ME. 0. W. RTCHARDSON OX THE ET.ECTRICAL CONDUCTIVITY 
positive cuiTent was invariably about 30 times as big as the negative. This was 
probably due to sodium having condensed on the wire electrode. For, although the 
wire had a surface per unit-length of less than one-hundredth that of the steel tube, 
the surface ionisation would be far easier to saturate; so that we should expect the 
currents in the tAvo directions under a mven voltae'e to have a ratio considerablv less 
than 100 to 1. The above high value of the ratio obtained initially Avould correspond 
to the stage when no sodium had condensed on the wire. 
In making these experiments the apparatus was first pumped down to a pressure 
of about "1 millim., ljut it was found that on heating the steel tube a considerable 
amount of gas was given off. At first the amount of gas evolved was so great that 
the pressure in the apparatus, the volume of wliich was very considerable, rose to 
several centimetres of mercury. This evolution of gas Avas noticed in CA^erA^ case 
Avhen sodium was heated, but by continued heating it usually became very small. 
In this jjarticular instance, eA^en after heating for several days, on pumping the 
apjiaratus out and heating again it Avas found that the pressure rapidly rose to about 
5 millims. It Avas thought that the gases from the furnace might perhajDS diffuse 
througli the steel tube. To prevent this the latter AAms covered with a layer of 
soluble glass, which Avas carefully dried oii; this seemed to liave the desired effect, 
for it Avas found that afterAvards there was no difficulty in keeping the pressure below 
a millimetre even Avhen the tube Avas heated to 450° C. 
Experiments Avere next made to investigate the Avay in Avhich the current varied 
Avith the potential Avhen tlie wim AjB, AA^as charged positively. It Avas found that 
the current E.M.F. curves Avere markedly 
different from those previously obtained AA-ith 
caiFon and platinum. The current Avas small 
at first and increased much more mpidly Avith 
the voltage than if the tAvo Avere proportional. 
In fact, the general shape of the current E.j\I.F. 
curves Avas much like tliat of the cui’A’es for 
current and tem])erature obtained AAuth carbon 
and platinum. There was no indication of 
saturation at any potential. 
These differences are to be attributed to the 
difference in the experimental conditions and 
especially in the shape of the electrodes. In 
the case of sodium Ave haA-e a larue ionisation 
produced at the inner surface of a Avide tube, 
and it is a Avell known fact that it is difficult to 
saturate the current to a wire inside the tube 
in such a case, owing to the Aveakness of the electric field near the surface. 
The accompanying curve (fig. 18) gives the relation l.)etAveen the current and the 
