Page:Philosophical Transactions of the Royal Society A - Volume 184.djvu/344

344 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 0.18 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:— The current was then reversed—the junction travelled downwards. 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.

The E.M.F. as measured by a voltmeter comes out $$\frac{88}{23.8}\times10.2 = 37.7$$ volts, which makes the potential gradient 2.73.

The velocity of the copper ion, under a potential gradient of 1 volt per centimetre, and through a solution of strength 0.18 grm. equivalent per litre, at a temperature of 15° C., comes out

The value deduced by from theory for a solution of infinite dilution at 18° C. is (‘Wied. Ann.,’ vol. 6, p. 206)