302 



Mr. H. A. M c Taggart on the 



The velocity is proportional to the potential gradient, at 

 least for differences of potential that are not very small. 

 Measurements of velocities in fields of less than 6 volts per 

 cm. were not made. The following table gives the time 

 taken by a bubble in moving 40 divisions under a field of 

 16 volts and of 8 volts. Readings were taken at six points 

 equally spaced between the electrodes to observe at the same 

 time the variation in the field. Near the electrodes the 

 times are shortest, i. e. the velocity is greatest. 





1. 



2. 



3. 



4. 



5. 



6. 



16 yolts 



8* 



8-3 



18 

 16 



17 



io| 



10 

 10-3 



21 



20 



20-5 



12 



HI 



11-8 



25 

 23 



24 



Hi 

 12 



11-7 



26 

 23 



24-5 



10J 



10-3 



24 

 20| 



223 



8| 



8-6 



20 

 17 



18-5 





8 volts 







When the voltage is doubled the time is halved, i. e. the 

 velocity is doubled. 



To get an estimate of the actual velocity per unit fall of 

 potential, it is necessary to know the average value of the 

 velocity. This may be got by plotting a curve with the 

 distances of a point on the axis from the electrode as 

 abscissae, and as ordinates the velocities or times to move 

 over a given distance about the point, and finding the average 

 ordinate for the curve. The value obtained for one set of 

 readings was 9'4 sec. which was the average time taken to 

 move 20 divisions of the scale each way or 40 in all. The 

 fall of potential was 22*8 volts per cm. This gives a velocity 

 in cm./sec./volt/cm. of 



40 



45-10 9-4 22-8 



= 4-1.10- 



An average of four sets of readings at another time gave 

 3*9 . 10~ 4 . It may be concluded then that the velocity of 

 small spheres of air in water of this purity is about 4 . 10 ~ 4 

 cm./sec./volt/cm. 



No effort was made to prepare water of extreme purity, 

 but rather to obtain by a fixed method of distillation water 

 of fairly constant quality. It invariably contained air and 

 carbon dioxide, to some extent. Under these conditions 

 spheres of hydrogen, oxygen, and illuminating gas all had 



