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An A.C. operated meter for small direct currents

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A.C. OPERATED METER FOR SMALL DIRECT CURRENTS
A 13ie8is
Presented to
the Faculty of the Department of Physics
University of Southern California
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
"by
Earle Clinton Enholm
June 1940
UMI Number: EP63331
All rights reserved
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The quality o f this reproduction is dependent upon the quality of the copy submitted.
In the unlikely event that the author did not send a com plete m anuscript
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a note will indicate the deletion.
Dissertation Publishing
UMI EP63331
Published by ProQuest LLC (2014). Copyright in the Dissertation held by the Author.
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unauthorized copying under Title 17, United States Code
ProQ uest LLC.
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This thesis, written by
EARIj:..CLIIITgiL..ElIH Cm ........................
under the direction of
h .% 3 .
F ac u lt y Committee,
a n d a p p r o v e d by a l l its m e m b e r s , has been
presented to and accepted by the Council on
Graduate Study and Research in pa r t ia l f u l f i l l ­
m e n t of the r e q u i r e m e n t s f o r the deg re e of
MASTER OF SCIE2mtCE
Dean
Secretary
JUHE,1940
F a c u lty C om m ittee
C hairm an
i.
? •
TABLE OF CONTENTS
CHAPTER
PAGE
INTRODUCTION............................
I. THE ELEMENTARY THEORY OF ELECTRONICAMPLIFIERS
FOR D.C. CURRENTS
...................
i
.
1
Sensitivity and its limitations • • ........
1
Effect of grid resistance ................
2
Current Amplification • • • ..............
3
Meter sensitivity ........................
3
Random fluctuations ......................
4
Choice of tubes ............................
5
II. DESIGN OF PRESENT AMPLIFIER............
7
III. ADJUSTMENT AND BALANCING OF AMPLIFIER........
14
IV. SENSITIVITY..................................
18
V. DIRECTIONS FOR U S E ...........................
20
Adjustment of Potentiometer
........
Directions for removing chassisfrom cabinet
Summary
.............
22
23
23
BIBLIOGRAPHY......................................
25
APPENDIX
26
...................
List of Parts
...........................
26
LIST OF FIGURES
FIGURE
PAGE
1 . Simple Electronic Amplifier............ • • • •
1
2* Simplified First Stage Circuit • • • • • • • •
7
3. Circuit to show ^-balancing • • • • • ........
9
4* Simplified Circuit of Two-stage Amplifier • • • •
9
5* Circuit of Amplifier, complete • • • • • • • • •
11
6 * View showing Interior of Amplifier • • • • • • •
15
7* Wheatsone’s Bridge ..................
• • • • • • 1 8
8 * Operating Panel of Amplifier • • • • • • • • • •
20
i
IHTRODUCTIOH
The problem of measuring very small currents and volt­
ages originating in high resistance sources has always been
& troublesome one.
Even when galvanometers of suitable sen­
sitivity are available, their efficient use is often preclu­
ded because of the condition that for maximum sensitivity the
internal resistance of the galvanometer must be of the same
order of magnitude as that of the external circuit.^
If the
external resistance is of the order of several megohms, this
requirement Is impossible for any galvanometer now available.
These small currents may be measured by passing them
through a high resistance and observing the potential dif­
ference developed across the resistor by means of an elec­
trometer.
This method is inconvenient and cumbersome.
The most versatile and convenient means for this type
of measurement is provided by the thermionic valve or vacuum
tube.
This device has a very high input resistance, so'that
no appreciable power is drawn from the source, and a moder­
ately low, easily matched output impedance.
If the vacuum
tube served merely as an impedance matching device between
source and galvanometer it would be exceedingly useful, but
in addition large amplification may be provided, extending the
i*.—
— ...
----------
_
H.B. Brooks: Sensitivity of a galvanometer as a function
of its resistance. Bur. Standards Jour. Research 4: 297 (1930)
ii
possibility oi measurement to ranges far below that provided
by the galvanometer alone*
The vacum tube when used to amplify small d*c. currents
i
is subject to many abberations. This has made necessary the
development of special tubes and circuits for the compensa­
tion and elimination of such abberations.
