12 



DISCOVI-HV 



within range which are adjusted to receive it — the 

 difficulty as regards wave lengths is of course reduced, 

 and some interesting demonstrations on these lines 

 have recently licen given in this country. On one 

 occasion last June, Dame Melba sang into a transmitter 

 at the Marconi Company's station at Chelmsford, and 

 the songs were heard clearly at many wireless receiving 

 stations on land and sea up to distances of several 

 hundreds of miles. It is possible that such a broad- 

 casting system might be utilised for news messages 

 to daily papers, and tests have recently been carried 

 out for the Press Association. These tests, however, 



were introduced. Even these, however, did not 

 produce radiation of exactly the sort required, and 

 it was undoubtedly the introduction of the Three- 

 Electrode Valve as a generator that gave wireless 

 telephony a fair start. 



The Three-Electrode Thermionic Valve (Fig. i) con- 

 sists essentially of a vacuum bulb similar to the ordinary 

 incandescent lamp, but with two metallic electrodes 

 inside the bulb in addition to the filament. When 

 this instrument is connected with suitable circuits 

 to a source of electric power, electric oscillations — that 

 is, electric currents oscillating at very high speeds — 



'Aeria! 



Aerial 



Fig. I. 

 A tlirce-clcctrode transmitting 

 valve. A is a plate of sheet 

 metal, b is a cylinder of .nar- 

 row mesh wire 'gauze, c is a 

 wire filament. The glass vessel 



is highly evacuated. FiG. : 



showed that such an arrangement was not suitable 

 for immediate use. 



In comparison with wireless telegraphy, the advances 

 in wireless telephonj' were e.xtremely slow until about 

 six years ago, when the Three-Electrode Thermionic 

 Valve was introduced as a generator of wireless waves. 

 Since then, however, such rapid progress has been 

 made that its great future is already well assured. 

 The principal reason why telephony lagged so far 

 behind was that a continuous radiation of uniform 

 ether waves which could be modulated by the voice 

 was essential, whereas in telegraphy a radiation of 

 groups of waves sufficed, a radiation such as was 

 obtained from the spark generators used in all the 

 earlier wireless telegraph transmitters. The first real 

 attempts at wireless telephony were made possible 

 when arc generators and high-frequencv alternators 



are generated, and when these currents are caused to 

 flow in an aerial, a continuous stream of uniform ether 

 waves is radiated. For telegraphy, these oscillations, 

 and hence the waves, are broken up into short and 

 long groups bj- a signalling ke\-, corresponding to the 

 dots and dashes of the Morse Code ; and for telephony 

 they are modulated by the voice speaking into a 

 microphone, just as the current in the line is modu- 

 lated in line telephony. A common form of micro- 

 phone consists of a small chamber filled with carbon 

 granules through which the current flows, the current 

 remaining constant so long as the granules are 

 quiescent, but varying in strength owing to the 

 varying electrical resistance, as the granules are made 

 to vibrate by the voice. 



The microphone transmitter may be placed as in 

 Fig. 2 at M in the aerial itself, so as to modulate the 



