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W 452
[FROM THE AMERICAN JOURNAL OF SCIENCE, VOL. XXXVI, Oct., 1913.]
UM A 1914
THE DISTRIBUTION OF THE ACTIVE DEPOSIT
OF RADIUM IN AN ELECTRIC FIELD (II)
By E. M. WELLISCH
Assistant Professor of Physics at Yale University
(Contributions from the Sloane Physical Laboratory, Yale
University, New Haven, Conn., U. S. A.)
THE
AMERICAN JOURNAL OF SCIENCE
FOURTH SERIES.]
ART. XXXII.-The Distribution of the Active Deposit of
Radium in an Electric Field (II); by E. M. WELLISCH,
Assistant Professor of Physics at Yale University.
Introductory.
1. The experiments described in the present paper are a
continuation of the investigation made by Wellisch and Bron-
son* on the distribution of the active deposit of radium in an
electric field. In that investigation radium einanation mixed
with air was introduced into a cylindrical condenser and the
relative amounts of active deposit which settled on the central
electrode and on the case were determined after equilibrium
had been established for different positive potentials applied to
the outer electrode.
It was shown that the part of the active deposit which settled
on the case (anode) was due to the diffusion of uncharged car-
riers; no evidence was found of the presence of negative car-
riers in any appreciable amount. It was found also that, when
the applied potential was not too small, the distribution of the
active deposit was independent of the quantity of emanation
employed, and that the fraction of the total amount which
settled on the cathode in general increased with increasing
potentials, although under the most favorable conditions there
was still about 10 per cent which was deposited on the case.
The passage of Röntgen rays through the gas during the expo-
sure was found to be without effect on the distribution except
when the applied potential was small, in which case the extra
ionization produced by the rays caused increased recombination
with the charged active deposit particles and in this manner
the cathode deposit was diminished. Finally it was found that
for potentials which were not too small the ratio of the equi-
* Wellisch and Bronson, Phil. Mag., ser. 6, xxiii, p. 714, 1912.
AM. Jour. Scr.-FOURTH SERIES, VOL. XXXVI, No. 214.--OCTOBER, 1913.
21
316 E. M. Wellisch-Distribution of the Active Deposit
librium ionization currents in the gas for two different poten-
tials was equal to the ratio of the corresponding cathode
activities.
The main object of the present series of experiments was to
extend the investigation in various directions ; in particular, it
was thought desirable to ascertain the effect on the distribution
of employing a containing vessel of different dimensions and
of mixing the emanation with gases other than air, and, in
addition, to make a detailed investigation of the distribution
FIG. 1.
E
K
с
- E
B
ELEC.
A
TEMTH.
P
1
mmmmm
wwwmmmmm
火 ​L
s'd
ways
Willi
Wholeh
-E
when small potentials were employed. The main experimental
results of the previous research have been confirmed, but the
fresli results which have been obtained necessitate a modifi-
cation of the theory which was suggested in explanation of the
phenomena.
Experimental Procedure.
2. The method employed for ascertaining the distribution
of the active deposit was the same as that which had previously
been employed." The radium emanation obtained usually from
of Radium in an Electric Field.
317
a quantity of carnotite, or in some cases from an aqueous solu-
tion of a radium salt, was passed into the containing vessel and
remained there under the desired conditions of potential, pres-
sure, etc., until radio-active equilibrium was established; in
general this period was about 3 hours. The emanation was
blown out by means of a strong current of air from a force-
pump, and the ebonite plug containing the central electrode
was then removed. A fresh electrode was suspended in the
vessel and the ionization current due to the case activity was
measured at 10 and 15 minutes after the emanation had been
removed; the activity on the central electrode was measured
by suspending it in a vessel of construction identical with that
which contained the emanation and observing the ionization
current at 20 and 25 minutes after the emanation had been
removed. The activity when in equilibrium with the ema-
nation was then calculated in the usual manner.
