pressure of either key should result in a corresponding clear signal tone in both telephones. If it does not work out in this way, go over the circuits in detail and be sure that they are correctly connected and sufficiently well insulated. The earth or ground connection may be made by wrap- ping a bare wire around a scraped water or gas pipe, or a steam radiator. For con- venience the ground lines may be discarded and a second line run between posts 3 of the two buzzers.
The diagram shows the variable shunting resistance at only the left-hand station; obviously the same plan may be used at both ends if the signals are too loud. The incoming signals are likely to be somewhat weaker than those made by the buzzer at the home station when sending, but if the line wire is of sufficient size and well enough insulated this difference in intensity should cause no trouble. Under some particularly poor conditions it may be found necessary to shunt the telephones while sending and
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