774 Sir Oliver Lodge on the Transmission 



o 



the effect of lengthening the wave without strengthening the 

 field at a fixed distance, and so must act deleteriously on 

 radiation intensity; though such coils will promote persistent 

 oscillation and tuning, and may therefore be advantageous up 

 to a point. Their use will always be of the nature of a com- 

 promise ; and this whether they are actually inserted in the 

 aerial or are inductively associated with it : so they should 

 be kept to a reasonable minimum. 



In an umbrella aerial whose ribs approach the ground, 

 charge from the region of greatest capacity has to ascend 

 before descending ; wherefore it fails to exert full magnetic 

 force at a distance, and conserves some energy which it 

 might radiate, thereby becoming more capable of tuning. 

 The amount of potential tuning necessary or desirable in 

 an antenna must depend on the method of excitation. With 

 shock-excitation some amount of it is essential; with main- 

 tained waves it appears to be unnecessary ; and accordingly 

 in the latter case the more closely an antenna can approach 

 to a simple Hertz oscillator — a copious radiator and absorber — 

 the better ; some kind of directivity being a still further 

 improvement when circumstances allow. 



A practical remark may here be interpolated : — The im- 

 portance of great height for the upper capacity area is so 

 striking, and the trouble and expense of securing it by 

 artificial structures so obvious, that I doubt if sufficient 

 attention has been paid — in the selection of a long-distance 

 site — to the possibility of finding a mountain or rock of 

 sufficiently non-conducting material to serve as the supporter 

 of the upper part of an antenna for a station at its base. 

 Experience would be necessary to ascertain whether the 

 surgings in such a hill, or pair of twin hills, need be any 

 more deleterious to efficient radiation and reception than 

 they are in the iron of the Eiffel Tower, for instance. 



Magnetic Radiation, 



A magnetic radiator only differs from an electric radiator 

 in having as moment a current area instead of a pair of 

 separated charges ; i. e. /ul AC instead of el. 



So if the value of C is made to correspond with en, which 

 would usually mean a very strong current since n is very 

 great, the two radiators would be equally effective when AC 

 in magnetic measure equals el in electric ; in other words 



when 27rA = ZX, 



i. e. in the case of a V aerial, when the efficient length of 

 upper arm equals \j2ir. 



