328 



SCIENCE OF HOME AND COMMUNITY 



of low voltage may be changed to one of high voltage. The 

 Ley den jars serve to store up electricity till a heavy charge has 



accumulated. When 

 sufficient electricity 

 has been stored in 

 the jars, it jumps 

 across the spark gap 

 with tremendous 

 force, and this starts 

 the electric waves, 

 which are transmit- 

 ted in every direc- 

 tion. In order to 

 make these waves 

 carry through long 

 distances, long wires 

 called antennae are 

 connected with the 

 sending apparatus. 

 In some stations 

 wires 200 feet high 

 are used. The 

 waves thus sent out 

 follow the curva- 

 ture of the earth, so 

 that the effect may 

 be felt by instru- 

 ments several thou- 

 sand miles away. 

 Receiving apparatus. The most important part of the 

 receiving apparatus is the detector, or coherer, as it is called. 

 The purpose of this instrument is to detect the presence of 

 the electric waves. Many different kinds have been de- 

 vised. One that has been much used by Marconi consists 

 of a small glass tube, plugged at each end, and having the 



Eorth 



FIG. 134. Wireless sending apparatus. 



