Page:Popular Science Monthly Volume 91.djvu/973

 Popular Science Monthly

���will be noted that the buzzer-sounds from each of the three stations possess a certain individuality, and that the identity of a sending station can be established even before the operator has given his station call, merely by noting the signal-tone character. If three of the same make or model of buzzers are used, the tones may sound very much alike, and it may be difficult to tell by tone which station is send- ing. If three different sizes or types are used, however, the signal tones are likely to be widely different. It is this variation in signal tone which makes possible the reduc- tion of station interference by the operator's concentration on the sounds themselves.

Fig. 17 shows a sig- nal station of the buzzer telegraph line. The buzzer Z\ has the telegraph key K\ and battery Bi connected across its binding posts, so that when the key is pressed the buzzer will operate. The left-hand binding post, which is con- nected with the vi- brating armature, is made fast to the earth

connection; and the contact-adjustment post is connected with the line wire through a pair of head-telephones of approximately 1000-ohms resistance. These telephones may be shunted by a variable resistance of about the same value, for the purpose of weakening the signals heard. The resist- ance is not usually necessary, however. The line wire may be extended in either direction to reach as many stations as de- sired, within reasonable limits of number and distance. Each station is connected as shown in Fig. 17, and the system is such that signals produced by pressing the key at any station are heard simultaneously in the telephones at all the other stations.

Using One Station to Make Interference

Obviously, such a system resembles a group of radio stations in many particulars. If there are more than two stations on the line, it is possible to arrange for one of them to interfere while the others are attempting to exchange messages. By choosing the buz- zer pitch of the interfering station so that it is different from that of the sending station, the easiest interference condition is created. By gradually adjusting the interfering note

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��Fig. 19 and 20. A typical buzzer of the iron frame type and a method of adjustment

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��to a sound closer and closer to that of the sending station, the difficulty of receiving is continually increased and practice in con- centrating on one note to the exclusion of others is secured.

A better way to develop systematically the power to eliminate or reduce station interference by tone selection is to install at one of the buzzer-telegraph line stations an equipment such as shown in Fig. 18. Here the left-hand portion represents the usual buzzer sending and receiving outfit, con- nected as in Fig. 17. The telephone shunt- ing resistance is. not shown, but may be added if found necessary. The right-hand portion of the diagram shows the inter- ference makfer, which consists' mainly of a second buzzer Z 3 hav- ing its own battery B 3 and a sending key or sending machine S connected across its terminals. From the contact-adjustment post to the left-hand (armature) binding post is connected a high resistance R 2 , which should have a value of at least 1000 ohms. A sliding contact on this resistance is connected with the line wire.

This interference-maker will produce interference signals whose strength, tone and occurrence are under control, and may be arranged so as to operate automatically. When first set up, it will be noticed almost immediately that sliding the contact along R 2 changes the strength of the interfering signal at all the stations on the line, and that the more resistance is cut-in between the line wire and the ground, the louder the signals are. In practicing, it is a good plan to start with fairly weak interfering signals, and then gradually to increase their strength until they are as loud as or louder than the messages it is desired to receive.

Varying the Tone of the Buzzer

The adjustment of the interfering or sending signal tone has been mentioned several times, but specific methods of making this adjustment have not been shown. In Fig. 19 a typical buzzer of the iron-frame type is illustrated. This is use- ful for this sort of work because of its cheapness and reliability. The mechanical arrangements vary in detail according to

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