and the Theory of Atomic Structure. 1079 



about 1'4 times the mean tension. Up to 10 k.v. a Braun 

 electrometer was used. At times it was necessary to work 

 with a rather low tension. In this case the secondaries were 

 connected in parallel, so that a larger current could be 

 obtained. In photographing the absorption discontinuities 

 of silver a maximum tension of not more than 5400 v. was 

 used, and with this tension the influence of the "space 

 charge" in limiting the magnitude of the maximum current 

 could readily be observed. As is known, this effect has been 

 experimentally and theoretically studied by Langmuir *, who 

 clearly showed that in an extremely high vacuum the charge 

 due to the electrons moving in the field, between the hot wire 

 cathode and the anode diminishes the rate at which electrons 

 may escape from the hot wire and enter the field. For every 

 tension there exists a maximum " saturation current " which 

 depends on the dimensions of the tube and is independent of 

 the temperature of the hot wire, if once a certain temperature 

 has been surpassed. In this experiment the saturation 

 current was less than 10 m.a. for a tension of about 5400 volt. 

 As with this small current a very long exposure is required, 

 it was desirable to get rid of the space-charge effect. This 

 may be done by working with a somewhat lower vacuum, as 

 in this case the positive ions of the gas neutralize the influence 

 of the electrons in the field. Fortunately I was able to 

 reduce the vacuum just sufficiently by an imperfectly sealed 

 join in the tube. Otherwise it would have been necessary to 

 lower the vacuum by regulating the speed of the molecular 

 pump. Under these circumstances it was possible to get a 

 current of about 50 m.a. at a tension of 5400 v. 



As fairly long wave-lengths were measured, it was necessary 

 to have also a vacuum in the spectrograph. The spectrograph 

 was exhausted by a Gaede pump which gave a pressure of 

 about O'l to - 2 mm. The slit of the X-ray tube, which was 

 immediately connected with the spectrograph, was covered 

 with goldbeater's skin in order to separate the high vacuum 

 from the low vacuum. This goldbeater's skin was coloured 

 with erythrosine to prevent the visible light as much as 

 possible from entering the spectrograph. The tube itself 

 was exhausted by a molecular pump in series with the low 

 vacuum pump. The pumps were able to give a very good 

 vacuum in about 10 minutes. The time of exposure for one 

 plate varied from \ to 1\ hour. 



§ 3. To get also the fainter lines of the spectrum, it 



appeared to be advantageous to use tensions several times 



higher than the critical exciting tension. Thus in several 



cases the maximum tension obtainable with the apparatus 



* Phys. Rev. II. ii. p. 450 (1913). 



