286 



BELL SYSTEM TECHNICAL JOURNAL 



completely equipped tube with filament at F, a row of targets which 

 can be moved consecutively into place at T, and the photosensitive 

 electrode at A with its gauze shields in front of it, come from the work 

 of Horton, Andrewes and Davies. A tube designed and used by 

 E. H. Kurth is shown in Fig. 7; the filament is seen in perspective at 

 C, the target at 7", and the sensitive disc at D\ the family of di\erging 

 straight lines represents a set of metal laminae, which being charged 



Fig. 7 — Kurth's apparatus for (litermiiiiiig cxiitatioii-potenllals. (Physical Review.) 



alternately to potentials and +135 volts gather in any ions which 

 start up towards the disc. The method can also be adapted to gases, 

 and this application has an interesting and important history; but 

 as nearly all the data respecting gases refer to wave-lengths superior 

 to 120()A, they fall out t)f the province of this discourse. Foote and 

 Mohler, however, [jenetrated to 2()A with the apparatus of Fig. 8, 

 filled with oxygen. The rilanuni is al A\ the electron-accelerating 

 voltage V is api)lit<i liiiwicn .1 and ihe gauze B, so that the target 

 is essentially a thin la>er of gas en\eloping B\ the photosensitive 

 electrode is the gauze C, the photoelectric current from which is 

 gathered in bN' the |)lalc /' (screened against positive ions b\' its high 

 potential). 



