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IOWA ACADEMY OP SCIENCE 
tion Induced by radium increases rapidly with increasing expansions from 1.33 
to 1.4. Above 1.4 the nucleation gradually increases and a maximum is not 
attained until the dp. is 34 cm. (dp. is the difference of pressure between that 
in the fog chamber after expansion and the ordinary atmospheric pressure. A 
dp. of 34 cm. corresponds to an expansion of 1.7.) Fig. 1 is a graph by Barns 
showing the nucleation due to radium at various pressures. In this graph the 
ordinates represent the nucleation (the number of condensed particles per cubic 
centimeter in the cloud in units of 1000) ; while the abcissae represent the dp. 
in centimeters. As will be noted, the nucleation begins at a dp. of 19 cm. 
and gradually approaches a maximum after a dp. of 22 cm. is reached. Now 
if Barns’ observations are correct then J. J. Thomson’s assumption that every 
water particle in the cloud at an expansion of 1.33 carries a single ion can 
hardly be accepted since such a result would only be likely at expansions giving 
the maximum nucleations. Barus observed that at the lower expansions nuclei 
carried a variable number of ions. My own determinations at an expansion 
of 1.33 also indicate this fact. 
Pig. 2 shows a curve very similar to Barus’, but one which was obtained in 
an entirely different manner. In this figure the ordinates give the time it took 
for the cloud formed in the fog chamber by expansion to fall a distance of 2 
millimeters under the action of gravity. It will be seen that as dp. increased 
from 16 cm. to 24 cm. the time increased from 3 second to 5.2 second. At the 
low expansion the droplets were heavy, falling rapidly. As the dp. increased 
the droplets became smaller with a corresponding diminished velocity. The 
density of the clouds also increased enormously with the high pressures. Com- 
paring the curve of figure 2 with that of Barus in figure 1, we note that the 
velocity of an ionized cloud under the action of gravity yaries, approximately, 
inversely as the nucleation. 
An account of H. A. Wilson’s method of determining e in the Phil. Mag., 
Series 6, Vol. 5, 1903, page 425. The method has the advantage over J. J. 
Thomson’s in that it is not necessary to know the number of particles in the 
ionized cloud. All that one needs to know is, first, the velocity of the cloud 
under the action of gravity, and, second, the increased velocity under the com- 
bined action of gravity and a static field of known strength. For instance, if 
the force of the static field is X and the charge of the particles in the cloud is e, 
then the total force acting when the field is on is equal to mg + xe; where m is 
the mass of each particle. When the field is not on, the force acting is mg. 
Since the rate of uniform motion of a sphere in a viscous fiuid is proportional 
to the force acting, we have : 
mg ^ Vq 
mg+xe Vi 
Solving e = 3.1X10“® (Vi— Vo) Vo^ 
And m = 3.1X10-®XVof 
Wilson’s determinations varied from 2x10-^® to 4x10-^® and his average was 
3.1x10-^®. All of his determinations were taken at a dp. of 17 cm. 
The correctness of Wilson’s result might be questioned for several reasons. 
First, he used X-rays as his ionizing source, which we know may be extremely 
variable. It is probable that this lack of uniformity of X-rays as an ionizing 
