GENERATION, CONTROL, AND MEASUREMENT 177 



Cadmium, zinc, and thallium may be operated as pure metals in quartz 

 tubes, with an outer insulating envelope very similar in design to the 

 medium-pressure mercury arcs. The metal is limited in amount so that, 



Table 3-12. Wave Length, in Millimicrons, of Principal Spectral Lines 

 OF THE Metallic Arcs and Helium Discharge 

 Cadmium Mercury 



228.8 184.9 



326.1 194.2 



340 .4 253 . 6 



346.6 313.2 



361.0 365.0 



467.8 366.3 



480.0 404.7 

 508.6 435.8 



643.8 546.1 

 1039.5 577.0 



579.1 

 Cesium 1014.0 



455.5 



459 . 3 Sodium 



794.4 330.2 



807 .9 568 . 8 



852.1 589.0 

 894 .4 589 . 6 



917.2 819.5 

 1138.2 

 1140.4 



upon freezing, the volume is not sufficient to crack the quartz envelope. 

 The wave lengths of the principal lines are given in Table 3-12. 



ALKALI-METAL ARCS 



The resonance lines of sodium are in the sodium doublet at 589.0 and 

 589.6 mijL and very close to the peak of sensitivity of the light-adapted 

 eye. These lines are excited most efficiently under low pressure, where 

 the mean free path of the sodium atoms is large. It is therefore impossi- 

 ble to obtain efficient sodium lamps of high intensity or brightness. 

 However, the luminous efficiency of the sodium lamp is high, being 

 approximately 50 lumens w~^ in commercial lamps (Buttolph, 1935). 

 Since practically all the energy is in the sodium doublet, it is very useful 

 as a source that is sufficiently monochromatic for many purposes. 



The lamp consists of an inner bulb of special glass resistant to metallic 

 sodium and an outer double-walled evacuated flask that thermally insu- 

 lates the inner tube. Since metallic sodium is a solid at room temper- 

 ature and the power input to the lamp is relatively small compared with 

 its volume, it takes about 30 min for the lamp to come to optimum tem- 

 perature and maximum brightness. The sodium lamps manufactured in 

 the United States are available in 180- and 28-w^ sizes. The 28-w lamp 



