of Ether Waves in Air. 773 



To express O in magnetic units we take /jl as 1, and to 

 express E in electrostatic units we divide by v. 

 Consequently, in the units most convenient, 



tm /3 cy 



while H =>/3.— |-. 



Thus for a simple vertical linear oscillator, the magnetic 

 force somewhat exceeds the electric in the region where 

 waves begin. Hence anything which tends to increase the 

 electric force, such as a tilted capacity area or the like, will 

 assist radiation in one general direction : — as initiated by 

 Dr. de Forest in 1904, introduced by Mr. Marconi in 1905, 

 and described by him to the Royal Society on 22 March, 

 1906 (Proc. Roy. Soc). 



Design of Aerial. 



The necessity for superposed electric and magnetic fields, 

 and the fact that radiating power is their vector product, at 

 right angles to the plane of both, explains the main behaviour 

 of directive antennae. A linear Hertz vibrator, for instance, 

 gives no axial radiation for two reasons, (1) because there is 

 no magnetic field at the axis, and (2) because the electric force 

 there is "end-on," so to speak, and could not contribute to axial 

 radiation anyhow. So also in the case of a closed-circuit 

 oscillator, say a large plane coil of one turn ; though the 

 magnetic force along the axis is strong, its direction prevents 

 its contributing to axial radiation. Hence if a linear and a 

 closed-circuit oscillator are combined, as they practically are 

 in a r shaped aerial, especially when the upper arm is very 

 long, the fore and aft radiation in the plane of the T, notably 

 that in front of the vertical part, will be copious compared 

 with that in a perpendicular direction ; and the interesting 

 figure-of-8 equal-intensity region, detected by Mr. Marconi 

 and his staff, is a consequence. 



All increase of capacity combined with height, in the aerial, 

 goes to increase both the electric and the magnetic fields at 

 a distance, and therefore will be advantageous (though ex- 

 pensive) notwithstanding the concurrent increase of wave- 

 length. In fact free capacity and height afford two separate 

 means of increasing wave-length without diminishing the 

 power of emitting radiation ; while both together increase that 

 power ; for radiation is proportional to the square of SVZ/X 2 , 

 and X 2 is proportional to S and also to I if the radiator is 

 straight. 



But insertion of inductance coils in the aerial will have 

 Phil. Mag. S. 6. Vol. 25. No. 150. June 1913. 3 G 



