248 Signor G. Marconi [Feb. 2, 



may be justified in saying that for several years the full importance of 

 the discovery of Hertz was realised 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. I 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 

 previously constructed — that is, a tube several inches long partially 

 filled with filings enclosed by corks — was far too untrustworthy to 

 fulfil its purpose. I found, however, that if specially prepared filings 

 were confined in a very small gap (about 1 mm.) between flat plugs of 

 silver, the coherer, if properly constructed, became absolutely trust- 

 worthy. In its normal condition the resistance of a good coherer is 

 infinite, but when influenced by electric waves the coherer instantly 

 becomes a conductor, 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 impossible 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 (see paper read before the Institution 

 of Electrical Engineers by G. Marconi, March 1899) that the distance 

 over which it is possible to signal with a given amount of energy 

 varies approximately with the square of the height of the vertical 

 wire, and with the square root of the capacity of a plate, drum, or 

 other form of capacity area which may be placed at the top of the 

 wires. 



The law governing the relation of height and distance has already 

 been proved correct up to a distance of 85 miles. Many months ago 



