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be expected. Thus the velocity chosen gives us a fair degree
of approximation to the ideal case where the emanation may
be uniformly distributed in the vessel and the deposit made
evenly along the length of the electrodes.

As is to be expected, experiments with lower pressures, such
as 9^' and 5^* of E.,SO^ gave smaller values of the percent-
age of cathode activity, due to edge-effect and other causes
which helped the case to get more than its share of the activity.

Mention might be made here of one or two experiments in
which it was found that the distribution of the activity was
perceptibly affected (the cathode percentage activity being
mcreased) in the case where the flow was asymmetrical witn



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546 Lucian — Distribution of the Active Deposit of

respect to the central electrode, three out of four outlet tabes
being stopped.

The effect of the irregularities of the field was examined in
a separate series of experiments. The variation of the electric
intensity in the cylindrical portion of the vessel and in the bot-
tom will not obey the same law, no matter what the construc-
tion of the bottom is. As a consequence, we would expect to
find discrepancies in the relative number of the neutral par-
ticles formed in the body of the vessel and near the bottom,
and diffusing to the electrodes. The bottom corner of tlie
vessel was tilled in with a curved piece to get rid of the edge
effect, and gauze bottoms of various shapes, flat and curved,
tried. It was then found that, although discrepancies in the
values of the cathode percentage for different shaped gauzes
occurred, these never exceeded the limits of experimental
error, and did not exhibit any consistent direction, as one would
be led to expect from the consideration of the different shapes
of the bottoms. Evidently the high velocity of the air stream
helped to smooth out the irregularities that would be expected
at lower pressures. Finally the curved gauze, shown in the
diagram, was adopted in order to match up with the curvature
of the corners. It will be noticed that the distance of the
electrode from the gauze is adjusted so that the average elec-
tric intensity at the bottom of the vessel would not by any
chance be smaller than in the body of the vessel.



III. Experimental Results,

Before giving the experimental results of this investigation
it may be useful to recapitulate the transformations which a
quantity of emanation undergoes, according to our present
state of knowledge.

The complete scheme of transformation, after Marsden and
Perkins,* is as follows : —



cc(5.7cms.) a{e.5)




^ ACTIVE OEPOSI-r



005 9ees.



^E. Maraden and Dr. P. B. Perkins, Phil. Mag., vol. xxvii, Apr., 1918.



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Aotinium in an Electric Field. 647

All the products after the emanation compose what is known
as the active deposit.

It will be noticed that the determining factor in the distri-
bution of the active deposit of actinium is Act. B. Act. A
being a short-lived product decays rapidly before any appre-
ciable amount of it readies the central electrode, for smaller
potentials. In fact it may be shown by calculation, using the
following expression for the fraction of the Act. A particles
that reach the central electrode,



2 FA?



Xlog|-(ft»-a')



1 - e 2 FA:



(6»-a')Xlog-

where,

F= applied potential,

cm'
A? = 1-54 p, mobility of the positive ion, assumed to

represent approximately the mobility of the deposit particle also.

a = -075 cm., the radius of the central electrode.

6 = 2*45 cms., the inner radius of the vessel.

X = 350 sec""*, transformation const, of Act. A.

that with 600 volts about 25 per cent of the deposit reaches
the wire as actinium A, with 1000 volts less than 40 per
cent, and with the highest potential used (1700 volts) less than
55 per cent. Since, as will be shown later, no increase of
potential difference above 1000 volts appreciably alters the
percentage of the cathode deposit, it may be assumed that
with increasing potentials the increased amount of actinium
A on the central electrode has no effect on the distribution of
the deposit; at least, in so far as the final result is concerned.
It seems probable, therefore, that actinium A and actinium
B are born with the same phvsical characteristics and exhibit
the same peculiarities in an electric iield.

The following experimental results were obtained in connec-
tion with the three special objects of investigation mentioned
in the introduction.

First of all, the dependence of the distribution of activity
on the amount of emanation employed claimed attention and
was made the subject of investigation. It was found that the
cathode percentage, which we shall call Ay (activity at poten-
tial V, referring to the cathode), depended to a great extent on



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548 Lucian — Distrihut/ion of the Active Deposit of



Fig. 2.



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Actinium in an Electric Field.



549



the amount of emanation used, for values of V up to 600 volts
or thereabouts. Above V = 600, A^ was independent of the
amount of emanation used, when the amount was not exces-
sively large. The curves in fi^. 2 represent the results of
these experiments and give an idea ox the ratio of amounts
used in tnese experiments. Very excessive amounts have not
been tried, but there are strong indications that the above
independence would no longer hold good.

