WIRELESS TELEGRAPH 



0321 



WIRELESS TELEGRAPH 



one knob connected to an aerial, or overhead, 

 wire and the other knob connected by a wire 

 to the ground or to metal pipes, such as gas 

 pipes, leading into the ground. A battery or 

 other source of electric current must be con- 

 nected to the primary winding of the induction 

 coil, and a key, such as a telegraph key, used 

 for making and breaking the circuit. The sim- 

 wireless receiving set consists of an elec- 

 tric circuit in which a battery current flows 

 through some form of detector (see below) 

 which is sensitive to the electric waves. There 

 must be in the battery circuit some instrument, 

 such as a telephone receiver, which will produce 

 a sound when the current is made stronger or 

 weaker. A simple outfit such as described 

 above can be used to send messages a mile or 

 more even with a small spark coil. 



A small but complete wireless telegraph sta- 

 tion which may be built at home consists of 

 the parts named above, with a switch arranged 

 to throw the aerial into the sending or receiving 

 circuit as desired. The receiving and transmit- 

 ting apparatus 

 cannot be con- 

 nected to the 

 aerial and ground 

 wires at tin 

 time, because the 

 powerful currents 

 of the transmitter 

 would injure the 

 receiver. A sys- 

 tem has been de- 

 vised for enabling 

 the operator to 

 send and r 



.-a me time. 

 In this system the 

 transmitting key 



INDUCTION COIL 



i l ' Key 



( 2 ) Condenser 



(3) Battery 



(4) Intern;, 

 (6) Core 

 (6) Primary 



(8) Spark gap 



rovided with a second set of contacts, so 

 that bet\\ lots and dashes the aerial and 



ml wires are automat ,r:illy connected to the 



receiving apparatus. This is called a "break iim- 



in-systcm," because it enables an operator to 



"brcak-m" v.l. mg a message without 



lessagc is completed and ask 



< certain parts repeated. 



tip- spark coil and the 



hmhi r tl. r a message can be 



sent. An amateur can usually receive messages 

 from a much greater distance than that 



h he can send, for the reason that it is a 

 simpler matter and less expensive to set up a 



-distance receiving set than it is to set up a 

 long-distance sending outfit. 



396 



INDUCTION COIL 

 ENCLOSED 



(3) Primary 



(4) Adjusting screw 



(5) Spring 



Detectors. The detector is the part of a re- 

 ceiving outfit that is sensitive to electric waves. 

 Various forms of detectors are employed, and 

 of these the following are most generally used : 



The crystal detector is one of the simplest 

 and most efficient. Crystals of silicon are 

 among those 

 commonly used. 

 The crystal is 

 placed between 

 two metal elec- 

 trodes, the con- 

 tact between the 

 crystal and one 

 of the metals be- 

 ing loose, a screw 



being used to (1) Secondary terminal 

 regulate the <?> Secondary terminal 

 pressure of this 

 loose contact. 

 The electric waves cause a rapid surging back 

 and forth of electricity through the aerial and 

 through the detector. This is called an oscilla- 

 tion. It is a form of alternating current \\ 

 the electricity is surging back and forth, trying 

 to go first in one direction then in the opposite 

 through the detector, the crystal then per- 

 mits the electric charge to go through in one 

 direction and refuses to permit it to go in the 

 opposite direction. Thus the current becomes 

 direct, flowing only in one direction, but is not 

 a uniform current. It is pulsating, flowing and 

 then ceasing to flow with each wave. Ii 

 pulsating current flows through a telephone 

 receiver it causes a buzzing sound. The oscil- 

 lation could not directly afiVct the ren 

 because it is of such high frequency that it 

 would be choked out by self-induction in 

 coils of the receiver, but there is no choking 

 effect with the direct pulsating current. Some 

 crystals that ne u-e.l are >ilu-on. carborundum, 

 iron pyrites, copper pyi -1:1 and lin 



The silicon detector is one of the best. 



the form of detector first used 

 by Marconi The coherer consists of two metal 

 plugs, preferably of silver, lilted in a glass tube 

 \\ith a small space between them, this space 

 being filled with fine filings of nickel and ml 

 Wires for electrical connection lead out from 

 the plugs. The air is pumped out of the tube, 

 and the tube is sealed. tic oscillations 



set up by the electric waves cause the filings to 

 cohere, or cling together more closely. Ins* 

 of one loose contact, as in the crystal detector, 



now have many loose contacts bet 

 filing?. A battery current flows through the 



