Page:Popular Science Monthly Volume 91.djvu/812

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��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.

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