54 Dr. R. T. Beatty on 



is no possibility of fracture due to sudden changes of 

 temperature. 



In constructing a glass cell the rubidium must be prepared 

 beforehand. The method found most suitable is shown in 

 fig.. 4. C is a copper vessel fitted with a narrower copper 



Fig. 4. 



tube bent at the end. The vessel is filled with the mixture 

 of calcium and rubidium chloride, and a copper end-piece 

 screwed on. It is then placed inside the glass tube Gr, which 

 is afterwards closed at one end. The apparatus is connected 

 to a Gaede pump, and when exhausted the tube around C is 

 carefully heated. At a red heat the softened glass shrinks on 

 to 0, but remains intact as long as the high temperature 

 is maintained. The rubidium vapour is delivered by the 

 bent tube and drops collect in the side tube. By gentle 

 heating the rubidium is made to flow into the bulbs B B, 

 which are then sealed off. 



The design of a glass cell is the same as that of the quartz 

 one shown in fig. 3, except that the electrodes are fused in 

 in the ordinary way. A bulb A (fig. 4), containing rubidium, 

 has the tip of the drawn-out end broken off, and is then 

 inserted in the side tube S (fig. 3). After closing the end 

 of S and exhausting, the metal is transferred to the cell by 

 gentle heating. Only the small surface of metal closing the 

 narrow end of the bulb becomes oxidized, so that the greater 

 part of the rubidium remains available. 



Method of using the Photo-electric Cell. 



The cell was placed with the window close in front of the 

 emergence slit of the monochromator (fig. 1). The cathode 

 was connected to the negative end of a set of small accumu- 

 lators, so that its potential was about two volts less than that 

 required to produce a luminous discharge in the helium. The 

 anode was connected to the gold-leaf of a double quadrant 