The diagram of connections is the same as that given in the
previous paper and is reproduced in fig. 1; in the present
investigation R and R' were wire resistances of 50,000 and
100,000 ohms respectively. The Dolezalek electrometer had a
platinuin suspension, and with 120 volts on the needle the sen-
sitiveness was 180mm per volt. C and B represent capacities
which could be added to the system by means of the key K,
and the total capacity of the system was then increased 21
times. A potentiometer device, not shown in the diagram, was
employed when measurements of the ionization current were
made; this device enabled the range of swing of the electrom-
eter needle to be so adjusted that its mid-point coincided with
the zero of the instrument, a precaution which was especially
necessary, when the applied potential was small. Reference
should be made to the previous paper for a more detailed
account of the apparatus and method of procedure.
Eeperiments with a Cylindrical Condenser of small diameter.
3. In the previous experiments the greatest amount of active
deposit that settled on the cathode was about 90 per cent of the
total; this occurred for a potential of 4000 volts. It was of
interest to ascertain the effect of applying a large potential
across a smaller distance so as to obtain very large values for
the electric field. For this purpose two cylindrical vessels
were constructed of the following dimensions :
Height
Inner diameter
Length of central electrode
Radium emanation mixed with air at 1 atmosphere pressure
was introduced into one of these vessels, and when a positive
potential of 3000 volts was applied to the case it was found
140 mm
1
-
1
19
101
318 E. M. Wellisch-Distribution of the Active Deposit
mm
that about 86 per cent of the deposit settled on the cathode.
In all probability some part of the cathode activity made its
way to the ebonite insulation, but there did not appear to be
any gain in the cathode deposit as a result of decreasing the
cross-section of the containing vessel.
Cathode Deposit in dry Air at different pressures.
4. Throughout the remainder of the experimental investi-
gation use was made of the two cylindrical vessels which had
been employed in the previous research. These vessels were
identical in construction and the dimensions of each were as
follows:
Heigbt (inside)
1400
Inper diameter
58
Exposed length of central electrode 132
Diameter of central electrode
1.83
The inner electrodes were made longer than those which
had previously been employed, and care was taken that no
appreciable part of tlie active deposit was able to settle on the
ebonite insulation. During the course of the present experi-
ments a fact was noted which had previously escaped observa-
tion. Discrepancies, in general small, occurred in the values
for the cathode deposit when the experimental conditions ap-
peared to be identical. Repeated attempts to ascertain the
cause of the discrepancies were for a long time unsuccessful,
but finally it was ascertained that the inconsistent results arose
from the presence of small quantities of water vapor in the gas.
In the previous research a test had purposely been made to find
the effect of neglecting to dry the gas with which the emanation
was mixed; this test appeared to show that the cathode deposit
was unaffected by omitting this precaution. However, the fal-
lacy of this result was shown by more thorough investigations.
The effect of water vapor is to diminish the cathode deposit
and is especially marked when the gas pressure is high and the
applied potential fairly sınall; in these circumstances an amount
of water vapor which was not sufficient to produce any percep-
tible increase in the recombination of the ions present in the
gas might easily diminish the cathode deposit by 30 to 50 per
cent. Further illustrations of this effect are given later, but in
future experiments extreme care was taken to dry the gas with
which the emanation was mixed. This was done by passing
the gas through several tubes containing P,0, and glass wool
before it entered the testing vessel.
The following table gives the values obtained for the cathode
deposit expressed as a percentage of the total deposit when the
emanation was mixed with dried air at pressures of 210mm and
760mm and various positive potentials were applied to the case :
of Radium in an Electric Field.
319
Potential in Volts
Percentage Cathode Activity
Air at 210 mm.
Air at 760 mm.
1
20
40
160
1030
2000
4000
88.7
88.8
89.2
88.8
65.3
74.8
83.9
S9.2
89.2
These values bave been corrected for the amount of un-
charged deposit that diffuses to the cathode during the expo-
sure; this correction was made by assuming that the uncharged
deposit particles were distributed on the cathode and the case
in proportion to the exposed areas, which were as 1:50. The
figures given represent accordingly the number of positire car-
riers of activity expressed as a percentage of the total number
of carriers.
