﻿Facilitating the Visible Electric Discharge in Vacuo. 71 



under favourable conditions, be passed through the vacuum 

 tube, using pressures of only 30 to 300 volts. 



It is generally held that the efficacy of the hot oxides in 

 this direction is due to their giving off negatively charged 

 ions or corpuscles. 



It therefore occurred to the writer to ascertain whether 

 similar effects could not be obtained by painting the cathode 

 with radium, and as radium gives off' corpuscles when cold, 

 it was anticipated that it might not be necessary to heat the 

 cathode. 



Jn the first experiment, however, this was found not to be 

 the case, as with a cold cathode, and using continuous 

 current up to 100 volts pressure, the radium did not seem to 

 have any appreciable effect in producing a visible discharge. 

 When, however, the radium-coated cathode was heated to 

 redness, the radium was found to have a very marked action 

 in facilitating the production of a luminous discharge. 



In the experiments a strip of platinum foil was used for- 

 the cathode, which, before mounting in the tube, was dipped 

 into a solution of radium bromide and dried. The amount 

 of radium on the foil must have been extremely small, but 

 with a suitable vacuum, and with the cathode heated to a 

 bright red colour, the discharge passed, and the gas in the 

 tube lighted up brightly with electrical pressures as low as 

 about 80 volts. 



In order to determine whether it was necessary that the 

 radium should be on the cathode itself or whether the mere 

 presence of radium in the tube would be sufficient to produce 

 the effect, a tube was constructed, as shown in the illustration 

 (p. 72), with two similar platinum-foil electrodes, either of 

 which could be used as a cathode, and both being arranged so 

 that they could be heated to redness by passing a current 

 through them, the anode being mounted midway between 

 the two cathodes. One of the cathodes was dipped before 

 mounting in a solution of radium bromide, while the other 

 was not so treated. 



Under these circumstances, it was found, that with pressures 

 up to 400 volts a visible discharge passed only when the 

 platinum strip that had been treated with radium was used 

 as cathode and was heated to redness, and that when both 

 platinum strips were heated to redness there was no visible 

 discharge when the untreated strip was used as a cathode. 

 Furthermore, it was found that the tube would only allow 

 visible discharges to pass in the direction that made the 

 treated platinum strip cathode, the tube acting as a uni- 

 directional valve in the same way as do tubes with cathodes 

 coated with oxides. 



