MISCELLANEOUS PRACTICAL APPLICATIONS. 



335 



at a distance due to wave motion as compared with the extremely 

 small energy manifestations at a distance due to steady actions, 

 it may be said that the only feasible method * of signalling at 

 moderately great distances is by means of wave motion. 



The use of the vocal organs for producing sound waves and 

 of the auditory organs for perceiving them at a distance, consti- 

 tutes the most familiar example of " wireless " signalling. The 

 term wireless telegraphy is applied particularly to the use of an 

 electric oscillator for producing electric waves and an electric 

 resonator or detector of any kind for perceiving the waves at a 



ground 



Fig. 37. 



distance. This system of electric wave signalling is due to Mar- 

 coni. 



The sending apparatus or oscillator, A charged body, an 

 expanse of metal, is suspended in the air so as to be thoroughly 

 insulated from the earth. This body of metal is usually made 

 of many strands of wire WW y Fig. 37, which are supported by 

 guy wires G G from poles, as shown, the guy wires being 



* Where the energy is not transmitted along a well-defined path like a wire, or a 

 pipe, or a string. 



