Page:Popular Science Monthly Volume 92.djvu/175

 Popular Science Monthly

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��1000 ohms, with a sliding contact, such as was shown at R~ of Fig. 18 in the Decem- ber article. This is connected in series with the battery B' and the rotating disk SM. The number of cells of battery can conveniently be varied by using a three- point switch as shown at S. One end of the resistance unit is connected with the ground at G' (which may be the same connection as used for the telegraph sta- tion at G), and the sliding contact is con- nected with the line wire. The roughened wheel is slowly revolved, by use of the clockwork or motor as explained above. When the battery is turned on, the imita- tion static will be heard in all the tele- phone receivers along the line. By slid- ing the mova- ble contact of the resistance toward the left or ground- ed end, the static sounds are made weaker. By increasing the amount of re- sistance be- tween the line wire and ground con- nections, the noises are strengthened. It may some- times be nec- essary to use more than three cells of battery, but this can only be determined for any particular buzzer telegraph line by actual trial.

Comprehensive Telegraph Practice

Having set up a buzzer line with at least two other students, and having made both the extra buzzer (with auto- matic sender) for imitating station inter- ference and the stray-maker for imitating atmospherics, the student is ready to work out a course of practice-study which will fit him for the Morse telegraphing part of the most difficult operating positions. Sufficient plain code practice, without interference, should first be carried on. When there is no difficulty experienced in sending and receiving messages sent at the rate of twenty-five words (one hundred

��and twenty-five letters) per minute, the next necessary step to be taken is copy- ing weak signals. By using the shunting resistance connected across the tele- phones, the signals are gradually reduced in intensity, and practice is continued until it becomes easy to read messages so extremely faint that the noise of a rattling window or of someone talking in the room makes it impossible tq hear them clearly. This corresponds to the practical radio case of receiving messages from a great distance.

Having perfected one's self in reading weak signals, copying messages at various tone-frequencies should be tried. The article immediately preceding this in the

series showed

LINE WIRE

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��S.M.

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��FIG. 25

��Connections for reading through both station and static in- terferences. They are obtained by combining apparatus

��how to ad- just the tone frequency of the buz- zer. By fol- lowing the plan given, as well as by us- ing various types of buz- zers, signal sounds rang- ing all the way from a low rattle to a high, pierc- ing musical note may be produced.

��The expert operator is able to read mes- sages sent with tones of any sort, and it is a good plan to practice on many different frequencies and with both loud and weak signals.

Drill in Overcoming Station Interference

The matter of station interference should next be taken up for drill. Using the automatic sender to produce inter- ference over the entire line, one student should send cipher messages consisting of five-letter words such as QSBVH MKUIL SHDYJ WUIPO. The station sending should transmit a certain number of messages of this sort, the cipher words having been written out in advance, and all the other stations on the line should attempt to copy the signals through the

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