The points on the curves represent the percentage of the
total activity which is deposited on the cathode, for a given
amount of emanation. As has already been mentioned, the
activities on the case (anode) and the central electrode
(cathode) were measured separately and from these measure-
ments the above values of the cathode percentage calculated.
A condensed table, in which the values have been interpolated
from the curves to correspond to given amounts of emanation
at points sufficiently near the experimental values, is given
below, for purposes of reference :

Table I.



Am otints

in
divs./sec.




Percentage Cathode Activity.



120


200


600


980


1700


volts


volts


volts


volts


volts


8.V1




93 7


94-7




83-9




93-t5






82-4


88-0








81-2


87-6


93-6


94-8




78-8


86-3


93-4




94-9


76 5


85-5


93 4


95




730


83-5


93


95-0


94-9



It is to be noticed that the curves for 600 volts, 980 volts
and 1700 volts are horizontal ; the last two being coincident.

Owing to the presence of neutral deposit particles, even at
the highest potentials employed, it is necessary to correct, for
diffusion, all the experimental values of the distribution of
activity. This was done by obtaining experimental values for
the distribution in the absence of an external electric field (0
volts), so that diffusion alone was operative. It was found that
the deposits on the central rod and the case were in the ratio
of 1 to Q'Q approximately. This ratio is quite different from
the ratio of tne areas of the central rod and case, which was 1
to 37. It is evident that if we corrected for diffusion on the
supposition that the diffusion distribution was proportional to
the exposed areas, we would get higher values for the cathode
percentage.



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550 Lucian — Distribution of the Active Deposit of

All the values given in the present paper refer to distribu-
tions for which the proper correction has been made.

The curve for volts in fig. 2 would be expected to remain
horizontal for all amounts of the emanation employed. The
upward slope of the curve for very small amounts may be due
to residual electric fields as well as to molecular agitation and
initial diffusion from the recoil-column which are of no effect
for larger amounts.

Some experiments of not a very high degree of precision
were tried with a negative potential applied to the case, the
central electrode being earthed. A typical set of equilibrium
values with added capacity for —600 volts on the case is as
follows :

Maximum activity on the central electrode - . = '08 - — *

sec.

Maximum activity on the case = 10-7 "

Total activity = 10-78 "

Cathode percentage = 99*36 ^

Corrected for diffusion = 94*6 "

The corrected value for the cathode percentage is quite in
accordance with the values for 4-600 volts when the central
electrode was made the cathode.

It is now evident that if we followed Walmsley's procedure
and obtained the distribution of the active deposit by measur-
ing the activity of the central rod, first as catliode and then as
anode, neglecting* the deposit on the case, we would ol>tain for
large potentials a cathode activity apparently very nearly 100
per cent of the total amount.

The experiments with a negative applied potential show
further that the activity deposited on the anode is due entirely
to neutral particles; for, if negative particles existed in the
vessel, the activity when the central electrode is made the
anode should be larger than the amount which settles there by
diffusion alone; whereas by a simple calculation, from the
value of diffusion on the case, we find that for +600 volts on
the case and a total amount corresponding to 10"78 mm8./8ec. the
activity diffusing to the central electrode is -113, which is
larger than the value obtained here ('08 » for —600 volts on the
case. This shows conclusively that no negative particles exist,
or at least, take part in the transfer of activity considered.

The above difference between the values of the active deposit
that diffuses to the central electrode as anode and cathode can

* A close examination of the figures given by Walmsley in Table I (loc.
cit.) seem to warrant the statement here made that he did neglect considera-
tion of the activity which diffosed to the case (anode) at high potentials.



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Actinium in an Electric Field.



561



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552 Lucian — Distribution of the Active Deposit of

be explained by considering the differeut relative distribution
of charged deposit particles and negative ions, the different
conditions of electric intensity and the consequent different
aroonnt of recombination near the central electrode and in the
neighborhood of the boundary of the vessel.

The curves shown in fig. 3 are plotted from those in tig. 2 by
taking the points in which the curves of fig. 2 intersect a verti-
cal line, corresponding to a given amount of emanation.

The earlier portions of the curves show clearly the increase
of volume recombination with the increase of the amount of
emanation employed. The gradual rise of the curves after
200 volts shows the presence of columnar or initial recombina-
tion. The curves of ionization coincide at about 600 volts;
the curves for the distribution of activity coincide at about
1000 volts (not shown in the fig.\ and thenceforth, no increase
of voltage alters perceptibly the percentage of the cathode
deposit. Thus there seems to be a definite limit (94*9) to the
value of the cathode percentage; this limiting value for
actinium is considerably greater than the value (88*2) found by
Wellisch for the active deposit of radium.

