HERTZIAN ^YAVE ]Y I RE LESS TELEGRAPHY. 439 



HERTZIAN WAVE WIRELESS TELEGRAPHY. IV. 



By Dr. J. A. FLEMING, F.R.S., 



PROFESSOR OF ELECTRICAL ENGINEERING IN THE UNIVERSITY COLLEGE, LONDON. 



WE have to consider in the next place the arrangements of the 

 receiving station and the various forms of receivers that have 

 been devised for effecting telegraphy by Hertzian waves. Just as the 

 transmitting station consists essentially of two parts, viz., a part for 

 creating electrical oscillations and a part for throwing out or radiating 

 electric waves, so the receiving station appliances may be divided into 

 two portions; the function of one being to catch up a portion of the 

 energy of the passing wave, and that of the other to make a record or 

 intelligible signal in some manner in the form of an audible or visible 

 sign. 



Accordingly, there must be at the receiving station an arrangement 

 called a receiving aerial, which in general takes the form of a long 

 vertical wire or wires, similar in form to the transmitting aerial. 

 There is, however, a distinct difference in the function of the trans- 

 mitting aerial and the receiving aerial. The function of the first is 

 effective radiation, and for this purpose the aerial must have associated 

 with it a store of energy to be released as wave energy ; but the function 

 of the receiving aerial is to be the seat of an electromotive force which 

 is created by the electric force and the magnetic force of the incident 

 electric wave. 



In tracing out the mode of operation of the transmitting aerial, it 

 was pointed out that the electric waves emitted consisted of alterna- 

 tions of electric force in a direction which is perpendicular to the sur- 

 face of the earth, and magnetic force parallel to the surface of the 

 earth. These two quantities, the electric force and the magnetic force, 

 are called the wave vectors, and they both lie in a plane perpendicular 

 to the direction in which the wave is traveling and at right angles to 

 one another, the electric force being perpendicular to the surface of 

 the earth. In optical language, the wave sent out by the aerial would 

 be called a plane polarized wave, the plane of polarization being 

 parallel to the magnetic force. Hence, if at any point in the path of 

 the wave we erect a vertical conductor, as the wave passes over it, it is 

 cut transversely by the magnetic force of the wave and longitudinally 

 by the electric force. Both of these operations result in the creation 

 of an alternating electro-motive force in the receiving aerial wire. 