By the use of
these circuits^ and by observing elaborate precautions cur­
rents as small as 10" * amperes, or a flow of electrons per
second h^ve been measured.
La;rge storage batteries and a
sensitive galvanometer are required for these ultra sensitive
circuits; but if, on the other hand, moderate sensitivities
—
to 10**12 amperes —
are stifficlent, an a.c. operated de­
vice, using a rugged milliamme,ter, is possible.
The con­
struction of such an instrument is the subject of this thesis.
In common with other recent deslgns,^*^ the Instrument
to be described has several advantages,
namely:
it Is port­
able, readings are easily and quickly made, and ordinary re­
ceiving £ubes are used, rather than the expensive electro­
meter tubes.
In addition, it Is a.c. operated, and uses a
rugged milliammeter Instead of a sensitive galvanometer.
£ Penick: Direct Current Amplifier Circuits for use
with the Electrometer Tube ( a bibliography is included)
Rev. Sci.Inst. 8 , 196-198 (1937)
® Roberts : A feedback Micro-micro Ammeter. R.S.I.
10:181-183 (1939)
^ Gabus and Pool: A portable Photo-tube unit using
an RCA 964 tube. R.S.I. 8:196-198 (1937$
•
*
*
111
The features which are peculiar to this design are as
follows:
the input resistance ia
20 megohms, thereby avoid!
ing the use of the somewhat unreliable resistors with values
of thousands of megohms used in other designs; unusually high
voltage sensitivity, making possible the use of the instru­
ment as a high resistance millivoltmeter; and a built-in
potentiometer for on-the-spot- calibration -- eliminating the
need for calibration curves and the need for recalibrating as
the tubes change their characteristics.
The instrument was designed for use as a d.c. millivolt­
meter of 20 megohms input resistance, or as a device to
measure small currents down to 10~^1 amperes, originating in
high resistance sources.
It is useful in measuring photo­
electric, leakage, or ionization currents.
will suggest themselves.
Many other uses
1
CHAPTER I
THE ELEMENTARY THEORY OF ELECTRONIC AMPLIFIERS
FOR D.C. CURRENTS
The simplest form of such an amplifier consists
essentially of a high resistance, Rg, through which the cur­
rent to he measured is passed, followed by a vacuum tube
connected as shown in Fig. 1, with a suitable meter M in
the plate circuit.
^
^
f
Xnfwt|
The voltage drop produced by the current
i
^
^
n
zizer»
^
passing through the input
resistor Rg is impressed upon
the grid of the vacuum tube,
and the corresponding change
i.
in plate current noted.
If the amplifier has been calibrated,
the current in Rg, or the voltage acrossi£t, is known.
As the circuit stands, all of the plate current passes
through the meter, necessitating the use of one which is
relatively insensitive* but by the use of another battery and
a simple circuit, the steady plate current can be bucked out,
so that only the changes in plate current are indicated by the
meter.
This permits a more sensitive meter to be used.
SENSITIVITY AND ITS LIMITATIONS
The current sensitivity of such a circuit depends upon
three factors, each of which is, however, dependent upon
limited by many other factors.
and
These are the grid or input
resistance Rg* the current amplification within the tube* and
the sensitivity of the meter in the plate circuit.
Effect of Rg.
That the sensitivity is directly
proportional to the magnitude of Rg is obvious, for if this
resistance be increased tenfold, the current to be measured
may be decreased tenfold without changing the IR drop
through the resistor.
The voltage on the grid of the tube,
and therefore the reading on the plate meter are the same as
before.
In a current measuring device, therefore Rg is made as
large as possible.
But the maximum value of Rg depends upon
the insulation of the tube itself and upon the amount of the
grid current.
The tube resistance acts as a shunt across the
input resistance Rg , so that Rg must ordinarily be small
compared to the resistance of the tube itself.
Grid current
also tends to lower the input resistance of the tube itself.
