November 7, 1912J 



NATURE 



295 



difference in the dielectric constant of tlie air at low 

 and at high levels would be sufficient to give the wave 

 front a sufficient tilt to make the waves pass far 

 above a receiving antenna 200 ft. liigh, and so weaken 

 immensely the received signals because the effect is 

 cumulative, and noticeable thereforeonly at very great 

 distances. As an experimental contribution to the 

 subject I have made some preliminary observations on 

 the dielectric constant of air filled more or less with 

 damp steam or warm mist. A tubular condenser was 

 constructed of such kind that steam from a small 

 boiler could be blown between the tubes, and the 

 tubes were so supported that the condensation of the 

 steam could cause no loss of insulation. The elec- 

 trical capacity of this condenser was determined care- 

 fully by the Fleming and Dyke capacity bridge, using 

 a telephone as a detector, and alternating currents 

 having a frequency of 2760.''' Employing all neces- 

 sar\' precautions, and comparing the dielectric con- 

 stant taken as unity of air nearly saturated with water 

 vapour, with air having water globules or damp steam 

 in it, we found that the dielectric constant of air filled 

 with water spherules varied from i'026 to i'oo4, 

 according to the amount of damp steam present in 

 the space. In other words, an electric wave would 

 travel more slowly in the steam-impregnated air th.Tn 

 in the ordinary saturated air in the ratio of 9S0 to 

 1000, or 998 to 1000, or anything between these limits. 



If, then, we consider a plane electromagnetic wave 

 travelling with plane vertical and its lower end in 

 ordinarv air and its upper end in air containing 

 minute water spherules, the upper end would travel 

 more slowly than the lower, and the wave front would 

 acquire a backward tilt sufficient, in a distance of a 

 few hundred miles, to carry the wave right above an 

 ordinarv receiving antenna. If the ionisation of the 

 upper air by ultra-violet light results in the production 

 of condensation nuclei, which condense water vapour 

 round them, then it is highly probable that the upper 

 levels of the air have a slightly greater dielectric 

 constant than the lower, and a difference of even a 

 very small fraction of i per cent, will be cumulative 

 in its action on the wave in giving the wave front a 

 backward tilt in travelling over long distances. 



Mr. Marconi states that over the Atlantic he has 

 found the maximum daylight effect to occur when the 

 shadow boundary was about half-way across the Atlan- 

 tic, the sending station being in daylight and the 

 receiving station in darkness. I think the well-known 

 "sunrise nick" in the signal intensity curve is due 

 to the fact that ihe effect is at a maximum when 

 the greatest difference exists between the state of the 

 upper air as regards ionisation and that near the 

 earth. Later in the day convection currents arise to 

 churn up the air and bring it more into a homo- 

 geneous condition, which, whatever may be the state 

 of ionisation, is unproductive of any tilt in the wave 

 front. 



It has been frequently suggested that an explanation 

 of long-distance radio-telegraphy may be found in the 

 reflection of the electromagnetic waves at the imder 

 surface of a layer of ionised air in the upper atmo- 

 sphere. No proof, however, has been given that this 

 hypothetical layer of ionised air has a sufficiently 

 defined surface to cause wave reflection. Hence, it is 

 improbable that anything like copious reflection of 

 long electromagnetic waves could take place at the 

 under surf.ice of a layer of ionised air producing an 

 inverted mirage effect. This point, however, is one 

 open for discussion. It Is true that refraction may 

 produce a change of ray direction which simulates 



'■■' Kor a description o^ this >iMd?e airi method of usin? it. see the paper 

 !>y J. A. Flemine and O. R. D» lie. on the power factor and conductivity 

 of dielectrics for alternat-ni; elertric riir'enrs o'" te'ephonic frequency and 

 i temperatur..s, Joiirn. Inst F,l c. Engineers, igi2. 



NO. 2245, VOL. 90] 



reflection, as in the case of the phenomenon of the 

 mirage. In this case the intense heat of the earth 

 expands the layer of air next to it, and lowers its 

 refractive index. Hence, the lower end of a plane 

 wave of obliquely incident light travels faster than 

 the upper end, and may do so to an extent sufficient 

 to swing the ray right round, as if it were reflected 

 from the layer of heated air. For a similar effect to 

 occur with radio-telegraphic waves, it would be neces- 

 sary for the upper end of the wave front to travel 

 much faster than the lower end. In other words, the 

 upper end must be in a region of less dielectric con- 

 stant than the lower end. 



Since the above remarks were put in type, a valu- 

 able paper has been published by Dr. Eccles (see Pro- 

 ceedings Roy. Soc, vol. Ixxxvii. a, p. 79, 1912, on 

 the diurnal variations of electric waves occur- 

 ring in nature, and on the propagation of electric 

 waves round the bend of the earth), in which 

 a theorv is developed of electric wave propagation in 

 ionised air. He gives a mathematical proof that under 

 certain assumptions as to the mass ot these ions the 

 wave velocity would be increased as compared with 

 that in un-ionised air, and hence that a plane vertical 

 wave front travelling with lower end near the earth 

 and upper end in air more or less ionised by sunlight 

 would be caused to lean forward by the increased 

 velocity of its upper end. Hence he proves that, 

 according to wave length and circumstances, the wave 

 may be better able to follow round the earth's curva- 

 ture, or mav be prevented from doing it. 



On this basis he has developed a theory of long- 

 distance radio-telegraphy and of the inhibition of day- 

 light upon it. The chief criticism to which I think 

 his theory is open is that he assumes that the dielectric 

 constant of the air is unaffected bv the ionisation or 

 condensation of water vapour on these ions. This is 

 not absolutely certain. We know that in the case of 

 solutions in a state of ionisation, such as dilute solu- 

 tions of metallic hydrates in water, the dielectric 

 constant of the solvent is considerably increased. _ As 

 a rule, anything which increases conductivitv ir. a 

 dielectric increases also the dielectric coefficient. 

 Hence ionisation may do so in the case of air. If the 

 dielectric constant (K) is increased by ionisation, then 

 in the expression given by Dr. Eccles for the wave 

 velocity that velocity may be more reduced by this 

 increase in K than it is increased by the presence of 

 the ions. 



It would seem, therefore, most necessary to settle 

 by experiment whether the wave velocity is increased 

 ot- diminished bv the presence of the ions due to 

 ultra-violet light' before we can base a theory upon 

 the constancv of the dielectric coefficient. 



There is no doubt, however, (hat the earth's atmo- 

 sphere contains something which acts at times towards 

 radio-telegraphic waves like a fog or mist towards 

 light waves." 



There are also occasions of unusual transparency 

 when waves of 300 or 600 metres in length seem to 

 travel round the world in an extraordinary manner. 

 .Ships provided with the ordinary ship transmitters 

 and receivers occasionally pick up signals sent 1000 

 miles away. This is not due to special operative skill 

 but to a temporary transparency of the atmosphere 

 to radio-telegraphic waves. 



The next question to which I should like to direct 

 attention is to the present state of the theory of 

 directive antenna?. I need not so very fully info the 

 earlv historv. Mr. Marconi pointed out in iqo6 the 

 special qualities of an antenna consisting of a long 

 wire arranged so that part is vertical, but the grea'ter 



1*^ I"terestinc ohs-r/ai-ons hav- he-n made on tliis matter by Admiral 

 Sir Henrv Jackson, F.R.S. See Proc. Roy. Soc, Lond., vol. Ixx 

 p. !54, 1902. 



