510 
ME. H. A. WILSON ON THE ELECTRICAL CONDUCTIVITY 
ionised thronghont the flame, the current should ha\m increased with the distance 
between the electrodes. To test this more completely, two electrodes of platinum 
foil, each 1’5 centim. square, were supported opposite one another in the flame so 
that the distance l^etween them could be easily varied. The salt solution sj^rayed 
was a g-Q normal rubidium chloride solution. With this arrangement it was found 
that, with a potential diflerence of 800 volts between the electrodes, moving the 
positive electrode did not affect the current between the electrodes unless it was 
moved so near to the side of the flame that it became comparatively cool, in which 
case the current was diminished. Moving the negative electrode usually affected the 
current, the current being greater the hotter the electrode appeared. If the negative 
electrode was placed at about the axis of the flame, then it could be moved several 
millimetres either way without appreciably affecting the current, but the effect of 
moving it to at all near the sides of the flame was to diminish the current, this effect 
being much more marked than in the case of the positive electrode. 
The amount of salt vapour passing between the electrodes is roughly proportional 
to the distance between them, so that since the potential difference used was fully 
enough to approximately saturate the gas, the current should have increased with the 
distance between the electrodes if the ionisation took place throughout the volume of 
the flame. 
If the two platinum electrodes just described were placed one on each side of the 
flame, just far enough from it not to be visibly Iieated, and about half-way up the 
flame, only a very small current could be passed between them, even when an E.M.F. 
of 400 volts was applied. This current, moreover, was only slightly increased when 
the flame was filled with a salt vapour. The following are the currents observed in 
one case with 45 volts E.M.F. :— 
(1.) Both electrodes just outside the flame and not visildy hot. Distance between 
the electrodes 2’5 centims.— 
Current without salt. 3 divisions. 
Current with ^ BboCOg.12 ,, 
(2.) Both electrodes just inside the flame and red hot. Distance between the 
electrodes 2 centims.— 
Current without salt.18 divisions. 
Current with -g-Q BlnCOg . 610 ,, 
It is clear that the heating of the electrodes enormously increases the available 
conductivity, exactly as though the ionisation did not take place unless the electrodes 
were red hot. There is, nevertheless, a very small amount of conductivity even when 
cold electrodes are used, as has l)een known for a long time. This is no doubt due to 
a small amount of ionisation really taking place throughout the volume of the flame, 
both in the case of the flame gases and of the salt vapour. 