It must be small enough so that, passing through the grid re­
sistor with the current to be detected, its effects will not
mask those of the applied current.
Special tubes, such as the General Electric S’.P. - 54
and the Western Electric D-96475 tubes, are available having
quartz insulation of the tube elements, and also designed so
as to have very small grid current:
Rg from 10^ to 1014 ohms.
these may use values of
But for receiving tubes operated
at ordinary voltages, 10 to 20 megohms is the limit.
current will be discussed more fully later.
Grid
3
Current Amplification*
The current amplification of a tube depends upon the
grid resistance Rg and the mutual conductance (gm) defined
by the relation
ip « plate current
gm = dlp/deg
eg = grid voltage.
But
i(j = Galvanometer current
k
= Galvanometer constant
d
Galvanometer deflect­
ion of meter
i *
input current
The current amplification A is
1q /1 » so that
The sensitivity S = d/i
*
SmRg/k
mny^amp
The last relation restates the facts mentioned above, that
the sensitivity depends upon the input resistance, the am­
plification provided by the tube, and sensitivity of the gal­
vanometer*
The current which will provide a deflection on one mil­
limeter is then given by the relation
l/S = k/gmRg
For an :FP-54 tube, gm s 25 micromhos, and if Rg * 109 ohms,
and k = 10"® amp/mm, a current of 4x10"^ amperes will produce
a deflection of 1 mm*
Meter Sensitivity*
Both the meter sensitivity and the
amplification within the tube are limited, for if either is in­
creased beyond a certain limit, it will be found that the in­
4
strument becomes subject to random fluctuations which in­
terfere with the usefulness of the device.
If the am­
plification is small, the fluctuations will be correspond­
ingly small, and a sensitive meter may be used* but if the
amplification of a given circuit is increased, the fluc­
tuations increase correspondingly, and a less sensitive
meter, must be used.
Random Fluctuations.
Since the occurrence of random
fluctuations places a limit on the sensitivity of the am­
plifier, it is necessary to mention the causes of these
fluctuations and to indicate methods for their reduction.
These causes are as follows:
1.
Lack of adequate shielding.
2.
Fluctuation of supply voltages.
3.
Microphonics within the tube.
4.
Ionization within the tube.
5.
Secondary emission.
6.
Uneven filament emission.
A great deal of work has been done along these lines.
Input circuits and tubes are carefully shielded, using iron
if there are strong magnetic fields.
Fluctuations in volt­
age supply are minimized by using large new storage batter­
ies or by devising special circuits which compensate for
voltage changes2).
Tube microphonics are reduced through
proper tube design and by cushioning the tube sockets.
Ionization within the tube may be reduced by using a highly
evacuated tube at low voltages - below the ionization po­
tential if possible.
The effects of secondary emission, if
iroltages high enough to produce it areused, are reduced by
means ofa 5)ecial grid ( #3 in a pentode tube.)
Uneven fil­
ament emission is minimized by using low temperatures on
the filament, stable current supply, by use of filament
material of maximum evenness of emission, and by special cir­
cuits.
Choice of Tubes
The general Electric F3? 54 Electro­
meter tube, which has been mentioned before, was especially
designed with these disturbing elements in mind*
it is
quartz insulated, and since the plate voltage is 12 volts,
the grid current ( due to ionization in the tube) is ex­
tremely low.
With a sensitive galvanometer to measure the
changes in plate current, currents down to 10 " ^ amperes can
be easily measured, and with great precautions and special
—18
circuits, measurements down to 10
amperes, may be made.
Where only moderate sensitivity isrequired, an a.c.
operated device using the less expensive receiving type tubes
and a rugged meter is more convenient.
Several types of
these instruments have been made, and are mentioned in the
references.
In selecting a receiving type of tube for this purpose,
one having the grid connection to the top of the tube rather
6
than to the base is preferable because of the better insu­
lation of the grid*
If the tubes are to be a.c. operated,
we have the choice of the following triple grid tubes: 954 ,
57, 6C 6 , and 1603.
The 954 is of the so-called acorn type.
