1905.] Oscillations into Continuous Currents. 481 



which are created in the receiving circuit, and since in the ordinary 

 transmitter these oscillation groups are separated by wide intervals of 

 silence, it is obvious that we can increase the sensitiveness of the above 

 described arrangement by employing a very rapid break or interruptor 

 with the induction coil. If, for instance, we employ a Wehnelt break with 

 the induction coil or a high speed mercury break or alternating current 

 transformer, we get a far better result as indicated by the deflection 

 of the galvanometer than when employing the ordinary low frequency 

 spring or hammer break. 



The point of scientific interest in connection with the device, however, 

 is the question how far such unilateral conductivity as is possessed by 

 the vacuous space is complete. The electrical properties of these 

 vacuum valves have accordingly been studied. 



A bulb containing a 12-volt carbon filament rendered brightly in- 

 candescent by a current of about 2 -7 to 3' 7 amperes was employed. 

 The filament was surrounded by an aluminium cylinder. The length 

 of the carbon filament was 4*5 cm., its diameter 0'5 mm., and surface 

 70 square mm. 



The aluminium cylinder had a diameter of 2 cms., a height of 

 2 cms., and a surface of 12 -5 square cms. The .filament was shaped like 

 a horse-shoe, the distance between the legs being 5 mm. This filament 

 was rendered incandescent to various degrees by applying to its 

 terminals 8, 9, 10, and 11 volts respectively. Another insulated 

 battery of secondary cells was employed to send a current through the 

 vacuous space from the cylinder to the filament, connection being made 

 with the negative terminal of the latter. The current through the 

 vacuous space and the potential difference of the cylinder and negative 

 end of the hot carbon filament were measured by a potentiometer. The 

 effective resistance of the vacuous space is then taken to be the ratio of 

 the so observed potential difference (valve P.D.) to the current (valve 

 current) through the vacuum. 



The following table records the observations. The column headed 

 P.D. gives the potential difference between the hot filament and the 

 cylinder, that headed A gives the current through the vacuous space in 

 milliamperes, that headed R the resistance of the space in ohms, and 

 that headed K10 5 is 100,000 times the conductivity. 



The result is to show that the vacuous space does not possess a 

 constant resistance, but its conductivity increases rapidly up to a 

 maximum and then decreases as the valve potential difference pro- 

 gressively increases. If we plot the current values as ordinates and 

 potential difference of the valve electrodes as abscissae, we find that the 

 current curve quickly rises to a maximum value and then falls again 

 slightly as the potential difference increases steadily. The conductivity 

 curve also rises to a maximum and then decreases (see fig. 3). 



The facts so exhibited are well-known characteristics of gaseous 



2 N 2 