In order to demonstrate experimentally that the activity
which appeared on the anode was almost entirely due to the
diffusion of uncharged carriers, several experimental deterini-
nations were made of the distribution of the active deposit
when a large negative potential was applied to the case. AS
an example of the results obtained in this connection it was
found that when the emanation was mixed with dry air at 1
atmosphere and with a negative potential of 160 volts, less than
2 per cent of the total deposit appeared on the central electrode
(anode), showing that no appreciable part of the active carriers
are negatively charged.
For potentials greater than about 40 volts the percentage
cathode activity is independent of the amount of emanation
employed unless the amount be inordinately large; over the
same range of potentials, moreover, it was verified that the
ratio of the two ionization currents obtained for any two poten-
tials was identical with the ratio of the corresponding percent-
age cathode activities. The values obtained for the percentage
cathode activity for air at 210mm pressure are greater than those
obtained in the previous research ; it is probable that this dis-
crepancy was due to the fact that in the previous experiment
some of the ebonite insulation was exposed to the emanation
so that the central electrode did not receive all the positive
carriers. The insulation would probably act as a partial con-
ductor, especially at the lower pressures when it would be
exposed to the a-radiation proceeding from a considerable dis-
tance. In the present experiinent this source of error was
carefully avoided, and the result appears to be that the same
320 E. M. Wellisch-Distribution of the Active Deposit
maximum value is obtained for the percentage cathode activity
both for the lower and the higher pressure.
In the previous work the assumption was made that 100
per cent was the limiting value which the cathode deposit ap-
proached as the potential was increased, and that even at the
low pressures the saturation attained was merely apparent.
This assumption was made chiefly as a result of the exper-
imental observation that the percentage cathode activity was
greater at the higher than the lower pressures. Since, however,
it has now been shown that the percentage cathode activity
has the same value (89.2) at the higher potentials for both
pressures, it appears much better to regard this as the true
limiting value. The gradual increase of the values for one
atmosphere for potentials above 40 volts shows that the phe-
nomenon of columnar recombination is present; the active
deposit particle recoils into the gas after the expulsion of the
a-particle from the atom of emanation and tends to recombine
with the negative ions which it forms along its path.
Experiments made with a steel instead of a brass central
electrode gave the same limiting value for the percentage
cathode activity, indicating that this value does not depend
upon the nature of the material of which the electrodes are
composed.
At low pressures, as is well known, a considerable number
of the active deposit particles may reach the walls of the con-
taining vessel before their velocity is sufficiently reduced to
enable them to be directed by the electric field. With air at
a pressure of 6min and with 180 volts the percentage cathode
activity was found to be 66.7.
Experiments with small applied Potentials.
5. The experiments described in the preceding section refer
to potentials for which the distribution of the active deposit
was independent of the amonnt of emanation employed. For
smaller potentials the distribution depends considerably on the
amount of emanation ; this arises froin the fact that with these
potentials recombination can occur between the positire par-
ticles and negative ions which are produced in the volume of
the gas, whereas for the larger potentials recombination can
only occur to any appreciable extent with negative ions which
are present in the same column as the active particle.
A number of experiments were performed to ascertain in
what manner the cathode deposit depended upon the amount
of emanation for any applied potential, and especially to see
whether the distribution would vary in the same way as the
ionization current which passed through the gas during the
exposure.
of Radium in an Electric Field.
321
In fig. 2 there are given two sets of curves, which rep-
resent the resnlts obtained in this series of experiments.
The abscissæ represent the ionization current in scale divi-
sions per sec (with added capacity) due to the emanation and
active deposič in equilibrium when a positive potential of
160 volts was applied to the case.
Inasmuch as this potential
afforded the same percentage (viz. 94:3) of the saturation
current whatever amount of emanation was employed, the
abscissæ (denoted by Ilco) serve as a measure of the saturation
current.
FIG. 2.