The dotted lines in fig. 3 show a number of curves represent-
ing the variation with voltage of the ionization current when
various amounts of the emanation were in equilibrium with
the deposit products. The readings of the ionization current
were obtained while the air current was passing and on this
account were not of a high order of accuracy as the amount of
emanation present in the vessel was subject to slight variations.
Hence a large number of these readings were taken for vari-
ous voltages and for different arbitrary amounts of emanation
used, and a set of average curves was plotted. All the ioni-
zation curves in fig. 3 were plotted by changing the scale of
ordinates to correspond to a saturation value of 94*9. It
should be noticed tliat the ionization current is assumed to
attain its saturation value at 600 volts. This is very approxi-
mately true ; at any rate the qualitative results that will be
drawn from the nature of the ionization and activity curves
are not invalidated by this assumption. At 1000 volts ioniza-
tion current readings showed no appreciable difference from
the values at 600 volts.

From an inspection of the curves of fig. 3 it will be seen
that for any given amount of emanation the "activity" curve
lies continually below the ionization curve. In other words,
the electric field is able to bring to the central electrode a
larger proportion of positive ions than of positively charged
deposit particles. For smaller voltages this can be easily ex-
plained on the supposition that the deposit particles and nega-
tive ions contained in the volume of the vessel combine much
more readily than the negative and positive ions among them-



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Actinium in cm Electric Field. 553

selves, i. e., volnme recombination takes place at a widely dif-
ferent rate for the two cases.

The same remarks hold true in the case of columnar recom-
bination also. The fact that the central electrode receives,
even for the higher potentials, a smaller proportion of charged
deposit particles than of positive ions shows that the deposit
particles are liable to lose their charge by recombination in the
columns more readily than the positive ions. The difference
in this particular phenomenon may be more strikingly shown
by considering the curves of ionization and activity correspond-
ing to an infinitesimal amount of emanation in the vessel.
These were obtained by the method used by Wellisch for the
case of radium by producing the curves of fig. 2 and of the
corresponding figure for ionization currents, so as to intersect
the axis of ordinates, and plotting these points of intersection
against the potentials, and are marked o in tig. 3 ; tliey may be
regarded as limiting curves, corresponding to the absence of
volume recombination. They show clearly that any given
potential is able to prevent columnar recombination of ions
much more easily than of active deposit particles. The two
curves approach at about 600 volts.

IV. Summary and Discitssion of ResuUa,

1. When actinium emanation is mixed with dust-free dry
air and allowed to come into equilibrium with its active
deposit the percentage of the deposit which is collected by the
cathode increases with increasing potentials, but even under
the most favorable conditions and at the highest potentials
applied there seems to be a definite limit to the percentage
of the active deposit which settles on the cathode. This limit
is 94*9 per cent, or 95 per cent roughly.

2. The remaining five per cent oi the active deposit consists
of neutral particles which reach the electrodes by diffusion.
It was shown also that no negatively charged deposit particles
take part in the transfer of activity.

3. For values of the activity distribution which are less than
this limiting value, the formation of the neutral particles is
explained on the view that the deposit atoms recombine with
the negative ions in the volume of the vessel for small applied

f)otentials, and with negative ions formed in the columns for
arger potentials. Thus both volume recotubination and initial
or columnar recombination have to be taken into consideration
for a complete explanation of the experimental results.

4. It has been shown that both volume and columnar recom-
bination take place at a greater rate between the deposit
particles and ions than for the ions among themselves. This
was shown by a comparison of the two sets of curves in fig. 3,
one for equilibrium ionization current and the other for the



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554 Lillian — Distribution of the Active Deposit of

cathode percentage of the equilibrium active deposit. This
behavior of the deposit atoms leads one to the conclusion that
they are of larger mass and size than the ordinary gas ions.

The ionization and activity curves of fig. 3, marked o, and
corresponding to absence of volume recombination, afford
further evidence as to the size of the deposit atoms. It will be
seen on inspection that the activity and ionization curves cut
the axis of ordinates at the points '36 and '66 respectively,
corresponding to 38 per cent and 69 per cent of the total num-
ber of ions and of deposit atoms. These numbers represent,
according to Wellisch and Woodrow,* the percentage of the
total number of ions and of deposit particles which escape from
the a-partiele and recoil columns as a result of molecular agita-
tion and diffusion. The above numbers indicate that, roughly
speaking, twice as many positive ions on the average escape
from the a-particle column as positively charged recoil atoms
from the recoil column. This relative slowness exhibited by
the deposit particles is naturally to be ascribed to their size and
mass as compared with the ions.

In all these particulars actinium active deposit seems to
behave, qualitatively at least, like the deposit of radium.

A theory has already been advanced by Wellisch in explan-
ation of the behavior of the radium active deposit in an
electric field. According to this view, after a deposit particle
recoils into the gas, it is subject to the chances of columnar
and volume recombination. But when both columnar and
volume recombination are avoided by the application of
sufficiently high potentials, the distribution of the active
deposit on the electrodes is determined by the relative number
of charged and uncharged carriers which result from the pro-
cess of recoil. During the motion of recoil the deposit atom
is unaffected by any applied electric field, so that initially the
relative number of charged and uncharged recoil atoms is
independent of the applied potential. The nature of the
charges carried by the deposit particles at the end of their
recoil path is determined by the continual process of gain and
loss which occurs during the recoil motion of the particles.

