C. A. Butman — Effect of Phosphorescent Material. 137 



The material was found to fatigue irrespective of the field, 

 when the light was on the substance. Fatigue occurred with 

 no field on and with positive or negative voltages up to 600. 

 It is important to notice that no deflection was ever obtained 

 when using negative fields on the case ranging from 2 to 600 

 volts ; however, if the light was on, the material fatigued, as was 

 demonstrated by taking a reading with no field before and 

 after exposure. 



The foregoing experiments were all made on CaBiJSTa. No 

 particular study was made of the photoelectric fatigue of Sidot 

 Blende, but in general it showed the same properties of fatigue 

 and recovery as exhibited by the CaBiNa. 



The results of these experiments may be summed up by say- 

 ing that the following hold when the plate is insulated : 



1. When there is light there is fatigue. 



2. When there is no light, there is recovery. 



3. High positive voltages accelerate fatigue, with light. 



4. Photoelectric fatigue and recovery is an inherent prop- 

 erty of phosphorescent material. 



5. The photoelectric fatigue is due solely to the incident 

 light. 



Velocity of Electrons from Phosphorescent Substances. 



It was found that the velocities of the electrons depended on 

 the state of the material. There are three states, (1) the elec- 

 trons have a velocity, (2) they have no velocity, (3) they 

 require an attracting field in order that any may leave the sub- 

 stance. These three states probably merge one into the other 

 as the material fatigues. The highest observed potential 

 necessary to prevent the electrons from leaving the CaBiNa 

 was — 0"35 of a volt. The field necessary to get any effect at 

 times was more than +4 volts. However, by applying a 

 strong enough assisting field I was always able to get an effect. 



The highest negative field necessary to keep the electrons 

 coming off from the Sidot Blende was — *075 of a volt. 



Another important fact should be noted, and that is that the 

 saturation values change with the state of fatigue. For in- 

 stance, the following results were obtained with CaBiNa, taking 

 the field as a measure of the velocity: 



Velocity —0*35 corresponded to saturation value +0*25 

 " ±0-0 " " " " +0-5 



" + -3 " " " " +1-0 



However, these saturation values appear to hold only for 

 voltages between to about 40 volts. Curves drawn from a 

 number of sets of reading would seem to indicate that com- 

 plete saturation would be obtained at 800 volts. A typical set 



