526 
MR. H. A. WILSON ON THE ELECTRICAL CONDUCTIVITY 
consequently continue to slope up, even with the greatest E.M.F.’s that could be 
applied. 
If the electrodes are connected to the galvanometer slmj)ly without any battery, 
there is a small deflection indicating a current from the colder electrode to the hotter 
through the flame. This is easily explained by supposing that some of the negative 
ions at the electrode diftuse out, owing to their high velocity leaving an excess of 
positive behind. The blast of the flame will assist this action by blowing away the 
negative ions as soon as they get away from the surface of the electrode. According 
to this the hotter electrode loses more negative ions than the colder, since there are 
more ions formed at it. In the same way a wire immersed in the flame becomes 
positively charged. This action is almost exactly analogous to the charging of a 
polished zinc plate, when ultra-violet light falls on it, described by Hallwachs, 
Righi, and others. 
If both the electrodes are hot enough to produce ionisation, and if the fall of 
potential at the upper electrode is great enough to make the ions from it come down 
against the blast, then there will be both sorts of ions present in the space between 
the electrodes, which will modify the fall of potential. Since the positive ions move 
so much more slowly than the negative ions, they remain in the gas much longer and 
so have a greater effect on the fall of potential; consequently the fall of potential is 
much greater at the negative electrode than at the positive electrode. The great 
velocity of the negative ions enables a much smaller slope of potential to drag them 
all out than is required to drag out the positive ions, so that unless the E.M.F. 
applied is very great the current is mainly carried by the negative ions. These 
points are very well illustrated by the experiments described above, on the effect of 
specially heating the upper electrode when it is positive. When it is heated the 
positive ions coming from it transfer the fall of potential from the positive electrode 
to the negative electrode, so that all the negative ions there are dragged out and the 
current attains its saturation value as far as the negative ions are concerned. 
If the jDositive electrode only is hot then the positive ions moving across will cause 
all the fall of potential nearly to be at the negative electrode, and owing to their 
small velocity this effect will be more comjflete than when the negative ions alone 
are present. Owing to this, and to the greater difficulty of dragging out the slow 
positive ions from the electrode, the current, when the positive electrode is hot, will 
be much smaller than when the negative electrode is hot. In this way it is easy to 
explain all the phenomena of unipolar conduction. 
In the case where both electrodes are hot, the fall of potential being mostly at the 
negative electrode, the current due to the negative ions attains its saturation value 
at a comparatively small E.M.F., but the current carried by the jjositive ions con¬ 
tinues to increase nearly uniformly with the E.M.F. 
According to this then, in the equations 
