1001)] 



OR Researches in Radiotehgraphy. 



673 



solved into Morse dot and dash signals by the key in the transmitter. 

 The operator usually wears on his head a double telephone, and listens 

 to these long and short sounds in the telephone and writes down each 

 letter or word as he hears it. The 

 reception of signals in modern 

 radiotelegraphy is most usually 

 effected by ear, by means of some 

 type of oscillation detector capable 

 of actuating a telephone. It is 

 important then to notice that, to 

 obtain the highest sensitiveness 

 when using the telephonic method 

 of reception, the spark frequency 

 or number of oscillation trains or 

 the number of interruptions of the 

 persistent train per second must 

 take place at such a rate that it 

 agrees with the natural time period 



of the diaphragm of the telephone used. An ordinary telephone 

 receiver is most sensitive, according to the researches of Lord 

 Rayleigh and M. Wien, for some frequency lying between 500 and 

 1000. Thus Lord Rayleigh (see Phil. Mag., Vol. 38, 1894, p. 285) 

 measured the alternating current in microamperes required to pro- 

 duce the least audible sound in a telephone receiver of 70 ohms 

 resistance at various frequencies, and found values as follows : — 



Fig 



-Walter's Tantalum 

 Detectoe. 



Both, however, agree in showing a maximum sensitiveness for 

 currents of a frequency between 600 and 700. This is due to the 

 fact that the frequency of the actuating current then agrees with the 

 natural frequency of the ordinary telephone diaphragm. Hence, alter- 

 nators for large power radiotelegraphic stations are now designed to 

 give currents with a frequency of about 300 or 600 alternations per 



