Popular Science Monthly 



159 



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 



rWuijW f^wiruW 



S.M. 



HlHiH 



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 



