344 
MR. W. C. D, WHETHAM ON IONIC VELOCITIES. 
boundary move under the influence of the electric forces in the same direction as the 
current. Suppose the current reversed. It will now cause a distribution of negative 
electricity over the boundary, which will tend to move it in the direction opposite to 
that of the current. But the current has now also been reversed, and, therefore, the 
boundary tends to move in the same direction relatively to the tube as at first. 
This effect is therefore eliminated by reversing the current, and when the velocity is 
observed to be the same in each direction, within the limits of experimental error, it 
shows that the effect we are considering is negligible. 
The first solutions used were those of copper and ammonium chlorides, with just 
enough ammonia added to each to bring out the deep blue colour of the copper. 
Their strength was about 0T8 grm. equivalent per litre. 
The current was first sent upwards from the copper to the ammonium solution— 
the junction travelled upwards, with the current. The following readings of its 
position were made by a kathetometer ;— 
Voltmeter at 
grade 23’8. 
Time. 
Position. 
8-8 
12.45 
27-20 ■ 
. = 3-62 in 90' = -0402 in 1' 
2.15 
23-58 j 
8-8 
3.55 
19-49, 
= 4-09 in 100' = -0409 in 1' 
The current was then reversed—the junction travelled downwards. 
Voltmeter at 
grade 23-8. 
Time. 
1 
1 
Position. 
7-5 
7-7 
12.30 
3.10 
26 - 83 } = 
If we calculate this for the same E.M.F. as before we get '0441 centim. per minute 
for the velocity coming down. The mean is ‘0423 centim. per minute. 
88 
The E.M.F. as measured by a voltmeter comes out —- X 10‘2 = 37’7 volts, 
2b O 
which makes the potential gradient 273. 
The velocity of the copper ion, under a potential gradient of 1 volt per centimetre, 
and through a solution of strength 078 grm. equivalent per litre, at a temperature of 
15° C., comes out 
0‘00026 centim. a second. 
The value deduced by Kohlrausch from theory for a solution of infinite dilution 
at 18° C. is (‘Wied. Ann.,’ vol. 6, p. 206) 
O’OOOSl centim. a second. 