No tests were
made on this tube, but its physical and electrical character­
istics certainly warrant an investigation as to its availa­
bility for this class of service.
The 57 and 6C6 are identical except for the filament, the
former operating on 2.5 volts, and the latter on 6.3 volts.
They vary greatly among themselves, and should be carefully
chosen.
The 1603 is identical to the 6C 6 electrically, but is of
low noise design for use in critical applications.
It is pro­
bable that this is the logical tube for the amplifier to be
described, but it was not tested because of considerations of
cost.
It may be placed in the amplifier at any future time
*
without making any changes other than normal readjustment.
If
the decrease in "noise11 should warrant an increase in sensiti­
vity, this may be easily accomplished by making a few changes
which will be described later.
The 6C6 was chosen on the basis of cost over the 954 and
the 1603, and over the 57 because, having been developed for
automobile service, it was considered probable that the
cathode would be more sturdy and less subject to microphonics.
CHAPTER 11
DESIGN OP PRESENT AMPLIFIER
The amplifier whose construction and operation are the
subject of this thesis has two stages of amplification.
The
first stage, which is similar to an amplifier devised by
Harnwell and Van Voorhis,^ is stabilized in two ways, which
will be considered separately.
As will be seen by the simplified diagram of the first
stage (Pig* 2) , in which grids #2 and #3, as well as tfee *x
balancing arrangement, are omitted for clarity, the circuit
is essentially a Wheatstone bridge in which two of the resist­
ors have been replaced with vacuum tubes.
As with the ordin­
ary Wheatstone bridge the various constants may be adjusted
so that the galvanometer M will show no current, and the cir­
cuit is said to be in balance.
If now a voltage is applied
to the terminals so that the grid of the upper tube is made,
say, more positive, the conduc­
tivity of the tube will there­
fore be increased and the current
5
through the upper portion of the
network will be increased.
Fit L
Be­
cause of the greater current
Showing first stage cir­ carried by R^ the potential of
cuit without /Lt-balance.
S
G. P. Harnwell and S.N. Van Voorhis, Balanced Electro
meter Tube and Amplifier circuits for small Direct Currents:
Rev. Sci. Inst: 5:244 (1934)
8
of point A will decrease and the circuit will he unbalanced*
The amont of unbalance as indicated by the galvanometer is
a measure of the applied voltage*
The advantage of using such a circuit lies in wellknown fact that the itfheatstone bridge is unaffected by
variations of supply voltage, for if the voltage EB is in­
creased, the voltage drop through
and R2 will change in
a balanced bridge, so that the potential of point A will
still equal that of Point B.
The change of supply voltage
is therefore not indicated by the galvanometer.
If the two
tubes were actually identical no further balancing other
than that inherent in the bridge circuitw ould be required*
But while the tubes were matched as nearly as possible at
the time of purchase, small differences in mutual conduct­
ance and in cathode heating rate prevent the two tubes from
behaving in an identical manner when the supply voltages are
changed:
perfect balance is therefore not achieved, and an
additional method of balancing the residual fluctuations is
required.
The method developed by Turner and called by him \i bal­
ancing is also incorporated in the first stage*
readily understood by reference to Pig. 3
It may be
Suppose the
plate voltage Ep to increase slightly due to an increase in
line voltage.
The increased voltage would increase the
plate current if no other factors are considered.
But the
current through ri and T£ would also increase, raising the
potential of the cathode.
As the
grid is now more negative to the cat­
hode, the current would tend to de­
crease.
And if
ijl
is the amplification
-W W W
factor of the tube, and if r^ and T2
are chosen so that there will be no
Fig 3
change in plate current with change in Ep.
A single stage amplifier embodying these two methods
of stabilization is very stable, and may be used with excell­
ent results using a micro-ammeter or a wall galvanometer.
In
order that a rugged mi H i ammeter might be used without loss
in sensitivity, a second stage was added.
This second stage utilizes a type 53 tube, which contains
two triodes in one envelope.
These are used to make a bridge
circuit similar to that of the first stage, the grids being
connected to what would be the galvanometer binding posts of
(ijl balancing, grids
§2 and 3, and volt­
age supplies omitted.)
of the first stage.
