EVENING DISCOURSE. 



WIRELESS ECHOES 



BY 



PROF. E. V. APPLETON, F.R.S. 



One of the most striking features of the history of wireless communication is the- 

 way in which practice has so often been ahead of theory. It is true that, at the 

 beginning, the theoretical predictions of Clerk Maxwell concerning the existence of 

 electric waves had been published thirteen years before they were practically realised 

 by Hertz. But such a striking precedence of theory before practice has not often 

 occurred since. Indeed, for the last thirty years things have usually been just the 

 opposite. Things deemed impossible or nearly so by the theorist have always been 

 happening, and we have been able to accomplish more than we could understand. 



This evening I want us to consider one of these fields of wireless investigation in 

 which unsolved problems abound. I refer to the subject of the propagation of 

 short waves. To justify our theoretical ignorance to some extent, I ought to say that 

 we can see at any rate why unsolved problems should exist. We know nowadays 

 that when wireless waves are emitted by a sending station they travel upwards into 

 regions through which we cannot follow them ; and it is only at the "etarting and 

 finishing points of their journey that we can make any observations on them. 



We may first consider a special case. Suppose we wish to send wireless signals 

 from one point on the earth's surface to another, say fifty mUes away. How do the 

 waves travel from one place to the other ? At first we might naturally be inclined 

 to say, ' Why, there is no difiiculty about it ! We can take a wireless receiver to all 

 intermediate points between the two stations and still hear the signals, so obviously 

 the waves have travelled straight along the ground from one station to the other.' 

 That would be quite, true, but it would be only part of the truth. It can be shown 

 that waves also reach the receiving station by much more circuitous routes. A little 

 consideration will show us that this may be possible. The earth, I need hardty 

 remind you, is not flat but round, so that there are two direct paths from one place 

 to another. The question may be asked, ' Why don't some of the waves come round 

 the earth the other way ? ' And the answer is that they do ; but, of coiu'se, since the 

 journey is much longer they take a longer time to do so. 



I have mentioned this particular example because it brings us to one of the most 

 fundamental points we have to consider to-night. If it is true that a wireless station 

 receives signals other than those which have travelled straight over the shortest 

 distance, how are we to find out where such wandering waves have been ? If we 

 wished to estimate how long a journey a traveller had made on a trip by train, we 

 could use the time taken on the journey and an estimate of the speed of travel. We 

 might also take into account the appearance of the traveller, fresh or otherwise, at 

 the end of the journey. Now such information is exactly that which we are able 

 to obtain from our wireless experiments. We are able to observe the times at which 

 these vagrant waves start out and arrive, and thus find the time they have taken on 

 their journey ; we also know their speed through the air, and from their strength or 

 weakness at the end of the journey we can make deductions as to the kind of time 

 they have had on the way. It sometimes happens that the waves which have made 

 a particularly long or difficult journey are so distorted on arrival as to be almost 

 unrecognisable, so that in justice to our railways we must admit that the analogy with 

 the railway traveller breaks down somewhat. One of the most frequent forms of 

 distortion is that in which waves sent out travelling in a normal erect fashion are 

 found to arrive travelling in a horizontal recumbent position, their bodies, as it were, 

 having been twisted through a right angle. Again we must gratefully admit that our 

 railway analogy has broken down. 



I mentioned that we know the velocity with which waves travel, namely 300,000 

 kilometres per second, or 186,000 miles per second — which is, of course, the velocity 

 of light. Actually, as we shall see, sometimes the speed of the waves, along some 



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