1.10
1000
160
1.00
Iy
1160
Av
A160
CONTINUOUS CURVES: IONIZATION
BROKEN CURVES: ACTIVITY
40
.90
20
!
1
1
.80
20
JE
.70
12
8
.60
.50
40
I
4
.30
.20
2
10
IONIZATION CURRENT AT 160 VOLTS : 1160
1.00
2.00
3.00
4.00
5.00
6.00
7.00
160)
The continuous curves in fig. 2 lave as ordinates Iv/1.60
i.
e., the value of the current obtained with V volts applied to
the case expressed as a fraction of the current obtained when
160 volts were applied.
The broken curves refer to the active deposit and have as
ordinates A, A., i. e., the cathode deposit obtained with V
volts applied to the case expressed as a fraction of the cathode
deposit obtained when a positive potential of 160 volts was ap-
plied. As mentioned above, the cathode deposit obtained for
a potential of 160 volts was only 83.9 per cent of the total
ainount, and the maximum amount obtainable on the cathode
for very large potentials was 89.2 per cent of the total.
E. M. Wellisch-Distribution of the Active Deposit
By plotting the curves in this manner the two sets become
comparable ; the continuous curves afford a measure of the
fraction of the total number of positive ions which reach the
cathode corresponding to any potential V, while the broken
curves similarly afford a measure of the fraction of the total
number of positively charged particles which settle on the
cathode.
A very large number of experimental results were used in
order to plot the curves ; for the sake of siinplification the
individual results are not recorded in the diagram.
The curves in fig. 2 all refer to the values obtained when
the air with which the emanation was mixed was thoroughly
dried as described in Section 4. The effect of a small amount
of water vapor was especially marked when the applied
potential was small. In illustration of this point some of the
results obtained for dried and undried air are recorded below:
A,
Iv/1,
1, 60
160
1 60
Air at 1 atmosphere :
V
= 8 volts
dried with special caution
containing slight traces
of water vapor
38.4
•61
3.21
31.2
.61
3.21
It is worthy of notice that the presence of small quantities of
water vapor does not appreciably diminish the fraction of posi-
tive ions which reach the cathode, whereas the effect on the num-
ber of positively charged deposit particles is considerable. It
has for some time been known that water vapor is effective in
causing increased recombination of ions, but the above results
serve to show that the ions are not nearly so sensitive to‘the
presence of vapor as the active deposit particles.
Referring again to the curves of fig. 2, it is seen that in gen-
eral the 'activity' curve for any given voltage lies below the
ionization curve for the corresponding voltage. This is almost
certainly to be ascribed to the increased recombination with
negative ions which occurs with the active particles as com-
pared with the positive ions even when the air with which the
emanation is mixed is thoroughly dried.
Cathode Deposit for very small quantities of emanation
in dry Air.
6. If the curves in fig. 2 are produced so as to intersect the
axis of ordinates, we obtain points which afford a measure of
the fraction of ions and of positively charged deposit particles
which would be obtained by the application of the corresponding
of Radium in an Electric Field.
323
potential when the air in the ionization vessel contains only a
very small amount of emanation. These points are plotted
both for ionization current and activity as separate curves in
fig. 3; they may be regarded as limiting curves which corre-
spond to the absence of volume recombination in the vessel
even at the smallest potentials employed. The upward slope
of the curves is due entirely to the fact that increasing poten-
tials prevent more and more the recombination of the positive
ions or particles with negative ions which are produced inside
the a-particle column.
It will be seen from the curves that any given potential
brings over to the cathode a larger fraction of ions than of pos-
FIG. 3.
1.10
1.00
Ly
: CONTINUOUS CURVE
Igo
Av
: BROKEN CURVE
A160
.90
.80
.70
.60
.50
40
.30
.20
.10
0
10
20
30
A0
50
60
70
80
90
100
110
120
130
160
140 150
VOLTS.
itively charged deposit particles ; at a potential of about 40
volts the two curves practically coincide. The difference be-
tween the two curves can be explained by supposing that the
negative ions which are produced in the column. recombine
with the positively charged active particles with greater facility
than with the positive ions.
