The potential gradient in the arc is very far from being uniform.
Thus we get the potential gradient, which averages about 150 volts per meter in the lowest part of the atmosphere.
A diminution in the number of positive ions would thus naturally be accompanied by a rise in potential gradient.
For the same current, therefore, the potential gradient is higher in the coloured solution and lower in the colourless one.
The potential difference between the two is recorded, and the potential gradient is thus found.