Page:Popular Science Monthly Volume 92.djvu/174

 158

��Popular Science Monthly

��come. It has also been demonstrated that the best signal-tone frequency for working through static interference is about 1000 per second, which is in the neighborhood of the second C above middle C on the musical scale. This frequency of 1000 has now been selected as standard for nearly all radio trans- mitters.

An Artificial Static Producer

It follows, then, that to get practice in reading signals through static we must have (in addition to the buzzers which produce the Morse signals themselves) some device which will imitate the sounds of static. The simplest way in which an

����s

��S.M

��"Wi

��FI&.25

��One way to associate the buzzer telegraph line and the stray-maker for practice work

��idea of atmospherics can be secured is shown in Fig. 23, where a telephone re- ceiver T is connected in series with a battery B and a coarse file F. The loose end of the wire from the telephone re- ceiver, W, may be rubbed along the rough surface of the file. The telephone will reproduce irregular rough and scratchy noises corresponding closely to some types of strays.

The file arrangement is scarcely uni- form enough in action to use for regular practice, and so it will be well to make up a "static producer" or "stray maker" of the sort shown in Fig. 24. A wooden base X has mounted upon it a block Y and two drilled standards, S' and S^ These standards aui)port a shaft upon which is fastened a brass disk or wheel

��BW, about }/2 in. thick and 3 in. in diame- ter. One end of the shaft carries a pulley P which permits the disk to be rotated slowly by belt B, a clockwork or back- geared motor being used as a source of power. The circumferential surface of the disk is roughened by cutting irregular diagonal V's across it with a sharp saw- file, so that its surface somewhat re- sembles an exceedingly coarse and rasp- like file. On the block Y is pivoted a soft copper wire W, about No. 12 in size, one end of which is bent up to rub upon the surface of the roughened disk at the contact point C. The other end of the wire extends out away from the disk, and a small lead weight W is fastened upon it so that the pressure of contact at C may be varied by sliding the weight back and forth along the wire.

If the terminals of the battery and tele- phone in Fig. 23 are connected with the disk and the wire of Fig. 24, so that the three elements are in simple series con- nection, and if the disk is then slowly turned, the telephone will produce sounds like those set up by static. By varying the speed of rotation (which must always be slow — not more than about one revolu- tion per second) and by changing the number of dry cells in the battery, almost any type of static can be imitated.

Connecting the Stray-Maker with the Telegraph Line

Now arrange the stray-maker in such a way that its imitation static can be impressed upon the buzzer telegraph line previously described. Thus, one static producer will afford practice to all the students using the line, and practice can be had in the actual exchange of messages under various conditions of atmospheric interference.

Fig. 25 shows one way of associating the buzzer telegraph line and the stray- maker SM. The left-hand portion of the figure represents any one of the stations along the line, and comprises the tele- phone T, the buzzer Z, the battery B, the key K and the ground connection which have been described in earlier articles. At any one of these stations (though pref- erably one near the middle of the line) the stray-maker may be installed by con- necting it as shown in the right-hand part of Fig. 25. i2 is a resistance of about

��I

�� �