518 
MR. H. A. 5VILS0X OX THE ELECTRICAL COXDUCTIYITY 
trodes, in which case the current was increased by the presence of the salt even with 
very small E.M.F.’s. 
It is difficult to form any estimate of the velocities from this experiment, because 
the ions must spread out from the electrode at which they start by mere diffusion, 
which will make the velocity appear too great. The fall of potential between the 
electrodes also is not uniform, and cannot be determined conveniently when the 
electrodes are so near together. Nevertheless, the results obtained were of the same 
order of magnitude as those obtained l)y the method described above. When the 
electrode was positively charged an E.M.F. of 0’25 volt was enough to Increase the 
current when the salt vapour was introduced. This gives for the velocity of the 
CBlltllllS 
negative ions 1200 If positively-charged electrode was placed outside 
the flame, then the E.M.F. necessary for the current to increase when the salt vapour 
was introduced at the other electrode was much greater than before. The following 
numbers show this. The electrodes were kept at the same distance aj^art, 1'5 
centim. in each case. 
(1.) E.M.F. required when both electrodes were in the flame . . 0‘25 volt. 
(2.) E.M.F. required when the charged electrode was at the side 
of the flame, so as to be only just very slightly red hot . . 1*2 volt. 
(3.) E.M.F. required when the charged electrode was outside the 
flame.2 "3 volts. 
The explanation of this increase in the necessary E.M.F. can be readily seen by 
considering tire results of the observations on the fall of potential between the two 
electrodes wlien one is much colder than the other. In such a case nearly all the fall 
of potential occurs near the cold electrode, so that the potential gradient available for 
dragging across the ions at the hot electrode is very greatly diminished. 
A third estimate of the velocity of the negative ions was obtained by diverting 
the current between two electrodes, one above the other, in the flame by means of a 
pair of charged electrodes jrlaced outside the flame. Two small platinum electrodes 
were fixed one al30ve the other in the flame, and 3 centims. apart. The two j^latinum 
foil electrodes already described were placed one on each side of the flame, and on a 
level with the lower of the small electrodes. The lower small electrode was put to 
earth and the upj^er one charged positively to of a volt, and the current from 
it measured. The two la,rge electrodes were then charged positively, when it was 
found that tlie current observed was diminished and reversed if they were charged 
to more tlian 0‘4 volt. Since the ol)served current was probably mainly carried by 
negative ions moving upwards witli the blast, l^ecause the E.M.F. used Avas not 
enough to drag down any positive ions, the stoppage of the current on charging the 
large electrodes must have been due to the negathm ions being dragged across from 
the lower small electrode to the large electrodes. These latter Avere 2 centims. apart, 
