336 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



Fig. 38. 



provided with insulating links II. The body of metal WW, 

 which is separated from the ground by a short air gap g, 

 is connected to one terminal of a high voltage induction coil, the 

 other terminal of which is connected to the earth, the body of 

 metal WW is charged until the air gap g breaks down, the 

 discharge which takes place is oscillatory in character as ex- 

 plained in Chapter IX, and electric waves pass out in all direc- 

 tions from WW. 



The receiving antenna. A long vertical wire is suspended by 

 insulating supports at the receiving station and connected to earth 



through a device which is called a 

 detector. The passage of the electric 

 waves causes electric charge to surge 

 up and down in this vertical wire or 

 antenna, and the weak alternating 

 current thus produced actuates the detector and produces the sig- 

 nal at the receiving station. The detector which was used in the 

 earlier days of wireless te- { 



legraphy was the coherer 

 of Branly. The essential 

 parts of this coherer are 

 shown in Fig. 38. It con- 

 sists of two short brass 

 rods between which is a 

 loose mass of metal filings. 

 This coherer is connected 

 across the air gap of the re- 

 ceiving antenna or resona- 

 tor as shown in Fig. 39, in 

 which is an ordinary 

 telegraph sounder. An 

 auxiliary device, not shown 

 in the figure, is used to 

 keep the metal filings vibrating slightly. Under these condi- 

 tions the filings do not conduct the battery current to any 



r i 





Fig. 39. 



to ground 



