704 



Professor J. A. Fleming 



[May 24, 



He found that such a bent antenna emits a less intense radiation at 

 any given distance in the direction in which the free end points, 

 than in the opposite direction. Also, since the law of exchanges 

 holds good for electric radiators, a similar form of antenna re- 

 ceives or absorbs best electric waves which reach it from a direction 

 opposite to that to which the free end points.* Hence two similar 

 bent antennae, when set up back to back, that is, with their free ends 

 pointing away from each other, form a system of radiator and receiver 

 which has greater range in that position than in any other for the 

 same distance, and hence has directive qualities not possessed by the 

 ordinary vertical antennae. 



Although I have given the mathematical explanation of the reasons 

 for this in another placet it is not difficult to translate the common 



+ K+h/ 

 -©-©- 



B E 



+H-h 

 -©-©- 



D P 



H+h' 



H-h 



Fig, 17. — Theory op Marconi Bent Antenna. 



sense of it into non-symbolic language. Imagine a square circuit of 

 wire half buried vertically in the earth (see Fig. 17). Let a current 

 be supposed to flow round it, in clockwise direction. Then it creates 

 a magnetic field, the direction of which along the surface of the earth 

 in a direction at right angles to the plane of the circuit, and at equal 

 distances from the centre, is towards the spectator on both sides. 

 Suppose, then, that a wire equal in length to one side of the square is 

 placed in contiguity to one vertical side, and that it carries a current 



* This is an extension to electric radiation of the principle known as 

 Prevost's Theory of Exchanges, as amplified by Balfour Stewart and Kirchhoff , 

 which forms the basis of spectrum analysis laid down by Stokes, Kirchhoff, 

 Bunsen, and others. 



t See " A Note on the Theory of Directive Antennae," Proy. Roy. Soc. 

 Lond., vol. IxxviiiA., 1906, p. 1. 



