288 WIRELESS TELEGRAPHY. 



of light and electricity, and the knowledge of how to produce and how 

 to detect these ether waves, the existence of which had been so far 

 unknown, made possible true wireless telegraphy. * I think I may be 

 justified in saying that for several years the full importance of the dis- 

 covery of Hertz Avas realized but by very few, and for this reason the 

 early development of its practical application was slow. 



The practical application of wireless telegraphy at the present time 

 is many times as great as the predictions of five years ago led us to 

 expect in so short a time. The development of the art during the past 

 three or four years and its present state of progress may perhaps 

 justify the interest which is now taken in the subject. Yet only a 

 beginning has been made and the possibilities of the future can as yet 

 be only incompletely appreciated. All of you know that the idea of 

 communicating intelligence without visible means of connection is 

 almost as old as mankind. Wireless telegraphy by means of Hertzian 

 waves is, however, very young. 1 hope that if I pass over the story 

 of the growth of this new art, as I have watched it, or do not attempt 

 to prove questions of priority, no one will take it for granted that 

 nothing is to be said on these subjects, or that all that has been said 

 is entirely correct. 



The time allowed for this discourse is too short to permit me to 

 recount all the steps that have led up to the practical applications of 

 to-day. I believe it will probably interest you more to hear of the 

 problems which have lately been solved, and the very interesting 

 developments which have taken place during the last few months. 



I find that a great element of the success of wireless telegraphy is 

 dependent upon the use of a coherer such as I have adopted. It has 

 been my experience, and that of other workers, that a coherer as pre- 

 viously constructed — that is, a tube several inches long partially filled 

 with filings inclosed by corks — was far too untrustworthy to fulfill its 

 purpose. I found, however, that if specially prepared filings were con- 

 fined in a very small gap (about 1 mm.) between flat plugs of silver, 

 the coherer, if properly constructed, became absolutely trustworthy. 

 In its normal condition the resistance of a good coherer is infinite, but 

 when influenced by electric waves the coherer instantly becomes a con- 

 ductor, its resistance falling to 100 or 500 ohms. This conductivity is 

 maintained until the tube is shaken or tapped. 



I noticed that by employing similar vertical and insulated rods at 

 both stations it was possible to detect the effects of electric waves of 

 high frequency, and in that way convey the intelligible alphabetical 

 signals over distances far greater than had been believed to be possible 

 a few years ago. 



I had formerly ascertained'' that the distance over which it is possi- 



» See paper read before the Institution of Electrical Engineers by G. Marconi, 

 March, 1899. 



