WIRELESS TELEPHONE 



lioL'S 



WIRELESS TELEPHONE 



the transmitter. As a matter of fact in most 

 ins the oscillations from the arc flow 

 through the primary coil of a transformer, ami 

 the oscillations from the secondary coil flow 

 through the transmitter and the antenna. The 

 transformer is for the purpose of increasing the 

 electric pressure which produces the oscilla- 

 tions. 



Detectors. A receiver of the same form as 

 that of the wire telephone is used, but there 

 must be in the circuit with the receiver a 

 detector which is sensitive to the strength or 

 litude of the electric waves. A coherer can- 

 not be used because if such a detector responds 

 at all it produces the same effect for weak as 

 for strong waves. 



Glowlamp Detectors. A very sensitive de- 

 tector which is used in most wireless telephone 

 ms is in the form of a glowlamp. A sim- 

 ple experiment illustrates the principle of this 

 detector. If a gas flame is made part of the 

 circuit of an electric battery and the circuit 

 includes also an induction coil connected to a 

 telephone receiver, then when a stream of elec- 

 tric waves comes along there is a click in the 

 receiver. The waves change the resistance of 

 the flame and so change the strength of the 

 current. The heated gas in the flame responds 

 to the electric waves. 



The Fleming oscillation valve and the De- 

 Forest audion are the most common forms of 

 glowlamp detectors. In each of these the small 

 amount of gas remaining in an incandescent 

 bulb takes the place of the flame in the ex- 

 periment described above. In addition to the 

 incandescent filament there must be in the 

 bulb either a single plate or a cylinder of metal, 

 as in the Fleming detector, or a plate and a 

 "grid" of metal, as in the DeForest audion. 

 The filament is made white hot by a current 

 from a storage battery. The small amount of 

 gas which remains in the bulb is ionized, that 

 is, the molecules of gas are separated into posi- 

 t ivory and negatively charged ions (see ION). 

 This ionized gas permits the oscillations in the 

 ai rial to flow only one way through the glow- 

 lamp, that is, from the metal to the white-hot 

 filament, the metal being always positive. The 

 ionized gas is also very sensitive to changes in 

 the strength or amplitude of the electric oscil- 

 lations. Thus it is that the current from the 

 glowlamp detector flowing through a telephone 

 receiver reproduces perfectly the sounds that 

 act upon the transmitter. 



Crystal Detectors. Crystal detectors which 

 rectify the oscillations, that is, permit the cur- 



rent to flow in one direction only, may be used 

 in wireless telephony. 



Fcsscndcn Detector. The Fessenden de- 

 tector consists of two coils of wire, one of 

 which is wound on an iron wire core and con- 

 nected to the antenna. The other is attached 

 to a mica diaphragm and has an alternating 

 current flowing through it of the same fre- 

 quency as that of the transmitter. The attrac- 

 tion between these two coils varies as the elec- 

 tric oscillations vary; this causes the mica dia- 

 phragm to vibrate and reproduce the sound 

 which acts upon the transmitter. This is the 

 only form of receiver in which the oscillations 

 produced in the aerial flow directly through the 

 coil of the telephone receiver. (See WIRELESS 

 TELEGRAPH, subhead Detectors.) 



Comparison with Wire Telephone. In the 

 wire telephone the electric current which trans- 

 mits the message is guided by the wire, while 

 in the wireless telephone the electric waves go 

 out in all directions from the sending station. 

 The electric waves may be detected by any sta- 

 tion having a properly tuned receiver. By a 

 system of tuning (see WIRELESS TELEGRAPH) it 

 is possible to connect a number of receivers to 

 one aerial and receive a number of messages at 

 the same time. 



The wireless telephone reproduces sounds 

 with greater accuracy and with better articula- 

 tion than does the wire telephone. The reason 

 is that, as a rule, the variations in the electric 

 current are slightly modified by the wire on ac- 

 count of induction. There is no such action in 

 the case of the wireless telephone, so the waves 

 arrive unchanged at the receiving station. For 

 this reason sounds heard in the wireless tele- 

 phone receiver are particularly sharp and clear 

 and free from the buzzing that often disturbs 

 conversation over the wire telephone. 



Lightning sends out electromagnetic waves 

 which have a disturbing effect upon the wire- 

 less receiver. These waves produce sounds in 

 the receiver which interfere with the receiving 

 of messages unless special means are employed 

 to prevent such interference. This is true to 

 such an extent that a wireless operator can 

 often predict a thunderstorm some hours in ad- 

 vance of its arrival. E.E.B. 



Consult Eccles' Wireless Telegraphy and 

 Telephony; Fleming's Principles of Electric Wave 

 Telegraphy and Telephony. 



Related Subject*. The reader is referred to 

 the following articles in these volumes : 



Current, Electric 

 Electricity 



Telephone 

 Wireless Telegraph 



