Page:Popular Science Monthly Volume 92.djvu/176

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��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.

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