The millammeter is connected into the
10
second stage in the usual manner.
"both stages is shown in Fig. 4.
A simplified diagram of
As the current am­
plification of the second stage is small* —
about 60, no
stablization other than that inherent in the bridge circuit
is used.
The entire circuit is shown in Figure 5.
It will be
noted that this includes a built-in potentiometer for use in
calibrating the instrument.
The current for this potentio­
meter is supplied by a dry cell, and since the current
drain is slightly more than one milliampere, the dry cell
provides good r egula,tion and long life.
This current is
adjusted by means of P-l, and checked for proper value by a
shunted 0-1 milliammeter.
The voltage in millivolts is read
directly on a dial mounted on P-2, a 600 ohm, potentiometer.
The other circuit constants for this portion of the circuit
are not given, as they must be determined experimentally in
each case.
This is because the dial does not cover the en­
tire winding of P-2, and because individual potentiometers
willin vary somewhat from their nominal values.
It should be mentioned here that the quality of parts
used in the amplifier and potentiometer is c ritical.
The
parts P-2 and P-3 are General type 314-A potentiometers,
having multiple contacts.
were of the same type.
Improvement might result if P-4
All other rheostats except P-6 are of
pot : a d j .
-6 874
S H I E L D ^
INPUT
•VvwwVt\r
VT-3
R-3
■WWvvV
S\ /-3
SHIELD
INPUT SWITCH
SW-2
SENSITIVITY
P I L O T LIGHT \
©
-
R r Z 2 .5 V
6C6
VT-2
603
CHOKE
E^6.3V
VT-5
©
R-9
V T -4
FH5-5
SW.4
TESTING SWITCH
^ GROUND TO CHASSIS
® CLOSED CIRCUIT JACK
12
General Radio make.
Resistors should he wired wound.
The
writer had set this circuit up over a year previously and
had abandoned it because of trouble which was later traced
to faulty action of smaller radio-type volume controls for
rheostat and potentiometer use.
The input to the amplifier leads to a three-position
input switch (Sw-2) by means of which the grid of the input
tube VT-1 may be grounded, connected to the input, or to the
calibrating potentiometer.
The grid resistors R 5 are 20
megohms, and are shielded by means of heavy brass cylinders
placed immediately against the tube shields for VT-1 and
VT-2.
All the input wiring is shielded, using aviation
ignition wire, although the recently introduced Amphenol
co-axial cable would provide improvement.
The operation of the amplifier is obvious from an ex­
amination of Rig. 5.
The constants of the tubes of the
first stage are individually variable through the use of sep
screen supplies (from P-5 and P-6 ) while the voltage is held
constant across the potentiometers by means of a voltage re­
gulating tube, UX874.
The ji-balance for VT-1 is provided by
R-7, P-3, and P-4 as fasr as the slider, corresponding to rg
of Rig.
r^.
The
3,
ijl
and the remainder of P-4 and R-5 corresponding to
balance for VT-2 is provided by the same resistor
network except that P-4 is replaced by two fixed resistors.
The sensitivity may be reduced by shunting resistors
13
across the plates of the first stage tubes hy means of Sw-3.
Jacks are provided hy means of which the plate current of each
of the first stage tubes and one of the second stage triodes
may be read, as well as a jack to allow the use of the first
stage independently of the second.
The type 874 tube not only maintains a constant volt­
age across P-5 and P-6 but provides a low resistance path
through thispart of the circuit for the cathode current of
they type 53 tube.
The otherwise high resistance would
cause excessive degeneration or “inverse feedback”.
Not shown in the circuit are two resistors of two ohms
each in each side of 6.3 volt filament supply, thus reducing
the filament voltage to approximately 4.5 volts.
This has
the effect of reducing the temperature and thereby making
filament emission more uniform and also increases the input
resistance of the tube.
The two Wheatstone bridge circuits are balanced by P-7
and P-8 .
These controls are marked “Zero Adjust, Pine" and
“Zero Adjust, coarse” respectively on the outside of the case.
