516 
MR. H. A. WILSON ON THE ELECTRICAL CONDUCTIVITY 
chlorides of the same metals the same result appeared to hold, but the much greater 
volatility of the chlorides made it difficult to obtain satisfactory results. 
To compare the velocity of the positive ion due to a potential gradient of 1 volt 
per centim. with the velocity of the flame gases, it is necessary to know the potential 
gradient between the electrodes when the P.D. is just enough to make the ions go 
down the flame. Diagram No. 13 shows the fall of potential in the flame without 
salt when this is the case with a P.D. of 107 volts between the electrodes 5 centims. 
apart, the upper electrode being positive. 
Diagram No. 1.3. 
The smallest potential gradient on this curve is 3’3 volts per centim., which is, 
therefore, the gradient necessary to make tlie positive ions of the alkali metal salts 
move against the gases of the flame. 
The mean velocity of the mixture of gas and air in the tube leading to the flame 
was approximately 206 centims. per second, as determined from the volumes of gas 
and air supplied to the apparatus. On entering the flame the gases of course expand 
very greatly, but the effect of this expansion is probably not great on the upward 
velocity of the gases, since the flame is free to expand laterally. Taking the velocity 
of the gases in the flame as 206 centims. per second, the velocity of the positive ions 
in tlie flame due to a potential gradient of 1 volt per centim. is therefore 
206 
•’.o 
, ^ centims. 
= 62 
sec. 
In determining the P.D. necessary to make the negative ions move down against 
the flame gases, a long wire through which a current was passing was used to supply 
the potential differences recpiired. Diagram No. 14 shows the results obtained with 
a bead of Na^COg. 
When the E.M.F. is less than I volt, there is a small current opposed to the 
applied E.M.F. At about I volt the current with the bead suddenly begins to 
increa.se rapidly, with the E.M.F. indicating that the negative ions have begun to go 
down against the blast of the flame. 
