816 



SCIENCE 



[N. S. Vol. XXVI. No. 676 



methods for producing Hertzian waves for 

 use in wireless telegraphy consisted in 

 charging up by means of an induction coil 

 a vertical insulated conductor, which was 

 allowed to discharge itself to earth by 

 means of a spark taliing place between its 

 lower end and another conductor which was 

 connected to earth. To detect the Hertzian 

 waves, Marconi employed an improved 

 form of the Branley filings tubes, which is 

 known as the "coherer." 



In order to transmit messages the radia- 

 tion is started and stopped so as to form 

 short and long signals, or dots and dashes 

 of the Morse code, out of which the whole 

 alphabet is built up in the well-known way. 



As I have already stated, the radiation 

 takes place round the vertical conductor 

 approximately equally in all directions. 

 Suppose that I set up my transmitting 

 apparatus here in Leicester, a receiving 

 station set up either in Nottingham, Derby, 

 Rugby or Peterborough would be able to 

 Teceive the message equally well. Should 

 I wish to send a message from here to Not- 

 tingham at the same time that Derby wishes 

 to speak to Rugby, then the receiving sta- 

 tion at Nottingham would receive both the 

 message from Leicester which it should re- 

 ceive and the message from Derby which 

 it was not required to receive. 



To get over this difficulty, known as "in- 

 terference," a large number of devices 

 have been patented. The most successful 

 in practise is syntony, or tuning: in this 

 method each station has allotted to it one 

 definite frequency or tune, and the appa- 

 ratus is so^ arranged at each station that it 

 will only be affected by messages which are 

 radiated by other stations on its own fre- 

 quency or tune, and not by any other radi- 

 ations. To take a musical analogy, sup- 

 posing I had somebody who was either deaf 

 to all notes of the piano except, say, the 

 middle C, or had such a musical ability 

 that he could tell at once when I struck the 



middle C; then I could transmit to that 

 person a message in the ordinary Morse 

 code by playing on the middle C, and that 

 person, whom I shall call Mr. C, would not 

 take any notice of the fact that I might 

 also be playing on the notes D, E, F, G, 

 etc., but Mr. C would confine his attention 

 entirely to what is being done with the 

 middle C. It is conceivable that I might 

 find a series of persons or train them so 

 that they -could each pick out and hear one 

 note only of the piano, irrespective of what 

 was being played on the other notes or of 

 any other noises that were taking place. 

 Taking an ordinary seven-octave piano and 

 neglecting for a moment the black notes, 

 this would give me fifty-six distinct notes 

 on which I could transmit messages; so 

 that, transmitting from Leicester, I might 

 send messages simultaneously to fifty-six 

 different towns. 



The number of possible simultaneous 

 messages depends on the number of octaves 

 there are on the piano used, and on how 

 close together the different notes are which 

 can be used without producing confusion. 

 For instance, it might be quite easy to train 

 • someone to distinguish with certainty be- 

 tween C and E, and pick out signals on C 

 at the same time that signals are being sent 

 on B. It is certainly more difficult to do 

 this with two notes that are closer together, 

 say C and D, and still more difficult if the 

 half-tones are used as well. The problem, 

 therefore, in wireless telegraphy is to ar- 

 range the receiving apparatus so that it can 

 hear, or perhaps I should say, more accu- 

 rately, so that it can only see, notes of one 

 definite frequency or pitch, and not be af- 

 fected by any other notes, even though of 

 but slightly different pitch. Another re- 

 quirement to obtain good working is that 

 we should use as little power as possible at 

 our transmitting station consistent with 

 obtaining enough power in our receiving 

 instruments to work them with certainty. 



