Page:Popular Science Monthly Volume 91.djvu/465

 Popular Science Monthly

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��to use them directly in the circuit (or shunted by an extremely high resistance) while receiving.

How the Call Signals Are Made

In taking up code practice over such a telegraph line, the usual wireless methods of calling and signing should be followed. Since every radio telegraph station has certain identifying call-letters assigned to it by the Government, the two practice stations should select call letters. Let us suppose that the left-hand station chooses the call signal KSW and the right-hand station KUR; these groups of letters will designate the corresponding station and that one only.

Having arranged the line and apparatus so that every pressure of the sending key at either station may be heard as a correspond- ing short or long buzz or tone in the tele- phones at the other, the sending of messages may be begun. The time for the first trial having been set, both students should be at their instruments, and listening to the telephones. Let us assume that it has been arranged that KUR, the right-hand station of Fig. 5, shall signal first. The operator there will follow the International calling method and first send the "atten- tion" signal, dash-dot-dash-dot-dash, two or three times as shown in Fig. 6. This is a general signal which precedes transmission of radio messages, and is merely to catch the attention of the receiving operator. Hav- ing signaled attention, the call letters of the station desired are now formed three times in succession : KSW KSW KSW, as is also shown in Fig. 6. This part of the call serves to notify the operator at KSW that he is wanted by some other station. The next part of the call is the French word "de," meaning from, which is immediately followed by the thrice repeated call letters of the sending station : KUR KUR KUR. At the close of this signature, which indicates the name of the station from which the call is sent out, the operator makes the finish signal, dot-dash-dot-dash- dot and then signals the letter "K." These two characters used in this way mean: "I have finished sending and will listen immediately. Please reply."

Answering the Call Signal

The operator at the other station, upon hearing this call, realizes that KUR desires to open communication with him. There- fore he sends an acknowledgment of the call

��signal and an invitation to transmit, in accordance with Fig. 7. This consists of the attention signal sent twice or three times, the signal letters of the calling station (KUR) sent three times, the word "de" (from), his own signal letters (KSW) three times, "K" several times (meaning "go ahead"), and the finish signal. The first operator, at KUR, then knows that the second station KSW is ready to receive the messages to be sent, and proceeds to transmit them. In the course of a message the sending operator occasionally makes a mistake; noting this, he at once stops

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��The usual wireless method of calling and signaling should be taken up in code practice

sending and signals a question mark dot- dot-dash-dash-dot-dot, then repeats the previous word correctly and continues with his message. If a receiving operator fails to receive the entire message correctly, in his following period of sending he requests a repetition of the words missed. This is usually done by sending the last word received correctly and following it by several question marks (dot-dot-dash-dash- dot-dot) and the next word correctly received. If the receiving operator gets correctly everything sent to him, he signals "R" (meaning "Received Correctly") sev- eral times at the beginning of his next period of transmission. Thus complete conversations are carried on and acknowl- edged or corrected. The detailed form of sending commercial messages is somewhat more complicated, but the simpler method just outlined will prove ample for practice work.

The practice messages should at first be made up of words including only the letters of the first three groups of Fig. 2, in last month's article. As soon as all of these letters are perfectly familiar and easily recognized when their Morse sounds are heard, the remainder of the alphabet may be memorized and worked into the code

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