Other controls needed far routine use are P-l and P-2, and
all switches except Sw-4.
side.
These are controlled from the out­
All others are located inside the case and are handled
except for every frequent readjustment.
14
CHAPTER III
ADJUSTMENT AND BALANCING OP AMPLIPIER
Note: The adjustment procedure to he described should
not be necessary unless the tubes have been changed or the
controls inside the cabinet tampered with.
Adjustment is
tedious, and care should be taken not to remove the tubes
nor to change the setting of the internal controls, as this
will make adjustment necessary.
In adjusting the amplifier, it is important to remember
that the grid bias on VT-4 is dependent not only oh the ap­
plied voltages, but also upon the plate current of VT-1 and
VT-2.
The grid potential is equal to the applied voltage
taken off R-7 diminished by the IR in P -8 and R-8 or R-9.
The cathode of VT-4 is connected to the high side of VT-3,
and its potential is determined by the setting of P-3.
The
adjustment is complicated by the fact that not only must \xbalance be obtained for VT-1 and VT-2, but that their plate
currents must be adjusted so that the grid of VT-4 will be
negative by the proper amount with respect to its cathode,
all with the same stet of controls.
However, once balance
has been obtained, further adjustments will not be necessary
until tubes are changed or they change their characteristics.
As the tubes are operated under low voltages, they should
last
almost indefinitely.
15
To make the adjustment, VT-1 and VT-4 are first removed.
By means of a cord fitted with phone plugs at both aids,
the
plate current jack of VT-2 is connected to the jack mounted
on the case of the meter M-l,
inside the cabinet*
taking
this connection automatically disconnects M-l from its reg­
ular circuit..
By means of P - 2 and P-6, VT-2 is adjusted to
^-“balance, the plate current of VT-2 to 0.53 milliamperes*
This is done by setting these controls so that when the test­
ing switch, Sw-4,
current.
is operated,
there is no change in plate
VT-1 is then put in place and similarly adjusted
using P-4, P-5, and to some extent for final balancing and
matching with VT-2, P-8.
Placing VT-1 in the circuit will
somewhat upset the operation of VT-2,
sd
that final balancing
must be done with both tubes in place..
Pig. 6
16
VT-4 is then placed in the circuit.
If the plate
current of the preceding tubes is correct, the tube will
draw from three to five milliamperes per plate.
The jack
which is connected with one of the triodes of Vt-4 is fitted
with a shunt so that it may be plugged directly into the
jack on M-l.
Any reading on the upper half of M-l indicates
a satisfactory plate current.
If the meter reads too low,
it is an indication that the first stage tubes are drawing
too much current* if the reading is too high, they are not
drawing enough.
The first stage must then be readjusted.
It is often helpful to have VT-4 in place and plugged into
a separate 0-1 milliammeter during the preliminary adjust­
ment, of the first stage.
On operating the testing switch, it will be seen that
the imperceptible movement of the meter when plugged into
the first stage is now considerably magnified.
Adjustments
are then made to reduce these deflections to zero.
Finally,
the testing cord is removed entirely, and adjustments con­
tinued for minimum movement of M-l when the testing switch is
changed in position.
This procedure eliminates instability due to changes in
plate or screen voltages.
Changes in filament eurrent are
partly compensated by the bridge arrangement and by the fact
that the tubes are operated at reduced voltages, but a sharp
change in line voltage will produce some drift due to the
17
fact that the heating rate of the first stage tubes are not
the same.
This drift could be reduced, if thought necessary,
either by using storage batteries or a saturated core trans­
former as a source of filament current.
The small residual fluctuations are due to “tube noise,1'
and will not interfere with the use of the instrument.
It
is probable that the use of 1603 type tubes in place of the
6 C6 would result in less of this type of fluctuation, thus
increasing the useful sensitivity, or enabling the use of the
present sensitivity with "quieter" action.
18
CHAPTER IV
SENSITIVITY
Because of small fluctuations which cannot be balanced
out the amplifier sensitivity was purposely limited.
