June 24, 1920] 



NATURE 



5*9 



Wireless 

 By Prof. W 



WIRELESS telephony has made such rapid 

 progress during the past six or seven years 

 that it must now be looked upon as a possible 

 rival to wireless telegraphy for communication 

 over distances up to a thousand miles. Although 

 telephonic communication demands on normal 

 occasions the expenditure of more power than 

 does communication by Morse '?igns, yet the 

 superior rapidity with which thought can be con- 

 veyed by voice transmission is a weighty advan- 

 tage ; and, besides, telephony often proves more 

 successful than telegraphy when strays and analo- 

 gous disturbances are bad, partly because the ear 

 is so skilful in followino- the voice in the midst of 

 other noises, and partly because the context 

 greatly assists comprehension. Many of the 

 recent improvements by which ♦^he present position 

 in wireless telephony has been reached are due to 

 the development of the thermionic vacuum valve 

 with three electrodes — called^, for short, the triode. 



The essential difference between wireless tele- 

 phony and wireless telegraphy is that the voice 

 is used instead of the Morse key to produce 

 alterations in the radiated electric waves. In 

 continuous-wave wireless telegraphy the Morse 

 key, and in wireless telephony the voice, may be 

 applied in two ways : (i) for altering the wave- 

 length, and (2) for altering the amplitude, of the 

 oscillations in the antenna. A distant receiving 

 station capable of sharp response to the normal 

 wave-length of the sending station picks up less 

 energy from the altered waves passing over it, 

 whichever type of alteration is in use at the send- 

 ing end ; for if the amplitude at the sender is 

 altered, the amplitude of the electric and mag- 

 netic fields produced at the receiver is changed 

 correspondingly, while if the wave-length at the 

 sender is altered, the receiving station responds 

 less, because the incoming waves are out of tune 

 with it. In many telegraph and telephone systems 

 both types of alteration occur together. 



Once the source of continuous waves is avail- 

 able, the main problem in wireless telephony is 

 to provide means of exciting the transmitting 

 antenna in accordance with acoustic vibrations 

 produced by the voice. The process of moulding 

 the oscillatory currents by means of the voice 

 has come to be called "modulation," and the 

 apparatus used, if distinct from the rest of the 

 transmitting apparatus, is called the "modulator." 



,The obvious method of modulating a given 

 high-frequency alternating current is to use the 

 familiar apparatus of ordinary line telephony. In 

 our ordinary line telephone services direct current 

 is passed through a carbon microphone, and is 

 constant in value so long as the granules in the 

 microphone are quiescent, but when the granules 

 are made to vibrate by the voice, the current is 

 correspondingly modulated, and may be made to 

 produce sound by the familiar telephone receiver 

 consisting of an electromagnet and an iron dia- 

 NO. 2643, VOL. 105] 



Telephony. 



. H. ECCLES. 



phragm or reed. In wireless telephony the micro- 

 phone may be used in a precisely analogous way 

 by being placed in the antenna as shown in Fig. i, 

 or in an earlier circuit as shown in Fig. 2. In 

 the apparatus of these diagrams the oscillatory 

 current may come from an arc, an alternator, or 

 a triode. 



A different method of modulating a given source 

 of supply was advocated, especially by R. A. 

 Fessenden in America, early in the history of wire- 

 less telephony. The essence of this method was 

 the employment of a condenser of which one 

 surface could be moved relatively to the other by 

 the voice, and this was usually associated with the 

 antenna of the sender. Alterations of the electrical 

 capacity of the condenser produce departures from 

 resonance, and therefore alter the amplitude ex- 

 cited in the antenna by the source of oscillations. 

 On the other hand, the condenser may be used 

 in the circuit generating the oscillations, especially 

 when the source is an arc or a triode, and in this 

 case the frequency of the oscillations supplied to 

 the antenna is modulated by the voice, and conse- 



highlre^ency 



quently both amplitude and frequency of the oscil- 

 lations in the antenna are modulated. The con- 

 denser has to be of special construction in order 

 that its capacity shall be variable at a frequency 

 of 1000 per second. In the recorded experiments 

 it has consisted of a thin diaphragm placed very 

 close to a fixed parallel plate, and the diaphragm 

 has been acted upon either directly by the voice 

 or indirectly by means of some magnified micro- 

 phone currents passing through an electromagnet. 

 The above two methods accomplish modula- 

 tion by variation of the resistance and of the 

 capacity respectively of one or other of the oscil- 

 latory circuits. It is natural to consider the possi- 

 bility of varying the remaining electrical magni- 

 tude — namely, the inductance, self or mutual. 

 The variation of self-inductance has been employed 

 by both German and American experimenters, but 

 perhaps the most successful is that due to 

 E. F. W. Alexanderson, of the General Electric Co. 

 of America. In a broad sense this experimenter 

 takes advantage of the dependence of the permea- 

 bility of iron upon the intensity of the magnetic 



