218 



COMPTON— ARC SPECTRA AND 



wire in this stream causes the atoms to recombine at its surface, with 

 the emission of the secondary spectrum and a heating of the wire.* 



Further Critical Potentials in Hydrogen. 

 The problem has been attacked from another angle by Dr. P. S. 

 Olmstead, who used a modification of the Lenard method of investi- 

 gating ionization potentials, illustrated in Fig. 4. Electrons from the 



j^imiu ,^1 n min* '"H'^^'^^'-^g 



^•w.kC/ 





Fig. 4. 



hot central portion of a tungsten filament F were drawn toward a 



wire net NN through an accelerating difference of potential V. 



Those which passed the net A''A^ encountered a retarding field Vr 



sufficient to stop them. 



The two distinctive features of the apparatus were the system of 



detecting electrodes P and W and a grid GG of twenty parallel 



tungsten wires. PP was a platinum plate and W was a fine platinum 



wire extending but a short distance in front of the plate PP. When 



the electrons from F collided with and produced radiation from atoms 



or molecules near the net NN a considerable amount of this radiation 



fell on the plate PP, whereas the wire W received only a negligible 



amount of radiation because of its very small area. Either PP or IV 



* Since this paper was written very decisive evidence on these points has 

 been obtained and is to be published by Dr. Duffendack in the Astrophys. Jour. 