This theory is susceptible to modification and further
development, especially with regard to the sign of the charges
acquired by the deposit particle as a result of and at the end
of the recoil motion, by taking into consideration the mechan-
ism of the process of ionization in the following manner. To
start with, the recoil atoms at their formation will acquire in
general an electric charge as a result of the simultaneous expul-
sion of an a-particle and a number of slow-moving electrons;
further, the deposit atoms are at least of ionic order of magni-
tude and perhaps larger and move with relatively small velocities.
♦WeUisch and Woodrow, this Jonraal, Sept., 1918.



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Actinium in an Electric Field. 555

We may also assume that the recoil atoms eflPect the process of
ionization by some sort of collision with or bombardment of
the gas molecules, expellinjs: negative electrons from the mole-
cules with much greater initial velocity than that imparted to
the main bulk which forms the positive ion. We may picture
to oureelves, therefore, the recoil deposit atom moving all the
time in and at the head of a column the core of which con-
aists of an overwhelming majority of positive gas ions and the
outer layer is constituted of negative electrons. Now, what-
ever the charges acquired by the deposit particles at their
formation, it is only reasonable to expect that travelling in
this core, lined on all sides almost exclusively with positively
charged gas ions, they should possess a greater probability of
emerging from the recoil columns with a positive charge than
otherwise. If the positive charge is acquired by the combina-
tion of the deposit atoms with the positive ions of the core,
then the resulting positively charged deposit particle assumes
molecular dimensions and becomes a cluster under favorable
circumstances; this agrees very well with the conclusions
arrived at from the discussion of the curves of fig. 3. Further,
the more complete the initial exclusion of the negative electrons
from the columns, the less will be the number of deposit atotns
emerging from the columns as neutral particles and hence the
higher the limiting value of the percentage of the cathode
deposit. It is also evident that no negatively charged deposit
particles, even if they existed initially in great numbers, could
emerge as such from the recoil columns.

The supposition here involved is that recoil atoms of diflPer-
ent initial velocity impart different amounts of energy to the
electrons which they expel in the act of ionization, and as a
consequence, for a very short time, during the motion of recoil,
a separation of positive and negative columns of ions actually
occurs. The fact that the limiting value of the OAthode
deposit for actinium is higher than that for radium is
exactly what would be expected when we consider that
the recoil atom from actinium A is expelled with a greater
velocity than that from radium A. Moreover, since the recoil
deposit atom of thorium has a velocity intermediate between
that of radium and actinium, we would expect to find the
limiting value for the cathode percentage of the thorium
active deposit to lie between 88'2 and 94'9. Experiments on
the distribution of the active deposit of thorium are now in
progress and will soon be published.

In conclusion I wish to express my gratitude to Prof. E. M*
Wellisch for suggesting this problem and for his continual
interest and advice throughout the course of the investigation.
I am also indebted to Prof. Boltwood for his helpful sugges-
tions.

Sloane Phynical Laboratory, Yale University, New Haven, Conn.,
May 18, 1914.

Am. Jour. Sci.— Fourth Series, Vol. XXXVIII, No. 228. -December, 1914.
88



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556 D. Todd— Amherst Eclipse Expedition,



Abt. LI. — The Amherst Eclipse Expedition / by David

Todd.

To observe a total eclipse successfully, one must have suit-
able instruments, he must get in the path of the shadow, and
he should Imve cloudless skies. In spite of both war and clouds,
the expedition secured results of value, though both these sin-
ister circumstances were operant against us. Fortunately we
had arrived in the middle of the eclipse belt just as mobili-
zation was beginning.

The instruments taken from the home Observatory were
intended mainly for a photographic record of the corona, and
were a selection from tnose that had been found most suitable
in previous eclipse expeditions. Unluckily they got stranded
somewhere between Libau, the port of entry to Russia, and
Kieff in Southern Russia, near which was the location of our
eclipse station. And although the officials of the Imperial
Railways did everything in their power to locate them, they
arrived at the station only the night before the eclipse ; too late
to be of any use.

Meanwhile, with many days of waiting at Kieff, itself very
near the line of central eclipse, though meteorologically not
very favorable, I had been successf lu in getting together an
auxiliary outfit for photographing the eclipse, and there was
abundant time for aajusting and testing it,

Kieff is a very old city, with more than half a million inhab-
itants, and every modern necessity. Exploring its interesting
sliops with Mr. George Martin Day, we very soon found a
Dalhneyer 6-inch portrait objective, the back lens of which



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