456 BELL SYSTEM TECHNICAL JOURNAL 



eighties, when air was regarded as an almost perfect insulator and 

 nothing was known that could make it conductive, was certainly 

 audacious. To speak of it in 1902 was still ingenious but no longer 

 daring, for by then it could reasonably be expected that the researches 

 on ions lately begun by Thomson and so many others would justify 

 the notion. 



Never was an expectation better founded. Within a few years 

 those researches had made it sure that the upper atmosphere must be 

 conductive, because of containing the raw material required for making 

 ions and one at least among the agents capable of making them: to 

 wit, atoms and molecules, and ultra-violet light from the sun capable 

 of ionizing them. The problem then became: what distribution and 

 what kinds of ions must be postulated for the upper atmosphere, to 

 explain (for instance) the reflection of radio signals? 



This problem could not even be attacked, without great forward 

 strides in both the art of experimentation and the mathematical 

 theory. These strides were rapidly made in the middle and late 

 twenties. Had theory alone gone ahead, it would have been little 

 more than a pretty exercise in mathematics. Had the art of experi- 

 mentation progressed by itself, the experimenters would at least have 

 found some interesting correlations of the data with such variables as 

 time of day and epoch of the solar cycle and presence of magnetic 

 storms; but the lack of theory would have been sorely felt. But 

 theory and experiment advanced together, and the interplay between 

 the two has seldom been so well exemplified. 



The advance in the art of experiment lay not so much in the inven- 

 tion of new apparatus (though this has not been wanting) as in turning 

 away from the practical problem of sending signals to great distances, 

 and instead designing the experiments for the purpose, first of proving 

 the ionosphere and then of "sounding" it. Three methods were 

 invented for this purpose, all based upon the fact that wireless waves 

 when sent into the sky come bouncing back from it. Two of these 

 will be scarcely more than mentioned in the pages to follow, since an 

 already great and ever-increasing proportion of the data is obtained 

 by the third. In this third a sharply-delimited signal or pulse or 

 wave-group is sent up, and a short time (a few milliseconds) later it 

 is detected coming back, like an echo from a cliff: the delay of the 

 echo is measured. This is done for many signals, and the delay is 

 plotted against the mean frequency of the wireless waves composing 

 the signals; and curves so plotted constitute the ultimate data. Usu- 

 ally the signal is sent vertically upward, the echo comes vertically 

 downward; and there is the quaint situation, that wireless telegraphy 



