116 



DISCOVERY 



taken as an example of signalling by light waves, 

 flashes of short and long duration representing the dots 

 and dashes of the Morse Code. These flashes consist 

 of trains of ether waves which radiate in all directions 

 at a speed of 186,000 miles a second, the oscillations 

 getting weaker and weaker as the distance from the 

 lamp increases, until at last they have dissipated their 

 energy and die away altogether. The eye can be used 

 to receive these signals, and the message in the Morse 

 Code can be read simultaneously from all directions by 

 any number of people within range, provided that there 

 are no screening effects caused by opaque bodies inter- 

 vening between their eyes and the lamp. 



Wireless signals are sent in a very similar manner 

 by short and long trains, or series of trains, of ether 

 waves which travel outwards from the sending station 

 in all directions at this same speed of 186,000 miles a 

 second. These trains of waves actuate, as dots and 

 dashes, according to their duration, the wireless receiv- 

 ing instruments at any number of stations which are 

 within range, and are adjusted to respond to the 

 particular wave length used. 



These long ether waves are able to pass readily 

 through bodies which are opaque to light or heat waves, 

 unless such bodies consist of, or contain, conductors of 

 electricity, in which case the waves produce currents 

 of electricity in the conductors, and so part with some 

 of their energy. 



Screening effects in the case of wireless waves are 

 therefore very different from those which obtain in the 

 case of light waves ; for example, buildings, etc., opaque 

 to light waves, may present no obstacle whatever to 

 the passage of wireless waves ; even mountains, and 

 indeed the obstacle caused by the earth itself, do not 

 affect these waves in the same way as thej' affect light 

 waves. 



For instance, there is no difficulty in signalling by 

 wireless waves across the Atlantic, whereas it is im- 

 possible to signal over distances of more than a com- 

 paratively few miles by light waves, owing to the earth 

 imposing, due to the curvature of its surface, an im- 

 penetrable obstacle to such waves. In the case of the 

 Atlantic Ocean this obstacle would represent a moun- 

 tain some hundreds of miles in height, and the waves 

 do not actually pass through this obstacle, but may 

 be considered as gliding over the surface. Like hght 

 waves they are subject to absorption, to reflection, 

 and to other forms of bending, as refraction and 

 diffraction, on account of which mountains or other 

 high land close to a wireless station may produce 

 serious screening in that direction, whereas the same 

 obstacle at a distance may have very little or even 

 no appreciable effect. Short waves are much more 

 affected by this land screening than are long waves ; 

 but in any case it is advisable to avoid siting a wireless 



station close to higher ground in a direction towards 

 which communication is required. 



Absorption and bending effects in the atmosphere 

 account for the fact that signals, especially when using 

 short waves, can be transmitted to much greater dis- 

 tances by night than by day, and that these night 

 ranges may vary considerably during the same night, 

 and from one night to another. 



For signalling by wireless, the sending station does 

 not transmit a number of ether waves of various 

 lengths at the same time, cis is the case when signalling 

 by light, but makes use of one wave length only for 

 that particular transmission. 



All wireless communication is not, however, carried 

 out on the same wave length, as if it were the receiving 

 stations would receive a jumble up of all the messages 

 being sent on that wave length from stations witfiin 

 range, instead of the particular message which it was 

 desired to receive. When the reception of a message 

 is thus interfered with by other messages being sent 

 at the same time, the message is said to be " jammed." 

 This jamming of a message may also be caused by stray 

 ether disturbances in the atmosphere itself. These 

 disturbances are called " atmospherics " or " strays," 

 and are specially strong when thunder-storms are in 

 the vicinity, Ughtning itself causing ver\- \iolent atmo- 

 spheric disturbances. Atmospherics often cause great 

 interference with wireless communications, and in 

 tropical countries are sometimes so strong as to prevent 

 any communication at all for hours at a time. They 

 arc strongest as a rule at night and in the afternoon. 

 In more temperate latitudes, as in this country, atmo- 

 spherics are usually at their worst during the night in 

 summer-time, but are seldom so troublesome as those 

 experienced in warmer climates. 



The actual choice of the wave length will depend on 

 various considerations — for example, high-power long- 

 range stations cannot be arranged to transmit short 

 waves, and low-power short-range stations, such as those 

 in ships, cannot be arranged to transmit long waves. 



All commercial ship and coast stations arc fitted so 

 as to be able to transmit on wave lengths of 600 and 

 300 metres, the intention being that communication, 

 if jammed on the one, may possibly be established on 

 the other. 



The S.O.S., or distress signal, is always sent on the 

 600 metres wave, as this is the wave length for which 

 the receiving apparatus in all ships is normally adjusted. 



BOOIvS RECOMMENDED 



Waves and Ripples in Waler, Air, and Aether. By J. A. 



Fleming, D.Sc, F.R.S. (S.P.C.K., 7s. 6d.) 

 Textbook on Wireless Telegraphy. Vol. I, General Theory and 



Practice. By Rupert Stanley, B.A., Temp. Major R.E. 



(Longmans, 15s. net.) 



