604 Dr. Mamie Siegbahn on 



tbe region of tbe long waves. Many types have been built 

 and tried, of metal and glass and combinations of both. 

 Tubes with cold and with hot cathodes (Wehnelt and hot 

 wire) have been compared. The definite type which resulted 

 from these investigations is to be seen in fie:. 2. 



This tube is entirely of metal with a wolfram spiral in a 

 cylindrical cathode. Only the anticatbode is isolated by 

 means of a glass tube. Cathode and anticatbode can easily 

 be detached when necessary. The wolfram spiral is sub- 

 stituted in a moment. By screwing the cylindrical cathode 

 up or down the focus-spot can be varied from the size of a 

 millimetre to the entire anticatbode face. 



To allow continuous work both cathode, anticatbode, and 

 tube are cooled by running water. In some cases a tube of 

 this type has worked steadily 10 to 15 hours with 40-50 

 milliamp. 10-15 k.v. without any interruption. Such long- 

 exposures were naturally unnecessary for the line-spectra 

 with which the following will exclusively deal. 



3. Experimental llesults. 



A. The Wave-length of Cu K a. 



In many experimental researches on X-rays the charac- 

 teristic radiation of Cu has been employed as standard. 

 From this point of view it seems of the greatest interest to 

 oive an exact determination of its wave-length. The value 

 given by Moseley for Cu K a was 1*549 . 10~ 8 , whereas our* 

 value was 1-539 + 0*003. 



The wave-length of CuKa has been determined with the 

 above-described apparatus on different occasions with new 

 adjustments of the whole instrument. The measurements 

 are therefore independent of one another, and give a good 

 illustration of the accuracy attained. 



-dt , Order of 



Plate. -r, a . 



Renexion. 



4 a. 5 1 



10 2 



10 3 



23 1 



84 1 



90 2 



112 1 



28 1 



28 2 



26 1 

 1 



On an average w T e obtain 



1537'358±0-033.10- n cm. 



* Siegbahn u. Stenstrom, Phys. Zs. xvii. p. 48 (1916). 



Wave-length. 



Reflecting 

 Crystal. 



153724. 



10- n em. 



Rock-salt. 



•34 





. 



•52 





„ 



•40 





. 



•21 





jt 



•44 





it 



•44 





,. 



•22 





Calcite. 



•35 





j? 



•49 





)i 



•29 







