WIRELESS TELEPHONE 



6326 



WIRELESS TELEPHONE 



electricity rushes into the condenser to charge 

 some of the current is thus taken away 

 from the arc by the condenser. When the con- 

 denser is fully charged it discharges back 

 through the arc, and thus electricity surges 

 back and forth into and out of the condenser 

 and likewise across the arc. This surging back 

 and forth of electricity is called an oscillation, 

 and every electric oscillation sends out waves 

 in the ether (see ELECTRICITY). The waves in 

 this instance are uniform, because the oscilla- 

 tions are equal in every respect. Such an arc 

 produces a musical note and is, therefore, some- 

 times called a singing arc; since it is used in 

 the transmission of speech, it is also called a 

 speaking arc. 



Early Attempts. An early form of wireless 

 telephone employed the speaking arc, with a 

 reflector which projected the light of the arc 

 like the beam of a searchlight. Along this 

 beam of light speech was transmitted, that is, 

 the beam of light transmitted the impulses 

 which reproduced the speech. The receiving 

 device consisted of a number of strips of sele- 

 nium (an element whose electrical resistance 

 is changed by the action of light), a telephone 

 receiver and a battery. As the light from the 

 speaking arc fell upon the selenium the battery 

 current varied in strength to correspond with 

 the changes in the sound from the arc. This 

 changing current produced the same sounds in 

 the receiver. Speech could be transmitted by 

 this means as far as a beam from a searchlight 

 could be projected. 



While the earlier wireless telephones used a 

 beam of light, the modern wireless telephone 

 makes use of a stream of electric waves which 

 are like light waves in every respect except 

 that they are longer. The short light waves can 

 be used only for short distances because they 

 are checked by many objects through which 

 the longer electric waves can readily pass. For 

 example, electric waves can go through a brick 

 wall, but light cannot do so. 



Theory of the Wireless Telephone. The 

 number of oscillations per second produced by 

 the musical arc varies from about 75,000 to 

 100,000. An equal number of electric waves go 

 out from the antenna, or aerial, in all directions. 

 At the receiving station these waves produce 

 oscillations in the receiving antenna similar 

 to those of the sending antenna. The oscil- 

 lations at the receiving station pass through 

 the coil of a telephone receiver. Whenever an 

 electric oscillation, that is, an alternating cur- 

 rent, passes through the coil of a telephone 



receiver it causes the diaphragm to vibrate; 

 hence, the continuous stream of electric waves 

 produces vibrations in the receiver, but they 

 are so rapid that they produce no sound that 

 can be heard. The reason is that the human 

 ear cannot hear a sound which consists of more 

 than about 20,000 vibrations per second, while 

 the musical arc gives out, as has been stated, 

 75,000 or more electric oscillations per second. 

 The stream of electric waves going out from 

 the sending station serves the same purpose as 

 the electric current which flows along the line 

 of the wire telephone. 



Telegraph and Telephone Contrasted. In the 

 wire telegraph the circuit is made and broken 

 for every dot or dash, so the telegraph current 

 may be called an interrupted current, the key 

 making and breaking the circuit. On the con- 

 trary the telephone current is not interrupted. 

 When a person is talking over the wire tele- 

 phone a current (alternating) must flow con- 

 stantly over the line wire. The sound of the 

 voice does not make and break the circuit, but 

 it changes the strength of the current and the 

 rate of alternation. There is the same differ- 

 ence between the wireless telegraph and the 

 wireless telephone. In the wireless telegraph 

 the stream of electric waves is interrupted for 

 every dot or dash, while in the wireless tele- 

 phone the stream of electric waves flows con- 

 stantly when the telephone is in service. The 

 sound of the voice varies the strength, the form 

 and the length of the waves. Just as the alter- 

 nating current in the wire telephone varies in 

 strength and fate of vibration to correspond to 

 all the variations in the voice, so does the 

 stream of electric waves of the wireless tele- 

 phone vary in form, strength and number per 

 second, to correspond to all the variations in 

 quality, loudness and pitch of the voice. 



It is for this reason that the transmitters 

 of the wireless telegraph and of the wireless 

 telephone differ. The spark coil of the wireless 

 telegraph sends out an interrupted stream of 

 electric waves; each spark discharge sends out 

 a stream of waves that fade out, as the sound 

 of a bell, which when struck once, gradually 

 dies away. On the other hand, the waves from 

 the speaking arc do not fade out, but continue 

 of the same strength as long as the current 

 flows across the arc, except as their strength is 

 varied by the sound of the voice acting upon 

 the transmitter. 



Each spark of the wireless telegraph spark 

 coil consists in reality of a series of sparks. 

 When the key is pressed the interrupter rapidly 



