ME. W. C. D. WHETHAM ON IONIC VELOCITIES. 
840 
Average upward velocity . 
,, downward velocity 
Centim. per minute. 
. = -1195 
. = -1205 
Mean .... = ’120 
Voltmeter, 10‘9. 
Therefore specific ionic velocity 
= 0'00059 centim. per second, 
winch is appreciably greater than for the stronger solutions. 
The success of these preliminary determinations and the closeness of their agree¬ 
ment with theory led me to consider whether any improvement in the method was 
possible, by which further and more accurate results could be obtained. 
In order to eliminate entirely the effect of the discontinuity of potential gradient 
it is necessary to use pairs of salts wdiich have the same specific resistance for the 
same strength of solution. This very much limits one’s choice, but an example will 
be found below. In such cases as that of copper and ammonium chlorides where the 
velocities in ojjposite directions are sensibly the same, it is fair to conclude that this 
efiect is negligible, but it was worth while to find one case at any rate where it could 
not in the least afiect the result. 
The next improvement was in devising a means of getting over the necessity of 
directly estimating the potential gradient. 
Consider the kation of one salt AC at the junction. Let its specific ionic velocity 
be ^ 1 , and its actual velocity v, under a slope of potential = dYJdx. Let r be the 
specific resistance of the solution. Let y be the total current passing, and R the 
total resistance of the column of llcjuid from the anode to the junction. 
Then we have v-= dYjdx. 
Since y is constant along the tube we get by Ohm’s law 
V = vyy 
(/E 
dx 
Now since the tube is of uniform bore near the junction d^jdx = the Increase of 
resistance which would occur if we supposed the column of liquid from the electrode to 
the junction, whose resistance we have called R, to be increased in length by unity. 
This increase of resistance = r/A, where A represents the area of cross-section of 
the tube at the junction. 
Therefore 
rfR _ »■ 
dx A 
and we get 
V = vy/vjK, 
or 
Vy = vkjyr. 
