1914] on Improvements in Long-Distance Telephony 109 



The treatment of this subject demands a lecture in itself and not 

 merely a few concluding sentences at the end of one. Suffice it to 

 say that telephony conducted without interconnecting wires has been 

 achieved over distances of several hundred miles as an experimental 

 feat, but cannot yet be said to have entered into the field of 

 practical everyday service. A brief outline of the method is as 

 follows. 



At the transmitting station we establish all the arrangements 

 now used for producing continuous long electric waves of rather less 

 than 10 miles in wave length, and preferably about 4 or 5 miles. 

 These waves are radiated from an antenna wire and must be per- 

 fectly continuous, that is, emitted without break or interruption ; 

 or, at least, if they are produced by spark discharges, the spark 

 frequency must be about 20,000 per second. At the receiving 

 station there are the usual arrangements for receiving by telephone 

 signals as in radio-telegraphy. 



The only difference is that at the sending station in the base of 

 the antenna is inserted a microphone of such current-carrying 

 capacity as to bear 5 to 20 amperes or more passing through it. 

 This microphone varies the resistance in the antenna circuit when 

 speech is made to it. This in turn causes a fluctuation in the 

 amplitude of the stream of continuous electric waves being emitted. 

 It, so to speak, makes waves on waves, and at the receiving station 

 the telephone is influenced by these secondary and slower modula- 

 tions of wave amplitudes. 



The frequency of the steady stream of waves emitted by the 

 sending station must be such that it is above the limits of audition, 

 that is, more than 20,000 per second, and preferably about 40,000. 

 An unbroken wave train of this frequency cannot affect the telephone 

 directly. If, however, the amplitude of the electric waves radiated 

 is varied in accordance with the lower frequency of speech, these 

 undulations of amplitude in the high frequency wave do affect the 

 telephone in the receiver, and the speech uttered to the transmitting 

 microphone is heard. 



The continuous wave-train can be produced either by a high 

 frequency alternator, such as that of G-oldschmidt, or by the 

 continuous-wave disc generator of Mr. Marconi, or by some form of 

 Poulsen or Moretti arc generator. 



The chief difficulty is that of constructing a microphone which 

 shall be able to carry the large oscillatory cun'ents which flow up 

 and down the antenna. 



Professor Majorana and Dr. Vanni in Italy have devised ingenious 

 liquid microphones. A multiple carbon microphone is commonly 

 used, and Dubilier has devised a water-cooled arrangement called a 

 relay microphone. 



The Vanni microphone is constructed as follows : — A jet of water, 

 which may be made slightly conducting by the addition of acid or 



