660 Drs. Foote, Mohler, and Meggers on a Significant 



In all the discussions so far published in regard to the 

 failure of the principle of selection, the field referred to has 

 been that applied between the two electrodes of a vacuum 

 tube. The " breaking through " of the principle with helium 

 is a trifle more pronounced in a high voltage capacity dis- 

 charge than in a lower voltage direct current discharge, 

 supposedly because of the greater field in the former case. 

 It is readily possible, however, to eliminate the effect of such 

 a field in a suitably designed discharge tube. One form of 

 such a tube is illustrated by fig. 1. This consists of a 

 central electrode closely surrounded by a cylindrical net 

 which is similarly enclosed by a cylinder of much larger 

 diameter. The net and outer cylinder are in metallic 



* R a di at ion 



Fig. 1. — Schematic representation of a discharge tube in which the 

 character of the radiation is not disturbed by the exciting- 

 electrostatic field. 



contact, and the field is applied between the central electrode 

 and net. Thus in the region between the net and cylinder, 

 where the character of the radiation is observed, there exists 

 no applied field. By use of an induction coil or a high 

 voltage transformer very high intensity of radiation is 

 excited, probably sufficient for fine structure analysis. 



It is desirable for certain types of work to replace the 

 central electrode by a Wehnelt cathode or heated tungsten 

 filament, in which case the applied field may be reduced to 

 the order of a few volts, to the ionization potential if arc 

 spectra are studied. Currents of 25 amperes and more at 

 20 volts are readily obtained with hydrogen. 



The radiation from such a tube containing potassium vapour 

 is shown in fig. 2 (PI. XIII.). The total applied field was 

 but 7 volts, and the spectroscope was sighted on the light 

 emitted in the force-free space between the outer electrodes. 

 With a current of 70 milliamperes leaving the filament, the 



