Applied Science 837 



tially a collection of apparatus in which regular electric oscilla- 

 tions can be set up. Now every such circuit has what can he called 

 its natural period of vibration, that is to say, there is a certain 

 rapidity of oscillation to which it most readily responds. If we 

 think of a heavy ball suspended by a string we know that we can 

 make this ball execute large swings by a number of light taps, 

 provided these regularly succeed one another at a certain definite 

 rate. Much heavier taps, if performed irregularly, will produce 

 much less resultant effect on the ball. If we make the experiment 

 we shall find that the period of the taps necessary to produce the 

 maximum effect depends on the length of the string. Every- 

 body who has swung himself in a swing makes use of the same 

 principle. It is for the same reason that soldiers are made to 

 break step in crossing a bridge, for if the slight shocks given to 

 the bridge by their tramping feet are in the natural period of the 

 bridge they may set up such violent oscillations as to break the 

 bridge. Now by varying certain apparatus in an electrical circuit 

 we can alter its natural period of vibration, just as by altering 

 the length of the string we can alter the period of vibration of the 

 heavy ball. The receiving circuit of a wireless telegraph set, when 

 so adjusted to the oscillations it receives as to show its maximum 

 response to them, is said to be tuned to them. When waves come 

 through regularly we hear a musical note, shrill, high-pitched or 

 low-pitched, the pitch depending on the number of arriving waves. 

 The "tuning" can be adjusted, just as the string of a violin, by 

 being tightened or loosened, raises or lowers its pitch. In radio 

 pitch is a matter of wave-length. Different sending stations send 

 out oscillations of different periods, so that a receiving circuit 

 which is tuned to one need not be tuned to the others. In this way 

 the sending station that is to be responded to can be selected from 

 all the others which may be sending out waves at the same time. 

 And a certain amount of privacy can be ensured in this way by 

 previous agreement as to the period of the oscillations which are 

 to be used. The receiving instrument is adjusted to receive waves 



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