362 Dr. Balth. van der Pol on Measuring without Electrodes 



bright parts of the striations. The same distribution of 

 absorption over the striae was obtained whether the wave- 

 amplitude was strong or weak. This fact may justify the 

 extensive use here of the term "conductivity." 



Fis:. 7 



40 cm, 



A series of measurements was finally made of the conduc- 

 tivity in the positive column when the discharge current, 

 bv means of an external resistance, was kept constant at 

 i = 2*5xl0 -3 amp., while the air pressure inside the tube 

 was varied from ^ = 0*400 to p = 0*140 mm. At the higher 

 of these pressures the positive column was continuous, and 

 as the pressure was lowered, it gradually broke up into 

 striations. 



In a preliminary experiment the absorption in the un- 

 striated column was found to decrease only slightly towards 

 the Faraday dark space. Therefore only a few observations 

 were taken in the unstriated column, these being sufficient 

 to indicate the position of the absorption curve. The result 

 is shown in fig. 8. 



It is seen from the measurements that when the pressure 

 is relatively high (0*400 mm.) and the column is continuous, 

 the conductivity inside the tube is small. When the pressure 

 is reduced to 0*215 mm. the conductivity, for the same dis- 

 charge current, is considerably increased. At this pressure 

 some faint striations began to appear at the cathode side of 

 the column. A further reduction of the pressure (to 0170 

 and finally to 0*140 mm.) causes the column to break up 

 into striae while at the same time the conductivity increases 



