Electrification at Liquid-Gas Surfaces. 



299 



must be due to the presence of an electric charge upon the 

 surface of the bubble, and its velocity will then be a measure 

 of the charge. An examination of these velocities will thus 

 provide some farther data regarding the electrification at 

 liquid-gas surfaces. In his experiments Quincke used glass 

 tubes of small bore (0'4 to 2 mm. diam.) in which to observe 

 the motion of the bubbles. One disadvantage in such small 

 tubes is the disturbing effect of the endosmose currents, 

 besides which there is the difficulty of keeping the bubble from 

 sticking to the wall. Instead of this it is possible to use a 

 glass tube rotating about its axis as shown in hg. 1. 



Fisr. 1. 



The ends are closed by metal electrodes which are counter- 

 sunk to receive the pivot points upon which the tube rotates. 

 When the tube is in motion the current is led in through the 

 liquid by means of the pivot supports. A bubble of gas in 

 the rotating liquid takes up a position on the axis if the 

 bubble is fairly small and if the speed of rotation is high 

 enough. The motion of the sphere of gas along the axis 

 due to an applied electric field may then be examined. This 

 is the method employed in this paper. 



To reproduce the effects observed by Quincke a preliminary 

 trial was made with a rotating cell of the form shown in 



fig. 2. 



Fig, 2. 



It consists of a piece of capillary tubing near the ends of 

 which two bulbs are blown. The inner surface from the 

 ends up to the dotted lines was silvered in order to provide 

 electrodes as large as possible. A small hole was left in one 

 of the bulbs through which a bubble of gas could be intro- 

 duced. This could be done very easily when the tube was at 



