Fijy. 2. 



Cadmium Vapour Arc Lamp, 357 



In filling the lamp, the cadmium containing 

 two or three per cent, gallium is placed in the 

 bulb F. It is necessary to maintain the pressure 

 in the lamp and connexions below 0*001 mm. of 

 mercury with the exception of that due to the 

 cadmium and gallium, throughout the process of 

 distilling. Owing to the fact that the volume 

 of the lamp is relatively small, the quartz capil- 

 lary at E should be of such a length as to permit 

 of sealing off in the shortest possible time. The 

 flame used for this purpose should be small, and 

 the heating of the tube on both sides of the 

 capillary should be prevented as far as possible. 



The method indicated above, if carefully 

 followed, will give a lamp with indefinite life. 

 One of this type has been in intermittent use for 

 over a year and shows no sign of deterioration. 

 Should traces of oxide or stains due thereto 

 appear during the process of rilling, they can 

 readily be reduced by introducing pure dry 

 hydrogen and heating. The lamp may be started 

 by heating with a flame to vaporize the metal. 

 It is in all cases advisable io have a current of 

 air blowing upon the lead seals to keep them 

 cool. If the blast is allowed to strike the body 

 of the lamp, the cadmium is condensed and 

 obscures the arc. The most convenient source 

 of energy for operation is the ordinary 110 volt 

 lighting circuit, on which it will operate con- 

 tinuously with a current as small as 3 am]). 

 and a drop of 14 volts across the terminals of 

 the lamp. The most satisfactory results, how- 

 ever, are secured with a current of about 7 amp. 

 and a drop across the terminals of about 25 volts. 

 Under this condition a practically pure cadmium 

 spectrum of great brilliancy is obtained. The 

 intensity secured is apparently equal to that 

 which would be obtained were the lamp filled 

 with cadmium alone. The map * of the spec- 

 trum of gallium given in figure 2 is interesting. 

 The wave-lengths and intensities of the lines 

 are given in Table I. in this connexion. It 

 will be observed that there are but five lines in 

 the visible spectrum and that from practically 

 4200 A to 6400 A there are no lines. 



* Eder & Yalenta, Atlas Tyjji<chen Spektr* 



