520 
MR. H. A. WILSON ON THE ELECTRICAL CONDUCTIVITY 
Fig. 5 shows the essential parts of the ap})aratns for getting the velocity of the 
ions in the stream of liot air. 
F is the liange on the platinum tube. An electrode, A, consisting of a grating of 
platinum wires, 0'25 millim. in diameter and 2 millims. apart, wound on pegs in a brass 
disk, 5'5 centims. in diameter, was placed opposite to the flange, and the stream of 
hot ail' kept the grating red hot. This electrode could he charged up by means of 
battery of small accumulators, and so an electric field maintained between A and F. 
A small head of the salt to be investigated was put midway between A and F in the 
current of hot air, and was supported by a platinum wire held by a glass tube, B. 
B 
<—^ WOT a/R ^ 
r/G. 6 
An electrode, C, was also introduced into the stream of hot air as near the mouth 
of the tube as possible. This consisted simply of a ring of fine platinum wire 
supported in a glass tube, which insulated the wire leading to the ring. 
The ring electrode was connected to a galvanometer of 11,000 ohms resistance, and 
through this to the platinum tube, which wars also connected to “ earth.” The bead 
of salt was slowdy volatilised in the current of hot air and its vapour passed through 
the grating. On the hot grating some of it was ionised, and if the electric field 
betw'een the grating and the ring electrode and liange "was strong enough, some of the 
ions moved against the blast of air to the ring electrode, so jiroducing a current -which 
was indicated by the galvanometer. 
It was thus possible to determine the P.D. necessary to make the ions move against 
the blast of hot air exactly in the same way as rvas done in the flame. If the velocity 
of the blast and the potential gradient corresponding to this necessary P.D. -were 
known, the absolute velocity of the ions could be at once calculated, but, unfortu- 