Emanation mixed with Carbon Dioxide, Hydrogen,
and Ethyl Ether.
7. When the emanation was inixed with dry CO, at various
pressures it was found that the maximum value obtained for
the percentage cathode activity was 80:7. In order to obtain
this value with potentials less than 1000 volts the pressure had
to be less than about 150mm.
324 E. M. Wellisch-Distribution of the Active Deposit
When the emanation was mixed with dry hydrogen the max-
imum value obtained for the percentage cathode activity was
89.2, the same as that obtained with air. This value could
readily be obtained with a potential of 160 volts and with
hydrogen at a pressure of 1 atmosphere, showing that there is
very little columnar recombination in this gas. Hydrogen was
found to be particularly sensitive to the presence of small
traces of water vapor; the effect of the water vapor was to in-
crease the potential necessary to obtain the same limiting
value.
Inasmuch as the presence of minute quantities of water
vapor resulted in a marked diminution of the amount of active
deposit which settled on the cathode, it became of interest to
ascertain the percentage of positively charged carriers which
would result from mixing the emanation with a vapor. For this
purpose ethyl ether was chosen; any gas which remained in the
vessel was swept out by a streain of ether which had previously
passed through P,0g. The following results were obtained :
Pressure
Potential
Percentage Cathode Activity
mm
82
85
128
235
Volts
160
1070
do
do
6.4
9.8
10.0
10.8
At the highest pressure there was a large current passing
through the vapor during the activation due mainly to the fact
that the ether being near the point of condensation was partly
conducting; this conduction current may have been respon-
sible for the increased amount of the cathode deposit at the
highest pressure. Apart from this it appears that for ether
vapor the limiting value of the cathode activity is approxi-
mately 10 per cent.
Summary and discussion of results.
8. When the emanation is mixed with any gas there appears
to be a definite limit to the fraction of the active deposit which
settles on the cathode. This limit is independent of the pres-
sure of the gas, provided it is high enouglı to prevent the
deposit particles from recoiling on to the walls of the vessel ;
it is in general dependent on the nature of the gas. This lim-
iting value is in general obtained only with large potentials;
with smaller potentials the fraction of the cathode deposit is
of Radium in an Electric Field.
325
1 mm
decreased as a result of columnar recombination of the posi-
tively charged particles with negative ions; and with very small
potentials the charged particles recombine with negative
ions in the volume of the gas. Small traces of water vapor
have a considerable effect in diminishing the number of posi-
tively charged particles; the water vapor appears to be effec-
tive in bringing about increased recombination, both volume
and columnar, between the charged particles and the negative
ions.
It has been shown in Section 5 that even in air which has
been thoroughly dried the recombination between the charged
deposit particles and the negative ions is greater than the
recombination between the positive and negative ions. This
result, which is in all probability to be ascribed to the larger
size and mass of the deposit particles, is not in accord with the
experimental result of H. W. Schmidt,* who came to the con-
clusion that as far as recombination and mobility are concerned
the active particles behave as positive ions.
The process which accompanies the deposit of the active
particles on the cathode appears to be most suitably explained
in the following manner. At the moment of expulsion of the
a-particle from the atom of emanation the residual part recoils
into the gas; in air at a pressure of 1 atmosphere the range of
this recoil atom has been shown to be about to As it
moves through the gas the recoil atom produces a large num-
ber of ions and in the act of ionization it is possible that the
recoil atom may lose its positive charge. On the other hand
recoil atoms which at any time are. uncharged may regain a
positive charge, so that if we consider a large number of recoil
atoms there will at any given moment be a certain fraction
which carry a positive charge, the remainder being practically
all neutral. The process is in many respects similar to that
which is known to occur in the case of canal rays. During tlie
motion of recoil the atom is practically unaffected by any
applied electric field, so that initially the relative number of
uncharged and charged recoil atoms is independent of the
applied potential. However, when the recoil atom has reached
the end of its path, if it be positively charged it may lose its
charge by recombination with a negative ion formed in the
column; this recombination can be prevented by increasing
sufficiently the applied potential. Moreover for small applied
potentials a positively charged recoil atom may recombine
with a negative ion in the volume of the gas.
