October 31, 1912] 



NATURE 



205 



rectitied current is in tlie opposite direction to the 

 tiiernioelectric current produced by heating the 

 junction. 



In addition to these glow-lamp and crystal rectifiers 

 another much-used detector is Marconi's magnetic 

 detector, in which a slowly moving band of iron wires 

 passes across the poles of a pair of horseshoe magnets. 

 The wire at that place is embraced by two other coils 

 of wire— one in series with the oscillating circuit of 

 the receiver and the other with a telephone. When 

 trains of oscillations are set up in the receiving 

 antenna a listener at the telephone hears a sound due 

 to the sudden change in the magnetic state of the 

 iron. The simplicity and absence of any difficult ad- 

 justments make this magnetic detector one of the 

 most useful for general purposes. 



The wireless message is thus picked up at the 

 receiving station by hearing telephonic sounds due to 

 a greater or less number of trains of high-frequency 

 oscillations in the transmitting antenna corresponding 

 to dashes or dots in the Morse alphabet. 



These long or short groups of oscillations in the 

 transmitting antenna create similar groups in the 

 receiving antenna, which, w-hen rectified, cause gushes 

 of electricitv in one direction through the telephone, 

 and therefore make sounds like ticks or musical note> 

 of long or short duration. The pitch of this note is 

 the frequency of the spark at the sending station. 



One of the practical difficulties not yet quite over- 

 come is the invention of a suitably simple and sensi- 

 tive call signal. At present the operators have to sit 

 with the telephone on their heads waiting for any 

 message which may begin, and this is expert work 

 which cannot be deputed to anyone else.' Another 

 requireinent is a simple and yet sensitive relay by 

 W'hich the messages may b3 printed down on paper 

 tape. The photographic method employing the Ein- 

 thoven galvanometer is effective but ratlier elaborate. 



The recentiv invented alternating-current resonance 

 relav of Dr. Kapp and Mr. H. von Kramer is sensi- 

 tive, and can be operated with an alternating current 

 having a frequency of about loo, and one-fifth of a 

 milliampere in value. What is required is a relay 

 sensitive to currents of a frequency varying between 

 50 and 500 or so, and a strength of about one-tenth 

 of a microampere. 



Having thus outlined the manner in which the 

 radio-telegraphic message is sent, I now pass on to 

 propound for your discussion certain imperfectly solved 

 scientific questions. The first of these is : — • 



Bv what mechanism or process are the signals 

 conveyed across the intervening space between the 

 transmitter and the receiver? Most persons would 

 sav, at once, by electromagnetic or Hertzian waves, 

 produced in the aether, and the answer is no doubt 

 correct so far as it goes. The action of the sending 

 antenna on the receiving antenna is not merely an 

 instance of one electric current inducing another in 

 a secondary circuit as in the magnetic induction form 

 of telegraphy. In radio-telegraphy the energv sent 

 out from the sender, no doubt, departs from it entirely 

 and exists for a time in a medium before it reaches 

 the receiver. The question is what is that medium? 

 The whole of the actions in the sending antenna by 

 which the distance effect is produced are consistent 

 with the assumption that electromagnetic waves are 

 sent out from it. But are these waves, strictly speak- 

 ing, Hertzian waves or space waves? What part, 

 of any, does the earth play in the process? Are the 

 very long distances which can be covered by modern 



3 The Marconi Co hav.> recently introduced a call instrument, in which a 

 Mgnal equivalent to a p'oloneed t/ash on the Morse code deflects a galvano- 

 meter, which in turn cloves a bell-bat'er^- circuit and rings a bell. The 

 difficultv is, however, to prevent atmospheric discharges from making a 

 false call, but render it sensitive only to a prearranged signal. 



NO. 2244, VOL. 90] 



radio-telegraphy consistent with the properties of pure 

 Maxwellian or Hertzian waves produced in the Eether. 

 These are the first unsettled questions I wish to throw 

 down for discussion. As soon as Transatlantic signals 

 had been received by the means already described, 

 physicists began to ask how such waves, if they are 

 true electromagnetic waves, are propagated one-eighth 

 of the way round the earth. Since then Mr. Marconi 

 has achieved the feat of receiving signals in South 

 America from his Clifden station in Ireland at a dis- 

 tance of 6000 miles. The problem now is to e-xplain 

 how this effect travels one-quarter of the way round 

 the earth. It suggests at once the query, could it 

 go half-way round? Can wireless signals be received 

 in New Zealand from England, and may we look 

 forward not merely to Transatlantic or Transpacific, 

 but to transterrestrial w'ireless telegraphy to the Anti- 

 podes as a practical possibility? The answer to these 

 questions is necessarily connected with that to the 

 more general question, how does the sending antenna 

 aft'ect the receiving antenna at any distance? In a 

 year or more, when the Imperial wireless scheme comes 

 into operation and the long-distance stations are com- 

 pleted, London will speak to Aden ; Aden to Bangalore 

 and Pretoria ; Bangalore to Singapore ; and 

 thence the step will be easy to .Australia and New- 

 Zealand. It is possible that we may yet communicate 

 from London direct to Melbourne without the inter- 

 mediate stations. In text-books and lectures it has 

 been usual, for the sake of simplicity, to treat the 

 problem of radio-telegraphy as if the earth were a 

 perfectly conducting sphere immersed in free aether. 

 A very little practical experience showed wireless tele- 

 graphists that the electric condition of the atmosphere 

 greatly affected it, and that the receiving apparatus, 

 so sensitive to waves intelligently sent out from trans- 

 mitting stations, picked up in addition all manner of 

 vagrant waves set going by atmospheric discharges. 

 .Also early attempts at long-distance radio-telegraphy 

 led Marconi to the discovery of the great influence of 

 daylight upon the distances attainable. If, however, 

 we leave out of account for the present these atmo- 

 spheric and daylight disturbances, to which we shall 

 return presently, we have still to face the fact that 

 the nature of the terrestrial surface between the send- 

 ing and receiving station affects the result very appre- 

 ciablv. 



Very earlv in the practical experience of radio- 

 telegraphy it was found that it could be conducted 

 more easily over sea than over land, and more easily 

 over ordinary wet soil than over very dry sandy soil. 



But apart altogether from this last effect, it has 

 alwavs been felt that there was something surprising 

 in the fact that it is possible to detect electromagnetic 

 waves created at a distance of one-eighth to one- 

 quarter of the way round the world. It has been 

 generally assumed " that this was wholly due to an 

 abnormally large diffraction effect. The first question 

 of importance is, then, whether diffraction can occur 

 to an extent sufficient to account for the observed 

 facts. The determining factor as regards diffraction 

 is the ratio of wave length to the earth's diameter. 



In the early attempts at long-distance wireless tele- 

 graphy wave lengths of 2000 to 3000 ft. were used, 

 but at the present time wave lengths from 10,000 to 

 20,000 ft. are employed, or, say, one-thousandth of 

 the earth's radius. 



Consider for one moment an optical analogue. The 

 mean w-ave length of visible light is about i /5n,oooth 

 of an inch. Suppose a luminous point of infinitely 

 small magnitude were placed at the pole on the surface 

 of a smooth sphere a quarter of an inch in diameter or 

 about the size of a pea, in a region otherwise not 

 illuminated. This corresponds to the case of electric 

 waves 1000 metres in wave length sent out from a 



