1905.] 



Oscillations into Continuous Currents. 



487 



kept cool by a jet of water playing in the interior. In this manner 

 [ have been able to draw off a continuous current of 3 or 4 amperes 

 from an alternating-current arc using 15 alternating-current amperes 



Returning, then, to the vacuum valve, we may note that the curves 

 in fig. 3 show that the vacuous space possesses a maximum conduc- 

 tivity corresponding to a potential difference of about 20 volts between 

 the electrodes, for the particular valve used. The interpretation of 

 this fact may, perhaps, be as follows : In the incandescent carbon 

 there is a continual production of electrons or negative ions by atomic 

 dissociation. Corresponding to every temperature there is a certain 

 electronic tension or percentage of free electrons. If the carbon is 



FIG. 4. 



made the negative electrode in a high vacuum these negative ions are 

 expelled from it, but they cannot be expelled at a greater rate than 

 they are" produced. Therefore, there is a maximum value for the 

 outgoing current and a maximum value for the ratio of current to 

 electromotive force, that is for the conductivity. 



This fact, therefore, fixes a limit to the utility of the device. The 

 current through the vacuous space is, to a very large extent, in- 

 dependent of the electromotive force creating it, and is at no stage 

 proportional to it, or at least only within a narrow range of electro- 

 motive force near to the maximum conductivity. 



Whilst, therefore, the device is useful as a simple means of detecting 

 electric oscillations, it has not that uniformity of conductivity which 

 would make it useful as a strictly metrical device for measuring them. 

 It can, however, perform the useful service of showing us how far any 

 device for producing electric oscillations or electric waves produces a 

 uniform or very irregular train of electric oscillations, and what 

 changes conduce to an improvement or reduction in the efficiency of 

 the transmitting device. 



