﻿946 Dr. F. H. Newman on a 



15 mm. The electrodes are iron rods 4 mm. in diameter, 

 and are sealed with sealing-wax. The current used must be 

 such that these rods never become so hot that the wax is 

 melted or softened. The liquid alloy of sodium and potas- 

 sium — two parts by weight of sodium and one of potassium — 

 is run into the bulb A, and the exit tube D then connected 

 to a glass tap, and the whole exhausted. The lamp can then 

 be disconnected from the pump. An electric discharge is 

 passed through the lamp, the alloy being made the cathode. 

 Under the action of the discharge the oxide on the surface 

 of the alloy disintegrates, and the surface becomes quite 

 clean. Tilting the lamp, some of the alloy flows into the 

 other bulb B. Using this method of introducing the alloy, 

 the part C remains quite free from the alloy. With direct 

 current the lamp works with a minimum applied potential 

 difference of 30 volts, although when once the arc is struck, 

 the fall of potential is only 10 volts with a current of 

 1*5 amps. The arc is struck either by tilting the lamp in 

 the same way that the mercury lamp is started, or one ter- 

 minal is connected to a small induction coil and a momentary 

 discharge passed. No external heat is required, as that pro- 

 duced by the current is sufficient to vaporize the sodium and 

 potassium. As the temperature rises the current decreases, 

 and the potential difference across the terminals becomes 

 greater. With currents smaller than 2*5 amps, the tempera- 

 ture of the quartz at C is never such that a piece of paper 

 held at this part is charred, and the wax seals do not soften, 

 however long the lamp is working. There is no "browning" 

 of the silica. As the applied potential difference is increased, 

 the radiation becomes brighter, and greater luminosity can 

 be obtained by warming the part C with a small gas flame. 

 This part of the apparatus is, of course, hotter than the other 

 parts, owing to the high current density. The lamp works 

 satisfactorily at any potential between 30 and 200 volts, and 

 the current can be regulated by a resistance in series. If 

 the current rises above 2*5 amps, there is the characteristic 

 " browning " of the silica. The lamp does not require con- 

 tinuous pumping while it is working. The sodium-potassium 

 alloy absorbs all gases, particularly nitrogen and hydrogen, 

 while the current is passing. In this manner a very good 

 vacuum is maintained, however long the lamp is in operation. 

 This fact, and the low voltage at which the arc is struck, are 

 two important improvements on the other forms of sodium 

 vapour lamp. The present form will not work satisfactorily 

 with alternating currents. 



With the lamp it was found that the potassium lines were 



