WIRELESS TELEGRAPH 



WIRELESS TELEGRAPH 



coherer. When the filings cohere the effect is 

 like that of tightening any loose contact the 

 current becomes stronger. Thus, when the 

 electric waves pass over the instrument the 

 coherer causes the battery current to become 



COHERER 

 (o) Electrode; ( b) filings; (c) electrode. 



stronger. A tapper then jars the filings so that 

 they are loosened again and the battery cur- 

 rent becomes weaker, becomes, in fact, prac- 

 tically no current. The coherer thus acts prac- 

 tically as a "make-and-break" in the battery 

 circuit, the "make-and-break" being operated 

 by the electric waves. The battery current can 

 be made to operate a telegraph sounder or to 

 produce a clicking sound in a telephone re- 

 ceiver. 



Electrolytic detectors are those that depend 

 on electrolysis. There are two principal forms, 

 the DeForest and the Fessenden, named for 

 their inventors: 



The DeForest detector consists of a tube 

 containing two metal plugs or electrodes, and 

 between the electrodes a paste of litharge and 

 glycerine or vaseline mixed with water or alco- 

 hol and a few metal filings. This detector acts 

 in the opposite way from a coherer. Under 

 ordinary circumstances a good connection is 

 made by electrolysis, the electrodes being con- 



RECEIVING APPARATUS 

 CD Receivers for ears (5) Aerial 



(2) Detector (6) Copper plate in 



(3) Fixed Condenser ground' 



( 4 ) Loose coupler 



nected by a "lead tree" (see ELECTROLYSIS). 

 The electric waves break this connection. 



The electrolytic detector used in the Fessen- 

 den system consists of two fine platinum wires 

 dipping into a liquid. Nitric acid has been 

 found best for signaling. The wires and the 



acid form an electrolytic cell, to which a bat- 

 t TV is connected. The acid quickly stops the 

 battery current, probably on account of polari- 

 zation. The electric waves cause a sudden 

 rush of the battery current through the acid. 



The magnetic detector used in the Marconi 

 system was invented by Marconi. In this de- 

 tector a 'band made of fine iron wire is kept 

 running on two wooden pulleys at the rate of 

 about three inches per second. Two horseshoe 

 magnets with two like poles together (as two 

 north poles) are supported above one side of 

 this band. Directly beneath the two magnet 

 poles just mentioned the wire band runs 

 through a glass tube. On this glass tube are 

 wound two coils of wire. One of these coils is 

 connected to the aerial and the ground; the 

 other is connected to a telephone receiver. 

 The wire is magnetized by the action of the 

 horseshoe magnets as it travels along. 



We should expect to find a south magnet 

 pole directly beneath the two north poles of 

 the magnets. This would be so were it not for 

 the fact that it requires a little time to mag- 

 netize iron; therefore, by the time this part of 

 the wire becomes a south pole it has traveled a 

 little beyond the magnets. 



Now, suppose the electric waves pass over 

 the aerial. An oscillation, that is, a rapidly 

 alternating current, is produced in the aerial 

 and in the coil to which it is connected. This 

 rapidly alternating current has the same effect 

 on the iron as a sharp blow with a hammer 

 would have (see MAGNET AND MAGNETISM). It 

 shakes up the molecules of iron so that the 

 iron quickly takes on the magnetism induced 

 by the horseshoe magnets. The south pole in 

 the wire then jumps back to a position directly 

 beneath the north poles of the magnets; thus, 

 the electric waves cause a change in the mag- 

 netism of the iron wire. Now the coil con- 

 nected to the telephone receiver feels this mag- 

 netic change, and a current is induced in this 

 coil (see INDUCTION, ELECTRIC), and this current 

 flowing through the receiver causes a click. 

 When the waves cease the south pole in the 

 wire jumps back to its former position and an- 

 other click is produced. 



Another form of detector, the glowlamp de- 

 tector, is described under WIRELESS TELEPHONE. 



Aerials. The aerial consists of a wire or a 

 number of wires suspended in the air, hence its 

 name. It is in the aerial that an electric surg- 

 ing, or an electric oscillation, is set up by the 

 electric waves. The aerial furnishes a large 

 conducting surface to catch the waves in re- 



