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) 



