Page:Popular Science Monthly Volume 91.djvu/974

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958

��Popular Science Monthly

��the make of instrument, but all have an armature G which carries a contact-spring F and is mounted to vibrate before the poles of the electromagnets KiK 2 . The con- tact-spring F normally presses against the tip of the contact-screw C, which is sup- ported by the contact-adjustment post E and held in place by the lock-nut D. The

���Fig. 21. An automatic sender made with a motor having a crank and pin on its shaft

electrical circuit is usually from the binding post A through the iron frame L and pivot H (or its equivalent armature- support) to the armature H and contact- spring F, thence through the contact-screw C and down its post E (which is insulated from the iron frame), from there by way of a wire to the magnet coils, through them and finally out at the insulated binding post B. The greatest control of the buzzer tone is normally secured by varying the number of battery cells used to operate it, and by changing the adjustment of the screw C. Where a pivoted armature is supplied, adjustment of the spring I will permit further variation in pitch or char- acter.

The buzzers of the iron frame class, and even the small nickel-plated types, usually have tones which are low compared with those in common use at radio stations. Practice on the high tones. For this pur- pose, either install special high-frequency buzzers which may be purchased for about $2.00 each, or adjust the cheap buzzer to produce a high tone. One way of making such adjustment is shown in Fig. 20, where M represents a bit of folded paper or a soft wooden wedge pressed in between the armature G and the contact-spring F. A little experimenting with the thickness of wedge, the setting of the contact-screw and the strength of battery will usually result in

��a clear high tone much like that of the modern radio stations. The standard tone of 1000 sparks per second seems to be of approximately the same pitch as the second C above middle C on the musical scale, and the adjustment of your high- frequency buzzer may easily be verified by comparing it with a piano.

Use of the Sending Machine

The sending-machine S of Fig. 18 con- trols the occurrence of the interfering signals, by stopping and starting the buzzer Z 3 . At first it is a good plan to run the buzzer continuously, in one long "dash," so that the interfering noise will be heard constantly. An experiment ' will show, however, that interference which stops and starts is more difficult to over- come than the steady-dash variety. Con- sequently your practice should be directed toward the kind which comes and goes, so that you will be prepared for the worst when you get into actual radio operating. The best way to get an imitation of bad station interference is to connect your automatic sender, of the tape or disk types described in the October article, at S; if you have no automatic sender, you can sometimes persuade a friend to take the interference key and make irregular dots and dashes for you.

A sending key K, Fig. 2 1 , mounted on a pedestal P and ba eB and connected with a clock-work or electric motor M will serve for use at 5 in Fig. 18. By attaching a wire W and a spiral spring S between the key lever and a crank-pin C mounted on a disk D on the motor shaft, as shown in Fig. 21, an automatic sender of dots or dashes can easily be made. By varying the speed of the motor, as well as the adjust- ment of the key, the length of dot, dash or space can be adjusted to suit.

Those who have no electric motor avail- able for use, may, of course, use a motor which is spring driven, instead. An old pho- nograph motor would be just the thing. Still another scheme would be to use your omnigraph to replace the key and motor arrangement.

The equipment described this month is sufficient to permit code practice, as well as extensive practice in reducing station inter- ference by concentration. Future articles will take up static interference, and the adjustment of the radio apparatus itself so as to minimize the difficulties. (To be continued)

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