680 



Dr. A. LI. Hughes on 



a 



frequently used as radiometers in the visible and in the ultra- 

 violet spectrum, a short account of this form of photo-electric 

 cell seems desirable. 



The cell is a pear-shaped flask, which is shown approxi- 

 mately to scale in the figure. A quartz plate Q, 4 cm. in 

 diameter, is joined to the flask by sealing-wax. It is im- 

 possible to avoid the use of sealing-wax, or some similar 

 cement, when an air-tight joint has to be made between 

 quartz and glass. Communication with the electrodes is 

 made by means of platinum fused into the glass, so that the 

 only waxed joint in the cell is that between the quartz plate 



■p 



to jvusmfi* cuncL otLoSuxrCL-il . 



and the flask. The illuminated electrode is a thin layer of 

 sodium which covers the whole of the inside of the flask, and 

 the other electrode is a brass rod projecting into the flask. 

 This is the electrode which is connected to the electrometer, 

 and therefore requires to be well insulated. The glass tube 

 around the platinum wire at P is warmed and covered with 

 sealing-wax, which is a good insulator. If the apparatus is 

 properly dried and evacuated, the inside of the glass tube 

 near P insulates perfectly. Sodium is introduced into the 

 bulb N, and is strongly heated to drive oft' gases which are 

 removed by a Toepler pump and charcoal in liquid air. 

 Meanwhile the cell is warmed to drive off condensed gases 

 and vapours from the walls. The apparatus is then tilted 

 so as to cause the melted sodium to pass through the traps 

 and constriction L into the small bulb M. The sodium in M 

 is now heated until the inside of the flask becomes covered 

 with a layer of distilled sodium. To prevent any deposition 

 on the quartz window, the following device was employed. 

 A piece of thin lead foil (shown obliquely at K) is cut so as 



