134 C. A. Bui man — Effect of Phosphorescent Material. 



feivnt pressures and it was found that with any vacuum better 

 than •1""" the maximum effect was obtained. 



The apparatus (tig. 1) consisted of a brass tube (B) closed at 

 one end by a quartz plate (C) and at the other by a vulcanite 

 cap (O). The cap was removable by melting the wax which 

 held it in place. (A) is a brass tube on which was fitted a 

 glass tube which led to a McLeod gauge and from thence to a 

 Gaede vacuum pump. (Q) is a brass wire net one inch from 



Fig. 1. 



plate (E). (E) is a brass circular plate on which the phos- 

 phorescent material was mounted. This plate was supported 

 by a brass rod which ran through an amber plug in the vulcan- 

 ite cap, being surrounded by a brass guard tube (F). The rod 

 went as far as (G), where it was soldered on to a rod connect- 

 ing it with the Dolezalek electrometer (H). The electrometer 

 and plate could be grounded by means of a connection at (G). 

 (J) is a brass tube surrounding the rod. 



A potentiometer (K) could be connected to the case (JB) by 

 means of a switch at (N). (M) is the battery. Thus by revers- 

 ing the connections accelerating and retarding fields could be 

 put on the case. (I) is the lamp and scale for the electrometer. 

 (L) is the source of light. A phosphorous pentoxide bulb was 

 placed at (P) to absorb the moisture. 



Method of Experiment. 



The phosphorescent material to be examined was ground to 

 a fine powder with alcohol in an agate mortar. The coarse 



