Apparatus for regulating current in moving-boundary experiments


Material Information

Apparatus for regulating current in moving-boundary experiments
Physical Description:
5 p. : ill. ; 28 cm.
Rathkamp, W. R
Baker, Philip S ( Philip Schaffner ), 1916-
Oak Ridge National Laboratory
U.S. Atomic Energy Commission
U.S. Atomic Energy Commission, Technical Information Service
Place of Publication:
Oak Ridge, Tenn
Publication Date:


Subjects / Keywords:
Electric current regulators   ( lcsh )
Potentiometer   ( lcsh )
bibliography   ( marcgt )
non-fiction   ( marcgt )


Includes bibliographical references.
Statement of Responsibility:
by W.R. Rathkamp, P.S. Baker.
General Note:
Cover title.
General Note:
General Note:
"June 8, 1954."
General Note:
"Work performed under Contract No. W-7405-eng-26"--P. 2 of cover.
General Note:
Work performed at the Oak Ridge National Laboratory.

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 005254622
oclc - 727982012
System ID:

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SAUG 14 1
ORNL -1704


W. R. Rathkamp
P. 8. Baker

June 8, 1954

Stable Isotope Research and Production Division
Oak Ridge National Laboratory
Oak: Ridge, Tennessee


Technical Information Service, Oak Ridge, Tennessee

Work performed under Contract W-7405-eng-26.

Issuance of this document does not constitute authority
for declassification of classified material of the same or
similar content and title by the same authors.
This report has been reproduced with mzinimumn altera-
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information contained herein.
Reproduction of this information is encouraged by the
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should be made with the author and the organization he
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Operated by
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AEC, Oak Ridge, TCenn.-W43670


W. R. Rathkamp and P. S. Baker

The measurement of transference numbers by the
moving-boundary method depends ultimately upon
the passing of a constant current of precise value
for a known period of time through a definite
volume of the moving-boundary cell. Since the
resistance is continually changing during an experi-
ment, the maintenance of a constant current is the
most difficult of the variables to control.
The early investigations were carried out by
hand regulating the current, either by gradually
increasing the applied potential or by decreasing
a series resistance.1 -The original apparatus of
Macinnes and co-workers2 and the subsequently
modified form have been used for much of the
work done in the field of transference-number meas-
urements. Their devices are, however, subject to
some error introduced by vibrations. Bender and
Lewis4 have suggested a fairly simple arrangement
for providing constant current in which the current
flowing through the cell is also the plate current of
a pentode tube, the magnitude of the current being
controlled by a grid bias. Occasional hand adjust-
ment is still necessary, nevertheless. Carsons
and Spiro and Parton6 have also described current
regulators of a similar type.
The purpose of this paper is to describe an
apparatus which incorporates some of the ideas of
Bender and Lewis but which is independent of
any regulation whatsoever during experiment. It
is not difficult to assemble, is oblivious to vibra-
tions, and the form described is capable of main-
taining constant currents with an average deviation
of about 0.04% from any desired value between 2
and 10 ma.
The entire apparatus comprises a half-wave
voltage-regulated power supply with 150-, 300-,
and 450-v tops (Fig. 1) and a modified Micromax

DO. A. Macinnes and L. G. Longsworth, Chem. Rev. 11, 188
20. A. Mdacinnes, i. A. Cowporthwarite, and K<. C. Blanchard,
J.3Am. Chem. Soc. 49, 1710 (1927)*
19, O 9 ngsworth and D. A. Marcinnes, J. Opt. Soc. Ame.
4P. Bender and D. R. Lewis, J. Chem. Ed. 24, 454 (1947).
5W. N. Carson, Anal. Chem. 22, 1565 (1950).
6M. Spiro and H. N. Parton, Tmran. Faraday Soc. 48, 263

(Leeds and Northrup Model C) which incorporates
the hi gh-transconductance tube of the Bender and
Lewis device. It should be mentioned in passing
that, since the peak voltage output of the power-
supply transformer is not much greater than 450 v,
the drop across the choke is enough to make the
VR tube work; hence no additional series res instance
is used.
The modifications to the Micromax are as follows:
the control switch and slide-wire assemblies for
the external circuit are removed, and the shaft is
cut back and insulated to accommodate a GR
potentiometer as shown in Fig. 2. The lead running
from the contact arm of the balancing slide wire
to the terminal TC+ is removed, as are the black
wires connecting the slide-wire assembly to a
resistor and a brass tie point. (The resistor also
is connected by a black wire to the standardizing
slide and carries, in addition, two red-and-white
trace wires; the brass tie point has one additional
connection a green-ond-white trace wire.)
In Fig. 3, the resistor string between A and 8
(100 R, 200 R, and 2 Sr) replaces the Micromarx
slide wire, A being the resistor lug and B the
brass tie point from which the black wires were
removed as described above.
One side of a dpst switch is inserted into the
lead between the fuse and the Micromax motor
terminal. The other side of this switch is SW in
Fig. 3. The input of the 6.3-v filament transformer
(D and D') is tied across the motor terminals. A
0.75-amp fuse replaces the 0.45-amp fuse sup-
Lead C is the existing yellow-and-blue trace
wire which runs to the galvanometer circuit. F is
a current-shunt resistor in this case, 72 ohms to
allow operation over a range from 2 to 10 me. E
is the power supply and should be of the voltage
required to obtain the desired current through the
moving-boundary cell.
To reduce the tendency of the Micromax to
"hunt," the sliding contact is caused to rotate,
not by the shaft directly, but by means of a yoke
connected to the shaft and fitted loosely around a
stud as shown in Fig. 4. Under these conditions,
if the Micromax overshoots and reverses itself,
the play in the yoke prevents transmittal of the

