��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