1920] The Thermionic Valve 161 



WEEKLY EVENING MEETING, 



Friday, May 21, 1920. 



His Grace The Duke of Northumberland, M.V.O., President, 



in the Chair. 



Professor J. A. Fleming, M.A. D.Sc. F.R.S. M.R.I., 



University Professor of Electrical Engineering 

 in the University of London. 



The Thermionic Valve in Wireless Telegraphy 

 and Telephony. 



The Thermionic Valve is an invention which has vastly increased 

 the powers and range of wireless telegraphy. 



It has provided simple and efficient means for conducting wireless 

 telephony over great distances, even across the Atlantic Ocean. It 

 has given to telephonic engineers a very perfect form of relay or 

 repeater by which it is possible to transmit speech through copper 

 wires of given section to greater distances than without it, or con- 

 versely to reduce considerably the amount of copper required for 

 good speech over the same distance. It has placed a new implement 

 in the hands of the scientific investigator, aiding him in various 

 branches of research, and lastly it presents itself as an object of 

 scientific enquiry by no means exhausted of all that it has to teach. 



Like many other inventions, the telephone for instance, it is 

 simple in its essential construction. It consists of a little electric lamp 

 comprising a glass bulb, very highly exhausted of its air, containing 

 a filament of carbon, or better tungsten, which can be rendered incan- 

 descent by an electric current. Within the bulb and around the 

 filament are fixed certain metal plates or cylinders, and it may be 

 spirals of wire or metal networks called the grid. 



To explain its origin in its simplest form I shall have to take you 

 back in thought to the days when the physical effects taking place 

 in incandescent electric lamps were first beginning to be carefully 

 considered. 



More than thirty years ago I had the honour of giving a Friday 

 Evening Discourse in this theatre on some " Problems in the Physics 

 of an Electric Lamp," and for the sake of those now present to whom 

 the subject is new, I shall repeat one or two of the experiments then 

 shown as a starting point for further explanations. - * 



* See The Proceedings of the Eoyal Institution of Great Britain, vol. xiii, 

 p. 34, Feb, 14, 1890, " Problems in the Physics of an Electric Lamp." 



Vol. XXIII. (No. 114) m 



