with Mercury contained in Tubes. 



51" 



the change becomes more noticeable. The period o£ lumi- 

 nosity becomes a smaller and smaller fraction of the whole 

 period, until in the typical fan effect it may be as small 

 as 1/25. In all the cases that have been examined, this is 

 the characteristic feature of the portions of the diagrams 

 below AC, and in all cases except those of very fine tubes, the 

 statement may be made that the arc goes out while the columns 

 are still separating. 



As in the case of the necklace effect, increasing the length 

 of the tube lowers the frequency. The relation between 

 frequency and length is not, however, the same in the two 

 cases. In the necklace effect, the length of the tube is 

 practically a measure of the moving mass in a motion which 

 is approximately simply harmonic. In the fan effect the 

 periodic time is only slightly greater than the period of ap- 

 proach, and is thus largely dependent upon the time required 

 for a column of mercury to grow a certain small length when 

 subjected to a constant difference of pressure between the 

 two ends. In this case the internal pressure between the 

 columns drops to a negligible quantity as soon as the arc 

 goes out. So that we should expect that with different tubes, 

 but under the same conditions of external resistance and 

 pressure and on the same voltage, the frequency would vary 

 approximately as the reciprocal of the length. Several 

 series of results bear out this view. The longest series is 

 2'iven below. 



Soda-Glass Tubes. *35 mm. bore, 6 mm. external diameter, 

 56 ohms in series, 105 volts across direct current mains. 



L. 



F. 



L.F. 



3 



100 



300 



4 



78 



312 



n 



59 



295 



r> 



55 



330 



7 



48 



336 



8 



44 



352 



9 



40 



3«0 



JO 



38 



380 



12-5 



33 



412 



15 



28 



420 



20 



24-5 



490 



25 



22 



550 



30 



21 



030 



35 



20 



700 



As a second approximation LF is a linear function of 

 Phil Mag. S. 6. Vol. 22. No. 130. Oct. 1911. 2 M 



