column and other 'phenomena connected with the discharge. 311 



distance increases, but the strength of current required to produce 

 it gets less, so that at pressures of a few tenths of a mm. (see 

 next section) that phase in which an increase in current produces 

 a separation of the striae may be nearly absent, and the current 

 required to produce the maximum separation is nearly equal to the 

 smallest current required to maintain the discharge. 



The presence of this maximum explains the observation of 

 Miiller and De la Rue (loc. cit.) that sometimes an increase in 

 current causes the striae to separate, while in other cases the 

 reverse effect is obtained. 



With air and nitrogen the effect is rather different. Starting 

 from a current just large enough to maintain the discharge, an 

 increase causes a rapid separation of striae nearly proportional to 

 the current until the latter is nearly four times its original amount, 

 after which the distance between is constant. 



The curve in fig. 3 is plotted from measurements taken on 

 the same tube as that from which the curves in the previous 

 figure were obtained, when it was filled with nitrogen at a pressure 

 of 1'16 mm. 



50 50 70 90 MO 130 



GAL . DE FLEX '/ON 



150 



170 



Fig. 3. Showing the relation between current and distance apart of striae in a 

 tube 12 mm. in diameter filled with nitrogen at a pressure 1-16 mm. 



Influence of the variation in pressure. 



7. In taking observations on the effect of pressure, from 

 what has just been said, it is seen that the current must be kept 

 constant throughout. Four sets of readings were taken at each 

 pressure, when possible, with different currents, but it was not 



