1909] Researches in Badiotelegrapliy. 651 



WEEKLY EVENING MEETING, 

 Friday, June 4, 1909. 



Sir William Crookes, D.Sc. F.R.S., Honorary Secretary and Vice- 

 President, in the Chair. 



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



Researches in Radiotelegraphy . 



Radiotelegraphy, popularly called wireless telegraphy, has out- 

 lived the tentative achievements of its precocious infancy and obtained 

 for itself a settled but important position amongst our means of com- 

 munication. 



This stage, however, has only been reached after a long struggle 

 with experimental difficulties and much labour in analysing the pro- 

 cesses involved. As many of these matters are of general scientific 

 interest, it is proposed, during the present hour, briefly to summarise 

 the results of some recent research. 



You are doubtless all aware that every radiotelegraphic station 

 comprises three elements. There is first the external organ called the 

 air-wire or antenna, by which the electromagnetic waves are radiated 

 and absorbed. This antenna consists of one or more wires extending 

 up into the air, either vertically or sloping, or partly vertical and 

 partly horizontal. These wires are insulated at the upper ends and 

 may be arranged fan fashion, or may form one or more nearly closed 

 loops, placed in a vertical position. The antenna is, so to speak, the 

 mouth or ear of the station, by which it speaks through the lether, or 

 by which it hears the ffitherial whispers coming to it from other 

 stations. The sether waves are produced by very rapid electric currents 

 moving to and fro in the antenna wires, and these, like the vil)rations 

 of a violin string, or the aerial oscillations in an organ pipe, set up a 

 periodic disturbance in the surrounding medium, which in the electri- 

 cal case consists of alternating electric and magnetic forces taking 

 place at each point in space around the antenna. 



There are, then, appliances in the station collectively called the 

 transmitter, which have for their function to create these powerful 

 electric oscillations in the antenna, and to control them so as to send 

 out short or long trains of aether waves in accordance with the dot or 

 dash signals of the Morse alphabet. Lastly, there is the receiving 

 apparatus which, when connected to the antenna, serves to detect the 

 presence in it of the very feeble oscillations which are being generated 

 in the antenna by the powerful oscillations in the antenna of some 

 far-distant sending station. It is usual to employ the same antenna 



