70 
PROF. II. A. WILSON ON THE ELECTRICAL CONDUCTIVITY AND 
Part IT.— Velocity of the Positive Ions of Salt Vapours in Flames. 
Some new experiments on the velocity of the positive ions of salt vapours in flames 
will now be described. 
The apparatus of the previous section with some modifications was used. The 
central part of the grating at the top of the tube, A A' (fig. l), was removed and an 
insulated platinum wire electrode supported in the gap, so that the grating formed 
a guard ring round it. The wire supporting this electrode passed through a-hole in 
the tube, AA', about 3 cm. from the top of it. This electrode was connected to 
a galvanometer, the other terminal of which was connected to 
the burner, B. The arrangement is shown in fig. 4. The upper 
grating electrode was connected to earth and the burner, B. was 
charged negatively. The galvanometer then indicated a small 
current passing down the flame from the upper grating to the 
electrode in the middle of the lower grating. Most of the current 
through the flame went to the guard ring and so was not indicated 
by the galvanometer. The potential gradient in.the flame could be 
measured with the wire, PP', as before. The velocity of the flame 
was found with the puffs of salt vapour, but the puffs were not 
sent into the flame except when its velocity was being measured. 
A bead of salt on a platinum wire could be introduced into the 
flame just below the upper grating, and the effect of the bead on 
the current through the galvanometer and on the potential gradient 
was determined. The distance between the upper and the lower 
gratings was about 7 cm. 
It was found that putting in beads of lithium, sodium, potassium, 
rubidium, caesium and strontium salts had no appreciable effect on 
the current or on the potential gradient below the bead with any 
potential difference from zero to 2700 volts. The potential gradient 
just below the bead was about 100 volts per centimetre when the 
P.D. used was 2700 volts. The velocity of the flame was 300 cm. 
per second. This shows that the positive ions from the beads 
did not move down the flame, even with the P.D. of 2700 volts, 
so that their velocities due to 1 volt per centimetre were less than 3 cm. per second. 
The distance between the upper and lower gratings was then reduced to about 
3 cm. It was then found that putting in a bead of any salt just below the upper 
electrode slightly increased the current and also slightly increased the potential 
gradient below the salt. The gradient in the salt vapour above the bead was very 
small, so that, since the salt vapour occupied about 1 cm. of the flame below the upper 
electrode, the effect of the salt was much the same as moving the electrodes 1 cm. 
Fuj.i. 
F, flame. 
B, .burner. 
AA', tube round 
burner. 
G, upper grating. 
W, wire e 1 e c- 
trode. 
