1909] 



011 Researches in RadioteUgraphy . 



Qll 



(see Fig. 17). The sharp bend upwards at one place impHes a larofe 

 increase in the current corresponding to a certain voltage, which 

 means that, corresponding to a certain potential gradient and therefore 

 velocity of the electrons, considerable ionisation of the residual gas 

 is beginning to take place. The current, however, would not increase 

 indefinitely with the voltage, but would before long become constant 

 or saturated. It will be seen, therefore, that at points on the curve 

 where there is a bend or change of curvature, the second differential 

 coefficient of the curve may have a large value. Hence, if we con- 

 sider the current and voltage corresponding to this point, it will be 

 seen that any small increase in the voltage increases the current more 

 than an equal small decrease in voltage diminishes it. If then we 

 superimpose on a steady voltage corresponding to a point of inflexion 

 of the curve an alternating voltage, the average value of the current 

 will be increased. This then points out two ways in which this 

 oscillation valve or glow lamp can be used as a radiotelegraphic 



200 

 



4 6 8 10 12 14 16 li 



Apjilied Voltage. 



Fig. 17. — Characteristic Curve op Rarefied Gas Ionised by Hot 

 Negative Electrode. 



detector. First, we may make use of the unilateral conductivity of 

 the ionised gas in the bulb and employ the glow lamp with cylinder 

 around the incandescent filament, as a rectifier of trains of oscillations 

 to make them affect a galvanometer or telephone. This method was 

 described by me in papers and specifications in 1904 and 1905. In 

 that case the valve is arranged in connection with a receiving antenna, 

 as shown in Fig. 18, and used with a galvanometer or telephone. 

 Mr. Marconi subsequently added an induction coil and condenser, and 

 employed in 1907 the arrangements shown in Fig. 19. In this case 

 the trains of oscillations set up in the antenna could not by them- 

 selves affect a galvanometer or a telephone, but, when rectified by the 

 valve, they become equivalent to an intermittent unidirectional current, 

 and can then afi^ect the telephone or a galvanometer, or any instru- 

 ment for detecting a direct current. 



On the other hand, we may take advantage, as I have more re- 



