1909] on Researches in Radiotelegraphy. 669 



the optical apparatus of a lighthouse, with catoptric or dioptric appa- 

 ratus, which projects the light from the lamp largely in one direction. 

 It is not yet possible to make with electric radiation of long wave- 

 length that which corresponds precisely 

 with a beam of light wholly concentrated 

 along a certain cone or cylinder, but it 

 is possible, by the use of a complex 

 antenna as described, to greatly limit the 

 diffusion of the radiation. Since radia- 

 ting and absorbing power go hand in 

 hand, it is obvious that such a directive 

 antenna also enables the position of a 

 sending station to be located. Messrs. 

 Bellini and Tosi have accordingly applied 

 their methods in the construction of a 

 radiogoniometer and receiving antenna, 

 by means of which they can locate the 

 direction of the sending station without 

 moving the antenna, but merely by -^^^- l^- 



turning round a secondary circuit into 



a position in which the maximum sound is heard in a telephone 

 connected with the receiver. By the kindness of Capt. Tosi I am 

 able to exhibit to you their ingenious apparatus (see Fig. 13). 



The space occupied by such closed antennae has hitherto pre- 

 vented their employment on ships. There is still therefore an opening 

 for the invention of apparatus capable of being used on board ship, 

 which will enable one ship to locate, within narrow limits, the 

 direction of another ship sending signals to it, and therefore of 

 ascertaining immediately the direction from which some call for 

 help is proceeding. 



Closely connected with this part of the subject is the question so 

 frequently discussed as to the isolation or secrecy of radiotelegraphic 

 communication. Up to the present moment the only really practical 

 method of isolating any particular receiver so as to make it sensitive 

 only to signals coming from a certain direction, is to avail ourselves 

 to the utmost of the principle of resonance and to tune the sending 

 and receiving circuits to exact correspondence. The question then 

 arises what is it which determines the effectiveness of this tuning. 

 If waves of one particular wave-length are impinging on a receiving 

 antenna and creating signals, by how much can the wave-length be 

 varied or the tuning of the receiver upset or changed without pre- 

 venting these signals being received ? It is clear that the narrower 

 this range, the more perfect the isolation of the receiver. It can be 

 shown that it depends upon the form of the resonance curve of the 

 sending and receiving circuits. If the sending station is emitting 

 waves of a certain constant wave-length and damping or decrement, 

 then in the receiving circuit of all other stations within range there 



