MR. W. C. D. WH-ETHAM ON IONIC VELOCITIES. 
343 
corks. The corks fitted loosely, and a vertical groove, filed in each, allowed any gas 
which might be evolved to escape. The junction tube had a length, from B to C, of 
12'6 centims., and a diameter of about 1'2 centims. If*we add one-half the diameter 
as a correction for each end we get 13’8 centims. as the effective length of the tube, 
which must be divided into the total difference of potential between the electrodes to 
give the potential gradient. 
The advantages of such an apparatus are shortly as follows :—• 
1. Any gas evolved at once escapes without interfering with the experiment. 
2. Since the liberation of ions and decomposition of salt occur only at the 
electrodes, it is (in general) only near them that alterations of concentration occur. 
Since they are surrounded by a large mass of liquid, these alterations will not 
appreciably change the resistance of the tube as a whole, and will be entirely without 
efiect on the resistance of the column of liquid in the narrow tube, which is the 
important point. 
3 . Changes in density at the electrodes will not appreciably accelerate or retard 
the velocity of the junction. Take the case of copper chloride when the current is 
passing down as a fairly typical example. The average current was -^ 5-0 in C.G.S. 
00 X 00 
units. The quantity of electricity which passed in one hour was, therefore, ----- - , 
and the weight of copper deposited, 
31*5 X X 10““^= ’0045 grm. 
2o50 ® 
Supposing that all this copper comes from the kathode vessel, and that when 
deposited it occupies no volume (both of which suppositions would increase the 
disturbing effect we are considering), half this weight of solution wmuld pass from 
anode to kathode to bring the liquids in the two tubes to their original levels. The 
area of cross-section of the tube is ()‘430 sq. centim. (see p. 347). Therefore the 
passage of '0023 cub. centim. of solution from anode to kathode in one hour would 
mean a velocity of '0054 centim. per hour. The actual velocity observed in this case, 
due to the ionic velocities, was 1'6 centims. per hour. The correction for differences 
in density is thus utterly negligible. 
4. A more important objection was pointed out by Professor J. J. Thomson. 
With salt solutions of equivalent strength there is, in general, a difference of specific 
resistance, although this is small in most cases. At the junction there will thus be a 
discontinuity of potential gradient, and, therefore, a distribution of electricity, which 
will be acted on by the electric force and introduce disturbances at the boundary. 
By a little consideration we shall see that this is a non-reversible efiect, and should 
be eliminated by reversing the current and taking the mean of observations in each 
direction. Suppose that the current is passing in the direction which gives a 
distribution of positive electricity over the boundary. This will tend to make the 
