Sensitive Plioto- Electric Cell. 681 



to fill the cross-section of the tube, and this foil is held up 

 by a thin glass rod, which in turn is suspended from the 

 winch W by a silk thread. When sufficient sodium has 

 been distilled, the lead foil is raised by means of the winch 

 and is crumpled up in the side tube out of the path of the 

 light. The blowpipe is then applied to the constriction B, 

 which collapses around the glass rod and is sealed off. 

 Finally the apparatus is sealed off at L. 



On looking into the cell, the sodium surface presented a 

 beautiful appearance. It consisted of an immense number 

 of minute solidified drops of sodium which scattered light in 

 all directions. 



A very convenient way of measuring the photo-electric 

 current was to balance it against a current tlowing through 

 a suitable xylol-alcohol resistance as described by Dr. N. R. 

 Campbell. A Wilson tilted electroscope was used as a null 

 instrument. When conditions were steady, the photo-electric 

 current remained constant to at least 1 part in 1000 over a 

 period of several minutes. 



The following test was made on the sensitiveness of the 

 cell. The wave-length X 4360 was isolated by a mono- 

 chromator, the aperture-ratio of the lenses being about /. 5. 

 The slits were 2 cm. by 2 mm. The energy absorbed by 

 the mercury lamp was 120 watts per second. The photo- 

 electric current obtained with the cell was as large as 

 580 XlO -11 amp. This appears to be considerably larger 

 than any current which has previously been obtained from 

 a photo-electric cell under similar conditions. The only 

 investigations which furnish sufficient data to effect a com- 

 parison are those of Pohl and Pringsheim. They illuminated 

 a smooth surface of sodium by means of a monochromator 

 which transmitted about the same amount of light as that 

 used in these experiments. Their lamp, however, apparently 

 consumed between 360 and 600 watts per second. 



The photo-electric current from the surface, when illu- 

 minated by \ 4360, was of the order 15 to 20 x 10~ n amp.* 



When it is remembered that the lamp used in my experi- 

 ments probably gave out at the most only one-third as much 

 light as theirs, it appears that this new photo-electric cell is 

 extremely sensitive. The sensitiveness is about 100 to 200 

 times that of a cell containing the usual smooth surface of 

 sodium which has been solidified from the liquid state. The 

 reason for this is two-fold. First, it has been shown by 

 Elster & Geitel and others, that a surface consisting of a 

 large number of minute solidified drops is more sensitive 

 than a surface prepared, in the usual way. Again, the 



* Pohl & Pringsheim, Verh. d. Deutsch. Phys. Ges. p. 54 (1912). 



