WIRELESS TELEGRAPHY 



currents in a large secondary coil which pass as sparks 

 across certain "spark-gaps" up along the air- wire of 

 the pole to the terminal at the top, whence they are 

 scattered broadcast. Some of these are gathered in by 

 the wires on the poles at the receiving station, are carried 

 down to the detector that sets in motion the Morse in- 

 strument, which sounds or prints the message just as in 

 the case of the ordinary land telegraph. 



From this it is obvious that the messages thus sent 

 are not really transmitted in any aimed direction, but 

 are scattered in all directions, and that they would be 

 received and recorded at other receiving stations than 

 the one intended. To prevent this a system of "tuning " 

 has been introduced, so that only instruments tuned 

 to a certain number of vibrations per second affect each 

 other. Unless the number of vibrations is nearly the 

 same, instruments tuned in this manner will not be 

 affected by messages from other instruments, and will 

 select out only messages intended for themselves and 

 sent out by similarly tuned transmitters. If some such 

 arrangement as this were not possible, wireless messages 

 would be so hopelessly jumbled as to be unintelligible ; 

 and indeed in practice serious difficulties are sometimes 

 encountered where a number of systems are working 

 within "striking" distance of each other. 



The progress of wireless telegraphy during the past 

 ten years has been so rapid as to seem like a succession 

 of triumphs. From the sending of wireless messages a 

 few hundred feet by Marconi in London, in 1896, until 

 the transmission of complete messages across the 

 ocean in 1902, seems but a series of rapid steps, cer- 



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