AN ACCURATE VACUUM-TUBE ELECTROMETER

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP80-00809A000600310137-5
• Release Decision: RIPPUB
• Original Classification: C
• Document Page Count: 3
• Document Creation Date: December 22, 2016
• Document Release Date: August 11, 2011
• Sequence Number: 137
• Publication Date: May 8, 1950
• Content Type: REPORT

Sanitized Copy Approved for Release 2011/08/17: CIA-RDP80-00809A000600310137-5
CLASSIFICATION CoNFmENTIALCONFIDENTIAL
CENTRAL INTELLIGENCE AGENCY REPORT
INFORMATION FROM
FOREIGN DOCUMENTS OR RADIO BROADCASTS CD NO.
SUBJECT Scientific - Electronics, electrical
measurements
HOW
PUBLISHED Monthly periodical
WHERE
PUBLISHED Moscow
DATE
TN IS DOCUMENT CONTAINS INFORMATION AFFECTING INK NATIONAL DEFINE!
OF THE UNITED STATES WITHIN THE MEANING OF ISPIONAGI ACT IT
A. I. C.. SI AND Il. AS AMINOSD. ITS TRANSMISSION ON TNI REVELATION
01 ITS CONTENTS IN ANY MANNER TO AN UN AVTHORII10 PINION IS /NO.
NISITED IT LAW. IKPRODUCTION Of THIS FORM 12 PRONIIITIO.
DATE OF
INFORMATION 1949
DATE DIST. 7 May 1950
NO. OF PAGES 3
SUPPLEMENT TO
REPORT NO.
THIS IS UNEVALUATED INFORMATION
SOURCE Zavodskaya Laboratoriya, Metallurgizdat, No 11, 191+9.
S. A. Ginzburg, All-Union
Inst of Aviation Materials
It is often necessary in research practice to measure potentials in high
ohmic, circuits. An example of this is the measurement of corrosion potentials
on grain boundaries, with the help of capillary electrodes. In this case, the
resistance of the circuit amounts to tens of megohms and, therefore, a greater
degree of accuracy is necessary than in ordinary instruments, i.e., of the order
of 0.1-0,2 millivolt.
It is difficult to obtain such accuracy because of the considerable effect
of electrostatic fields on the high ohmic grid circuit of an electrometer tube,
and also due to possible fluctuations of plate current in this tube. The author,
in conjunction with G. Ya. Lion, has designed a vacuum-tube electrometer of high
accuracy which satisfies these requirements. The instrument is constructed on
the basis of the USSR-produced four-electrode electrometer tube. The grid cur-
rent of the control grid of this tube is not more than l0-11+ amperes.
In making measurements, the positive voltage applied to the control grid
is compensated by a ve'> ge from a three-stage measuring voltage divider (po-
tentiometer), consistir~, of two potentiometer-type units and a rheochord, con-
nected in one of the filament leads. The total range of voltage measurements
on the potentiometer is 1.1 volts. The first unit enables measurements to be
made with an accuracy of 100 millivolts, the second with an accuracy of 10 milli-
volts. The rheochord has 100 divisions, each of which corresponds to 0.1 milli-
volt. The resistances of the potentiometer must be made with great accuracy.
Calibration against a "Raps" standara potentiometer showed that the potentiometer
described gives an error of not more than 0.05 millivolt.
To increase the range of measurement and to measure potentials, which might
change their sign in the process of reasurement, a reversible switch is included
which enables a normal cell to be introduced into the potentiometer circuit in
either polarity. One arm of the switch also serves as the negative terminal for
the input voltage to the tube.
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CONMENTIAL
The difference between the measured voltage and the voltage on the potentio-
meter is applied to the control grid of the electrometer tube which is located
in a special aluminum, hermetically sealed shield containing a vessel with a
moisture absorbent. A push button is mounted on the upper cover of the shield
which, when pressed, connects the control grid of the tube with the grid lead-
out. The latter is carefully insulated from the grounded housing of the shield.