When both columnar and volume recombination are avoided
by the application of a sufficiently high potential the distribu-
tion of the active deposit on the electrodes is determined
* H. W. Schmidt, Phys. Zeitschr., ix, p. 184, 1908.
326 E. M. Wellisch-Distribution of the Active Deposit
entirely by the relative number of charged and uncharged car-
riers resulting from the recoil of the atoms of Ra. A in the gas.
Under these circumstances we should expect that the distribu-
tion should be independent of the pressure of the gas because
the recoil atom will meet the same number of gas molecules
before it is brought to relative rest. Of course if the pressure
is too low an appreciable number of active deposit particles
will recoil on to the walls of the vessel and in this manner the
cathode deposit will be diminished.
Although nothing has been established in this research with
regard to the velocity of the recoil atoms when moving under
the influence of an electric field, nevertheless there is distinct
evidence that, as far as diffusion and recombination are con-
cerned, the recoil atoms behave differently from the positive
gas ions. It has been shown in Section 5 that, when recom-
bination occurs between negative ions on the one hand and
positive ions or positive recoil atoms on the other hand, a con-
siderably smaller fraction of recoil atoms than of positive ions
is received by the negative electrode. This is especially the
case with inoist gases, but even in gases which had been dried
with the utmost care the difference is well marked. An
examination of the curves of fig. 3 seems to afford further
information in this connection. The curves may be regarded
as giving the fraction either of positive ions or of positively
charged recoil atoms that is received at the negative electrode
for any given potential, volume recombination being supposed
to be entirely absent. It will be noticed that these curves cut
the axis of ordinates at the points marked .7 and 4, these
points representing respectively 66 per cent of the total num-
ber of positive ions and 38 per cent of the total number of
positively charged deposit particles. This type of curve has
already been treated by Wellisch and Woodrow* for the case
of the columnar.recombination resulting from a-particle ioniza-
tion. It was shown by them that the ordinate of the point of
intersection represents the fraction of the total number of ions
which escapes from the a-particle column as a result of molec-
ular agitation and diffusion. Inasmuch as volume recombina-
tion is absent these ions are brought over to the electrodes by
a very small electric field. If we draw through the point of
intersection a straight line parallel to the axis of potential and
if we refer the curve to this straight line as a new axis of
potential, then the new ordinates will indicate to what extent
the electric potential is effective in preventing recombination
between those ions which still remain in the column after the
initial diffusion has occurred. If we treat the curves of fig. 3
in a similar manner we see that, whereas in the vessel employed
* Wellisch and Woodrow, this Journal, September, 1913.
of Radium in an Electric Field.
327
66 per cent of the positive ions on the average escaped from
the a-particle column, the corresponding figure for the posi-
tively charged recoil atoms was only 38 per cent. This slow-
ness with which the recoil atoms diffuse is readily ascribable to
their relatively large size and mass. Of those ions and recoil
atoms which do not escape by diffusion from the column
approximately the same fraction is brought over by any given
potential; it seems that there is here some compensating influ-
ence at work; probably the greater tendency of the recoil atoms
to recombine with negative ions is partly balanced by the
smaller number of encounters with these ions.
The existence of a definite limiting value to the percentage
cathode activity has been ascribed above to a continual process
of gain and loss of charge which occurs during the recoil
motion of the active deposit particle. It is to be expected
that this limiting value will depend upon the nature of the gas
into which the particle recoils; the experimental determination
showed that this was in general the case although the limiting
value for hydrogen was within the limits of error the same as
that for air. The fact that the value for ether is as sinall as
10
per cent is surprising and is in all probability to be ascribed
to the ease with which the molecules of ether are ionized.
UNIVERSITY OF MICHIGAN
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