2S8 Prof. J. A. Fleming : JS T ote on the Photoelectric 



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then attached to a mercury pump for the purpose o£ making 

 a high vacuum in it, and should be supported in a slanting 

 position on a sheet of asbestos cardboard and covered by an 

 iron trough. The tube should then be exhausted, and at the 

 same time the metallic potassium and sodium heated so as to 

 melt the lumps into a mass of liquid alloy which, however,. 

 will be covered with crusts of oxide. 



When a good vacuum has been produced, the whole tube 

 being hot and dry, the constriction in the quill tube may be 

 melted with the blowpipe and the glass tube sealed off from 

 the pump. If this is properly done, the part of the tube 

 having in it the platinum plate will remain quite clean, and 

 the molten metal or liquid alloy will be contained in the other 

 portion of the tube. It is then easy to tilt the tube and 

 transfer the clean mercury-like alloy of K and Na into 

 the part of the tube containing the platinum plate, so as 

 to make a pool of liquid alloy on the bottom of the tube 

 having an electrical connexion with the outside by means 

 of the platinum wires sealed through the tube, and having 

 over it, and not far from its surface, a platinum plate also iu 

 connexion with the outside by sealed-in wires. In so doing,, 

 care should be taken that the liquid alloy is not splashed upon 

 the platinum plate, but that the latter is kept quite clean and 

 free from adherent drops of potassium-sodium. 



It is better not to attempt to seal off that part of the tube 

 in which the alloy is melted from that part to which it is 

 decanted over, because the glass nearly always cracks and 

 spoils the apparatus in so doing if there is the slightest 

 particle of alloy smeared on it at the part heated in the 

 flame. 



A tube so prepared should be carefully handled, as if it is 

 broken the liquid K and JNa alloy is spontaneously inflam- 

 mable and deflagrates violently on coming in contact with 

 moisture. It is convenient to mount it on a wooden stand, 

 which should be placed in an iron tray in case of an. 

 accidental breakage. By the aid of such a specimen of 

 photoelectric alloy we can show many interesting experiments* 

 If the tube is supported in a horizontal position, an electric 

 arc contained in a projection lantern, equipped with the 

 ordinary condenser-lens, can be so placed and tilted down- 

 wards as to converge on to the brilliant mercury-like surface 

 of the pool of alloy a very concentrated beam of light. If,. 

 then, we connect the platinum plate and the alloy by 

 means of wires with a sensitive mirror galvanometer, we 

 find that the impact of the light upon the surface of the alloy 

 not merely facilitates the escape of negative electricity from 



