956 



Popular Science Monthly 



ference. The selection of the free wave- 

 lengths, on which no interference exists at 

 the time, is made by trial; and the actual 

 adjustment which limits the transmitters 

 and receivers to certain definite wave- 

 lengths is made by control of parts of 

 the apparatus according to the process 

 generally known as "tuning." The proper 

 use of these various wavelength channels of 

 radio communication will be taken up later, 

 when the limitations of tuning or wave- 

 length selection will be pointed out. 



In addition to selection by wavelength, 

 the characteristic of tone frequency or spark 

 sound is used to discriminate between the 

 signals of several stations which are heard 

 at the same time. If a pair of senders of 

 equal power are 

 about the same dis- 

 tance from some re- 

 ceiving station, and 

 if both transmitters 

 use the same wave- 

 length, under normal 

 conditions their sig- 

 nals will be heard 

 with about the same 

 intensity. If, now, 

 these two signals 

 sound alike, it will 

 not be possible for 

 the operator to dis- 

 tinguish between 

 them. However, if 

 the dots and dashes 

 from one sender are 

 heard as an inter- 

 mittent rough, low- 

 frequency sound, 



and if the signals from the second are 

 high and musical in character, it is easy to 

 see that the receiver can concentrate on 

 either and decipher its messages without 

 being disturbed by the other station. The 

 condition is comparable to that in which a 

 fife and drum corps is heard in the distance ; 

 it is easy to count the strokes on the bass 

 drums without noticing the fifes at all, or to 

 note the air played by the fifes without 

 being disturbed by the booming, deep 

 sounds of the drums. 



Practice in Concentration 



Skill in reducing station interference by 

 tone selection is mainly the result of 

 operating practice. How well the man in 

 charge of a receiving station can con- 

 centrate on the signals produced by some 

 given transmitter from which he wishes to 



Fig. 18. At left is the ordinary type of 

 buzzer outfit connected as in Fig. 17, 

 and at right is the interference-maker 



receive, while he at the same time dis- 

 regards other signals Or other tones which 

 are present in his telephones at the same 

 time, depends almost entirely on his 

 experience. This is one of the most im- 

 portant qualities which an operator can 

 develop, and it is worthy of much practice. 

 Fortunately enough, practice in tone- 

 selection for reading messages through sta- 

 tion interference can easily be carried on 

 without wireless apparatus. It is only 

 necessary to make use of the buzzer 

 telegraph line described in the earlier 

 articles of this series. For the period of the 

 war it is not permissible for experimenters 

 or others to use wireless apparatus without 

 specific permission from the Navy Depart- 

 ment ; nevertheless, 

 many of the branches 

 of radio operating 

 can be covered thor- 

 oughly by using the 

 instruments which 

 have been and are to 

 be explained in this 

 series. Thus it be- 

 comes possible to 

 learn the most diffi- 

 cult and perhaps the 

 most important parts 

 of wireless operating 

 without experiencing 

 any difficulties on 

 account of the pres- 

 ent embargo on radio 

 experimenting. 



The Buzzer Tele- 

 graph Line 



If you have not yet arranged with a 

 friend for building and operating a buzzer 

 telegraph line, you should do so at once. 

 Study by oneself is practicable in beginning 

 operating and for certain later divisions of 

 the work, but it is almost impossible to get 

 the practice which is so essential unless 

 several students cooperate in the use of a 

 buzzer telegraph line. The best plan is to 

 have at least three stations, in three nearby 

 houses, connected by wire as explained in 

 the October article. If you cannot arrange 

 for this, set up two or three stations in as 

 many different rooms of your own house, 

 and connect them by wire in the way 

 described. Then, by inviting one or two 

 companions to work with you, you can all 

 secure the practice which is so needful to 

 the ambitious radio man. 



In working with a three-station line it 



