160 



Popular Science Monthly 



artificial interference produced by the 

 extra buzzer and sending machine. There 

 will thus be a race to see who can get the 

 greatest number of words correctly, as can 

 be determined by comparing the sent and 

 received copies at some later time. To 

 make things fair for all the stations along 

 the line, the sending should be done from 

 the station at which the interfering buzzer 

 is located; otherwise the ratio of in- 

 tensity of interfering and desired sigrals 

 will be different at the different stations. 

 Sometimes it will be found that the same 

 mistake is apparently made by several 

 receiving operators. When this occurs, it 

 is nearly always safe to assume that the 

 sending student has made an error. Thus 

 practice of this sort is seen to be helpful in 

 developing accurate sending as well as the 

 ability to receive through interference. 



The first station-interference practice 

 should be made with the extra (interfer- 

 ing) buzzer adjusted to a pitch different 

 from that on which the desired messages 

 are transmitted. Likewise, for the first 

 trials, the interfering signals should be 

 made comparatively weak. When mes- 

 sages are received correctly under these 

 conditions, the interfering signals are 

 made stronger and stronger, until they are 

 about as loud as or even louder than those 

 which are being copied. After this point 

 of skill in receiving is reached, the inter- 

 fering buzzer may be adjusted until its 

 tone approaches more closely that of the 

 station sending messages. Work of this 

 sort is most valuable in preparing the 

 student to meet the actual difficulties of 

 radio operating. 



Practice in Reading Through Strays 



The next step is to practice in reading 

 through static or atmospheric inter- 

 ference. Here the same plan is followed, 

 except that the static-maker is substituted 

 for the extra interfering buzzer. By 

 sending code messages while the stray- 

 maker is working, the strength of the 

 interference is increased day by day until 

 it is possible to receive messages correctly 

 through severe disturbing noises. In 

 this practice the desirability of using a 

 high, clear signal-tone should be noted. 

 Although the low tones sound much 

 louder when no static interference is 

 present, it is surprising to note that the 

 higher and weaker signal tones stand out 



more clearly as soon as the static begins to 

 grow strong. With practice, the student 

 will find that he can read a high, clear 

 musical signal through disturbing static 

 noises many times louder than the mes- 

 sages. This is one of the most curious 

 phenomena which is encountered in prac- 

 tical radio telegraphy, and explains the 

 selection of 1000 per second frequency for 

 most modern radio stations. 



Practice in reading through both sta- 

 tion and static interferences may be 

 secured by combining Figs. 18 and 25, as 

 shown in Fig. 26. Here the sending 

 operator manipulates key K, while inter- 

 fering signals are produced from buzzer Z^ 

 and atmospheric interference is set up by 

 the disk SM. With the skill which can be 

 attained by faithfully practicing in ac- 

 cordance with the plan outlined above, no 

 operator need dread the difficulties of 

 telegraphing which he may encounter in 

 actual work. He needs in addition to 

 this operating ability some measure of 

 knowledge of the radio apparatus itself, 

 and this matter will be taken up in 

 succeeding articles. 



{To he continued) 



A Small Motor Used to Open 

 Large Doors 



LARGE doors like the ones used on 

 ^ garages are difficult to handle, and 

 for this reason I made the attachment 



D.P.O.T. SW. OR 

 REVERSING SW. 



A motor which opens 

 and closes a heavy 

 door automatically 



illustrated, which may be operated with a 

 push-button. 



I attached a motor of suitable size and 

 power at the top of the door on the inside, 

 its shaft being supplied with a small 

 pinion which meshes into teeth on a seg- 

 ment fastened to the wall or other suit- 

 able support.— H. B. Pearson. 



