RADIO COMMUNICATION 251 



widening circles run to the shore of the pond, and there set to 

 rocking the weeds or grasses that are growing along shore. A 

 bell when set in vibration will cause the surrounding air to be 

 set in motion, and this wave motion when it strikes the ear will 

 set up there a similar vibration which is transformed to nervous 

 impulse and transmitted to the brain, so we hear the sound 

 (Fig. 166, p. 327). 



Sometimes it has been noticed that certain notes struck on a 

 piano will cause objects in a room to vibrate; other notes will 

 seem to have no effect on these same objects. Thus, if a violin 

 string be tuned so it gives off the C note if bowed and this note be 

 struck on the piano, the violin in the same room will be found to 

 also sound this note faintly. The violin string is set in motion be- 

 cause the sound waves regularly striking it have the same period 

 of vibration as is now natural to it, and so gradually produce in it 

 the same rate of vibration as the vibrating wire originally struck 

 in the piano. The violin is said to be "in tune" with the note, 

 and so will respond to notes of this rate of vibration. Similarly, 

 the radio receiving apparatus must be adjusted so as to be "in 

 tune" with the sending station. This adjustment may be 

 changed so that, although many stations may be sending out 

 vibrations at the same time, only the one with which the receiving 

 apparatus is "in tune" will produce noticeable effects. Sending 

 stations are also capable of adjustment so that at different times 

 they may send out vibrations of different rates. 



Radio apparatus then consists of two types of appliances: 

 those that create the waves, the large transmitting and broad- 

 casting stations, and the appliances which receive the waves, 

 or the many thousands of small receiving sets distributed over 

 the country. 



The sending station consists of apparatus which will produce 

 electrical vibrations of such high frequency that they will set 

 the ether into vibration, and thus radiate through space in every 

 direction from a point. Hence the expression "radio broad- 

 casting." A system of control must also be included so that the 



