﻿388 Prof. H. A. McTaggart on the Electrification 



the worm E. The pointer rests by its own weight on the 

 top of the post, but at any instant in its motion over the fixed 

 dial D it may be raised and stopped by a small electromagnet 

 controlled by the key B. When released it falls back on the 

 post and begins to record time with the same regularity as 

 the tuning-fork. It forms a very convenient stop-watch if 

 velocities are to be measured. 



A travelling microscope M measures the distance travelled 

 by any bubble on the axis of the rotating cell. 



The water used was twice distilled — the second time in 

 " Pyrex " glass and condensed in a silver coil. 



The thorium nitrate was by Merck, and was assumed to 

 have 12 H 2 — water of crystallization. 



A stock solution was made up containing 4 x 10~ 6 equiva- 

 lents per c.c. (1/250 normal), and from this other solutions 

 were made by successive dilation. 



A first series of readings was taken with various concen- 

 trations of the salt, but with bubbles of nearly the same size 

 in order to reproduce the effects previously observed — the 

 method of working being to fill the cell A with the desired 

 solution, introduce a single bubble of air with the gas pipette, 

 and set the cell in rotation. The bubble very soon takes up 

 a steady position on the axis, and its motion under any fall 

 of potential L may be examined. 



Very small concentrations sufficed to reduce to zero the 

 natural negative charge found in pure water and to give 

 the small sphere of air a positive charge. 



The following readings are typical : — 



Fall of potential 34 volts per cm. 



Diameter of bubble 0*3 mm. 



Concentration. ^. f Velocity of 



Equivalents ^ n ot bubble. 



per c.c. c lar S e - cms./sec/volt./cm. 



4X10~ 7 + 5xl0~ 4 



4X10-8 m . „ + slower 



8X10" 9 + very slow 



(1-5X10- 4 ) 



5-7X10'" 9 - slow 



47X10- 9 - faster 



4xl0 -9 - faster 



Pure water — 4xl0 -4 



