166 Professor J. A. Fleming [May 21, 



The formula also shows that this thermionic current increases 

 very rapidly with the temperature. At a constant temperature and 

 in a very high vacuum the thermionic current increases as the 

 1*5 power of the potential difference of the anode and hot cathode. 



It is remarkable that although this emission of electricity from 

 incandescent substances bad been studied by leading physicists for 

 more than a quarter of a century, none of them (not even Mr. Edison) 

 made any practical application of it prior to 11)04 in telegraphy or 

 telephony, or indeed in any other way. At that date I was so 

 fortunate as to discover a new and totally unexpected application of 

 this thermionic emission in wireless telegraphy. By that time 

 wireless telegraphy by electromagnetic waves had been brought into 

 a condition of considerable utility, chiefly by the work of Senator 

 Marconi and his collaborators. 



I shall assume that the broad general principles of it are familiar 

 to all present, and that it is understood that this kind of telegraphy 

 is accomplished by electromagnetic waves, which differ only from 

 ordinary light in that their wave-length is reckoned in hundreds or 

 thousands of feet rather than fractions of an inch. The waves sent 

 out from the great Carnarvon Wireless Station, for instance, have a 

 wave-length of 50,000 feet, whereas yellow-green light has a wave- 

 length of 3 0^-ooth part of an inch. 



When these long electric waves strike an elevated aerial wire they 

 create in it very feeble alternating electric currents, and these have 

 to be appreciated by sensitive instruments called detectors. 



Before 1904 only three kinds of detector were in practical use in 

 wireless telegraphy — viz. the coherer, or metallic filings detector, the 

 magnetic-wire detector, and the electrolytic detector. The first 

 operated in virtue of the fact that feeble electrical oscillations can 

 alter the electric conductivity of collections of metallic particles. 

 The second depended upon the power of electric oscillations to shake 

 up the molecules or groups of molecules of an iron wire and promote 

 magnetisation or demagnetisation ; and the third by reason of the 

 fact that rapid alternating currents can improve the contact between 

 a small platinum wire electrode and an acid electrolyte in which it 

 is immersed and by which a small direct current is passing into the 

 liquid. 



The coherer and the electrolytic detectors were both rather 

 troublesome to work with on account of the frequent adjustments 

 required. The magnetic detector was far more satisfactory, and 

 in the form given to it by Senator Marconi is still used. It is 

 not, however, very sensitive, and it requires attention at frequent 

 intervals to wind up the clockwork which drives the moving iron 

 wire band. 



In or about 1904 many wireless telegraphists were seeking for 

 new and improved detectors. 



I was anxious to find one which, while more sensitive and less 



