70 



TRANSACTIONS OF WAGNER 



ETHER WAVES AND THE MESSAGES THEY BRING 



the currents in this ground wire is the important piece of apparatus in all wire- 

 less work. 



The Marconi coherer was one of the first as well as one of the simplest 

 of such devices. Although it is now obsolete, it illustrates well the use of these 

 waves for wireless telegraphy. The metal plate A (Fig. 4) is a resonator and 

 corresponds to the aerial in a working wireless set. A wire runs from this 

 plate to the coherer, C. This consists of a glass tube nearly filled with a mix- 

 ture of iron and nickel filings, which are held in place by a small stopper at each 

 end of the tube. The wire from the aerial runs through the stopper at one end 



M^ 



Tx 



1^^ 



Fig. 4. — Diagram showing use of Marconi coherer. 



and is buried in the filings. Another wire runs from the other end of the tube 

 to the earth. The ends of the wires in the tubes are separated by about one 

 centimeter of the loosely packed filings. An ordinary bell circuit with two cells 

 in it also passes through the coherer, using the same terminals at the tube as 

 the aerial. Now ordinarily the resistance of the filings is so great that the bell 

 will not ring. If, however, an electric spark occurs, even at some distance, 

 the alternating current set up in the wire from the aerial causes the filings be- 

 tween the two wires to cling together and the resistance is so diminished that 

 the bell will ring. A device like the clapper of an electric bell, working in the 

 same circuit as the bell and tapping the tube, shakes up the filings and stops 