DWG. 49622A


0.75 AMP


6.3 v o 3 PILOT


Fig. 1. Stabilized D-C Power Supply.

full travel to the slider; the resulting effect is
about one-fifth the full travel of the shaft. Except
under the most extreme conditions, this reduced
travel will not permit overshooting in the opposite
direction and suffices simply to bring the setting
nearer -to the correct operating point. Usually the
next increment of travel is small enough to bring
the Micromax readily to balance. Since this yoke
mechanism operates only during reversals of
direction, it does not affect the operation when the
Micromax is responding to any large changes in-
volved in bringing the circuit back to balance,
To permit estimation of the current, voltage, and
resistance during transference-number measure-
ments (and during the evaluation of the current
regulator), a meter box was assembled containing
suitable resistainces, shunts, and a switch to give
readings within 1% of the true value at any time.
The arrangement is shown in Fig. 50. The meter
shown is a 100-pao G-E type D-058 which is easily
read to within 1%. The microampere movement
minimizes the shunt current around the cell when
voltage measurements are being made. -The par-
ticular choice of resistors gives full-scale readings
of 500 v or 10 ma on the meter; the corresponding

signals to the potentiometer are 50 and 100 mv,
The terminals + and shown in Fig. 56 allow
the use of an external potentiometer (volt box)
across built-in precision resistors for more precise
measurements than can be obtained with the meter
Table I shows typical values for several currents
as the resistance is changed over a considerable
Table 2 includes various values for currents and
the corresponding resistance rangesiof the instru-
From Table 2, it is seen in the case of a 5-ma
current with the 450-v supply, for example, that
the resistance may be changing as much as from
1,500 to 85,000 ohms. Since constant currents of
from 3 to 6 ma were required during resistance
changes of from about 5,000 to 50,000 ohms, the
circuits were designed to handle them. Figure 6
shows the operating ranges for various voltage
supplies. Actually, the apparatus can be made
operable over almost any desired range by suitably
changing resisteinces of the circuit and the poten-
tial divider.

OWG 19624A1

DWG 19623A


pig. 3. Constant Current Supply.

Fig. 2. Modified Potentiometer.

o soIIIC

DWG. 19625A


-- YOK(E

Fig. 5. Ammeter-Voltrmeter Box.

Pig. 4. Detail of Mod~fied Potentiometer, Fig. 2.

110 V

Table 1, Valriations in Current Values with Changes in Resistance

150-v Supply, 300-v Supply, 450-v Supply,
Resistance Current Resistance Current Resistance Current
(ohms x 10- ) (ma) (ohms x 10 3) (ma) (ohms x 10- ) (ma)

77.7 3.052 77.7 5.032

57.4 2.027 57.4 3.052 57.4 5.035
43.0 2.027 43.0 3.055 43.0 5.032

35.0 2.027 35.0 3.055 35.0 5.037

25.3 2.030 20.3 3.057 20.3 5.037

15.0 2.027 15.0 3.057 15.0 5.035

5.0 2.027 5.0 3.055 5.0 5.035

DWG. 19628

0 4 2 3 4 5 6
CURRENT milliamperess)

7 8 9 0

Fig. 6. Approximate Maximum and Minimum Resistancsa at Yarious Constant Currents.

Table 2. Approximate Resistance Ranges of the Instrument for Various Current Values

Voltage Approximate Voltoge Approximate
Supply Current Resristnce Range Supply Current Resistance Range
(V) (ma) (ohms x 10-3) (V) (ma) (ohms x 10- )

150 2 1.5-62 450 4 3-104

300 3 1.5-95 450 5 <1.5-85

300 4 1.5-67 450 6 <1.5-70

300 5 1.5-45 450 7 <1.5-58

450 9 <1.5-40

111111111111111111111111 111111111111111111 IIIIIUHI IIIII

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