This type of shield design serves simultaneously as an electrostatic shield and
desiccator. Before installing the tube, the surface of the bulb is carefully
wiped with alcohol, so as to prevent surface leakage.
and in order to improve stability in the shield, a condenser is connected be-
of the measuring potentiometer be maintained constant. Both these c,irrents are
In the given circuit the galvanometer used has a sensitivity of 5 x 10-9 amperes,
The initial negative bias on the control grid of the electrometer tube is
obtained by the voltage drop created by the filament current through a cathode
resistor. To regulate the bias, two identical and mechanically interconnected
rheostats are used in the filament circuit. When these are turned together, the
total resistance of the filament circuit and the measuring potentiometer does
not change, and a new adjustment of the working current is not required. (In
practice, a small adjustment is sometimes required.)
The electrometer tube is connected in the circuit as one arm of a bridge
circuit formed by two fixed resistors, the plate load resistor of the tube, and
the sections of two voltage dividers (potentiometers). Due to this arrangement,
it is possible to balance the bridge with great accuracy. The balance point is
determined by the galvanometer and the double-pole double-throw switch.
It should be pointed out that where the requirements for accuracy and sta-
bility of measurement are hibh, the quality of the contacts of the potentiometer
units in both the bridge and filament circuits is of exceptional importance.
Therefore, in building the instrument, special attention was paid to selecting
high-quality potentiometer step-switches, and, in addition, the rheochord and
rheostat contacts were specially designed.
The electrometer tube is operated as follows: before beginning measurements,
negative bias is established by means of the rheostats. Then the working current
of the filament and potentiometer are checked against the normal cell by means
of the galvanometer. During this time, the push button shoUd be pressed down
and the control grid connected to ground, i.e., the established arias is applied
to it. In this position, the bridge is balanced with the aid of the voltage
dividers and the galvanometer, which at this time is connected in the diagonal
of the bridge. To carry out the measurement, the push button of the shield-
desiccator is pressed down. When this is done, the control grid of the tube is
disconnected from the ground, and the positive voltage to be measured is applied
to it. In this manner, the negative bias on the grid is decreased, the internal
resistance of the tube also decreases, and the equilibrium of the bridge is up-
set, which can be observed by the deflection of the galvanometer. The position
of the arms and the nccochord of the filament potentiometer are then altered un-
til the galvanometer returns to the zero position. This indicates that the volt-
age being measured and the voltage on the potentiometer are equal. The voltage
being measured can then be obtained from the position of the potentiometer arms
and the rheochord reading.
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GOI FIBE IT9M
In the case of high ohmic electrodes ,tens of megohms), a phenomenon may
be observed whereby, despite the absence of a difference between the measured
and balancing potentials, the galvanometer gives a deflection when the push
hut';on is pressed down. This defhction depends on the resistance of the elec-
trodes, but is not more than 0.?-0,3 ml li-,o]': and vanishes completely when
the push button is released. Therefore, the measurement can be made by observ-
ing the deflection when the push button is up
The vacuum-tube electrome+er de_cribed is located in a wooden box, shielded
with sheet aluminum on the inside. The shield-desiccator, arms, rheochords,
and rheostats are mounted on an inclined duralumin panel which is faced with
getinax (laminated insulation). The galvanometer with a luminous scale is
mounted in a cut-away portion of the panel.
The use of this instrument for measuring potentials of microinclusions
under the microscope with the help of a thin capillary showed that it enables
measurements of potential to be carried out reliably with electrodes of high re-
sistance (30-50 megohms), giving an accuracy of ? 0.2 millivolt.
(Editor's note; Instruments for potentiometric measurements are necessary
for accurate determination of hydrogen ion concentration, for potentiometric
titration, for the study of corrosion processes, etc.
The assembly, testing, and calibration of such instruments in institutes
and laboratories, even when detailed descriptions are provided, takes much time
and demands a considerable amount of work. It would be much simpler and cheaper
to centralize the manufacture of such instruments, whose use in scientific re-
search and plant laboratories would facilitate considerably the solution of many
current problems.
The Editor draws the attention of the Ministry of Machine and Instrument
Building to the necessity of organizing series production of instruments for
potcntiometric measurements in 1950.)
fExterior view of the apparatus available
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