﻿the Photoelectric Effect. 621 



altogether wrong. For this reason, and as they are extra- 

 polations and not actually observed values, they are enclosed 

 in square brackets. 



Sodium. 



Time from 

 preparation of 

 surface. 



[0] hours. 



1 „ 



Value of N 



for First 



Maximum (a). 



[47xl0- 4 ] coul./cal. 

 17-1 xlO" 4 



Value of N 



for Second 



Maximum (b). 



[33 xlO" 4 ] coul./cal. 

 19-7 xlO- 4 



2-5 „ 



3-6 xlO- 4 



8xlQ- 4 * ,. 



18 „ 



0-59 xlO"" 4 



2-44 xlO" 4 



C cesium (loc. cit. p. 564). 



Owing to an oversight the capacity used in the original 

 measurements with this substance has not been recorded, 

 and as we had not time to repeat the measurements before 

 leaving Princeton, the data we are able to furnish have 

 only a qualitative significance. It appears from the 

 possible values of the capacity, and the other data, that 

 the value of N for the maximum point on the curve at 

 ^ = 1*2 X 10 15 sec. -1 must lie between the limits : — 

 IxlO" 5 and 5 x 10" 5 coul./cal. 



The value of the unit of N varies of course between 

 corresponding limits. 



These numbers, although very wide apart, are of consider- 

 able interest. They show that caesium under the conditions 

 of the experiments previously described is very inefficient 

 photoelectrically. It is very much worse than sodium, con- 

 siderably worse than aluminium, and comparable with 

 platinum in this respect. In view of the highly electro- 

 positive character of caesium, this result seems very 

 surprising and rather points to the view that the caesium 

 used in these experiments was already much fatigued before 

 the observations commenced. This position is supported by 

 the following additional considerations : — 



(1) The absence of fatiguing during the observations 

 which was remarked in the previous paper. 



(2) The absence of the expected first hump, which, if the 

 experiments with sodium may be taken as a guide, dies out 

 more quickly than the second. 



(3) If, as seems to be the case in general, photoelectric 

 fatigue is more rapid the more electropositive the metal, we 

 should expect it to be accomplished more quickly with 

 caesium than with sodium; so that it well might be practi- 

 cally complete before the actual observations commenced. 



In our opinion it is desirable to examine the photoelectric 

 * This number may be 10 per cent, too high (see above). 



