796 



Popular Science Monthly 



ber, the check, the address, the text and the 

 signature. Similarly, when a number of 

 relaying stations or a complicated route 

 is involved, the preamble will have to be 

 even more extensive than that given in the 

 sample message. All these matters will 

 develop as practical operating is taken up, 

 but it is essential that the student appre- 

 ciate thoroughly as much as has been given 

 of the routine of message handling. 



Interference in Radio 



Thus the matter of forms of transmitting 

 may be disposed of for the time being. 

 By following out the system as outlined, 

 the student will soon become accustomed 

 to the necessary formalities and will begin 

 to recognize the portions which may be 

 omitted under some particular circum- 

 stances without causing confusion. Having 

 this in mind, and remembering that the 

 matter transmitted in addition to the mere 

 address, text and signature is necessary to 

 keep a proper record of the messages and to 

 make a correct accounting of its costs of 

 transmission, the learner may pass on to 

 one of the most interesting and important 

 branches of radio operating, namely, the 

 reduction of interference. 



In radio telegraphy there are two main 

 classes of interference, both of which tend 

 to make it difficult to exchange messages 

 without interruption. The first of these is 

 that caused by other radio stations, and 

 the second that set up by natural or non- 

 radio electrical disturbances. The second 

 type of interference may be subdivided 

 to some extent, so as to separate troubles 

 caused by lightning, by passing trolley- 

 cars, by dust storms, etc., as will be ex- 

 plained more fully in later articles of this 

 series. The first type, or "station inter- 

 ference" should probably be taken up first. 



Reducing Interference Between 

 Radio Stations 



Since all radio telegraph and radio tele- 

 phone stations use the same basic medium 

 of communication, namely, the hypothetical 

 "ether" of space, it is only natural to expect 

 that under some conditions there will be 

 confusion if many messages are passing 

 from station to station at the same time. 

 Speaking very roughly, the use of this same 

 ether by all the stations is somewhat like 

 the use of the same air by a large number of 

 people talking at once. We have all ex- 

 perienced the difficulty of speaking in a 

 noisy crowd. The person to whom we 



talk hears not only what we say, but also 

 the voices of ten or a dozen other people 

 who are near by. Unless we speak loudly, 

 or in a tone different from that used by the 

 others, we are not heard clearly. It is 

 much the same in radio. When one station 

 sends its signals out, the waves spread in 

 all directions, and some arrive at the de- 

 sired receiving point. If a dozen other 

 stations are sending at the same time in the 

 same neighborhood, their waves also reach 

 the receiving station in question. The re- 

 sult is bound to be confusing unless the 

 first mentioned plant sends its signals more 

 strongly, or in a different tone or pitch 

 from that used by the other installations. 



Thus the two fundamental ways of avoid- 

 ing station interference are indicated: (i) 

 by the use of large transmitting power, 

 which corresponds to speaking loudly when 

 in a crowd, and (2) by using signals of a 

 character different from those common to 

 the interfering stations. This second 

 method has been developed to a great ex- 

 tent in the technology of radio telegraphy, 

 and is of course the preferable plan. Its 

 advantages are apparent as soon as one 

 considers that "talking loudly" may get a 

 single set of messages through interference, 

 but is certain at the same time to create 

 still greater disturbance for other trans- 

 missions which may be going on simul- 

 taneously. 



On the other hand, reducing interference 

 by making a distinction between the kinds 

 or characters of signals used helps the other 

 stations in their parallel problems of send- 

 ing. In succeeding articles, both of these 

 will be discussed in greater detail, and in 

 the next issue experiments on the buzzer 

 telegraph line (described in the earlier in- 

 stallments) will be explained to illustrate 

 various effects of interference and ways to 

 reduce their troublesomeness. 

 (To be .continued) 



Testing for Trouble That Causes Poor 

 Automobile Lights 



THE method of procedure for locating 

 trouble in the lighting system of an 

 automobile is as follows: Test the electro- 

 lyte of the battery to see if it registers 

 1275 or 1300 specific gravity. See that 

 the ammeter registers a charge when the 

 engine is running, otherwise the battery 

 may become discharged. The lamps may 

 be out of focus. A set screw back of the 

 reflector adjusts the focus. 