The
voltage gain of the first stage is limited by the fact that
R-8 and R-9 are set at 100,000 ohms.
If this value were
raised to 250,000 ohms the sensitivity would be increased
3.3 times.
Nor are R-13 and R-14 the optimum values to de­
liver maximum power from VT-4 to M-2.
For maximum power
into the meter, the following re­
lation should exist:®
1
Fig 7
Rgi
= _ 1_
+
1
RI+R3 RS + R4
Rq2 is fixed, and is approximately 33 ohms.
R]_ and R3 are
the tube resistances, and are 22,000 ohms each.
Rg and R4
are the resistances in the plate circuit, and are equal.
Solving for Rg and R^ we find 17 ohms the optimum value for
these resistances.
The voltage sensitivity of the amplifier as built is
2/3 millivolt per division of M-l.
The current sensitivity
is, since 20 megohm input resistors are used:
2/3 x 10-3 x 1/2 x 10“7 = .33 x 10“10 or 3.3 x 10”11.
amperes/division.
The voltage range of the instrument is limited to a
®
Page and Adams:
Principles of Electricity:
p 180
19
maximum of 100 millivolts.
This is due to the fact that an
input voltage ts much greater than this, after amplifi­
cation by the first stage, is sufficient to cause the tube
elements of ttee VT-4 to block.
The current sensitivity is
therefore limited to the range from 5 x 10“^ amperes as
the maximum to a minimum detectable of lO*1^ amperes.
20
CHAPTER V
DIRECTIONS EOB USE
CAUTION:
DO NOT TAMPER WITH ANY CONTROLS INSIDE THE
CABINET.
M -
L
L>
Pig 8
1*
Turn the sensitivity control to the left.
2*
Turn on the power (extreme lower left switch).
Due to
the fact that the tube heaters warm up at different rates,
the amplifier meter M-l (on the left) will fluctuate widely.
Keep the needle on scale as nearly as possible by use of the
two adjustment controls.
The "Pine” adjustment will have
the most effect in this case.
The large fluctuations of the meter will soon stop,
leaving a fairly steady drift which will stop within a few
minutes, when temperature equilibrium is established.
The
21
residual small fluctuations are due to tube HnoiseM and will
not interfere with the use of the instrument.
3.
Connect the source of voltage or current to he measured
to the terminals on the right side of the case.
The leads
should he well shielded, and of very high resistance between
the grounded shield and the conductor.
Amphenol coaxial
cable is recommended for uhis purpose.
Connections should be made so that the positive lead
^oiee to the porcelain bushing.
ed.
The binding post is ground­
With the connections made, but with the E.M.P. on,
and adjust the sensitivity so that the meter will give as
large a deflection as possible, but remain on scale.
Chang­
ing the sensitivity will require a resetting of the zero
point.
4.
With the sensitivity properly set, and the zero set with
input circuit in place, turn on the unknown E.M.P., and see
the reading of the amplifier meter.
Turn on the E.M.P. and
note whether the meter again reads zero.
If not, readjust
and take another reading.
5.
Turn on the potentiometer (Sw-l) which has been previous­
ly adjusted as described below.
Turn the input switch to
"Potentiometer" and adjust the amplifier to zero, being sure
that the potentiometer is set for zero.
Then turn the pot­
entiometer dial until the amplifier meter gives the same
reading as that produced by the unknown E. M. P.
The dial then
22
given the same reading as that produced by the unknown
E. M.P. measured across the 20 megohm input resistance in
millivolts.
The current may he calculated by Ohm’s law.
Adjustment of Potentiometer
1.
Note zero reading of
the potentiometer meter M-2 (on right).
This reading is
not the zero of the meter, but the first small division past
the zero mark.
This may be adjusted with a smaL1 acrewdriver
if necessary.
2.
Turn on the potentiometer.
As the winding on the potent­
iometer is not quite uniform on the lower end, it is nec­
essary to have slightly different currents passing through
the windings for different regions on the dial.
These are as follows15
Dial Reading, Mv
Pot. Meter Reading
0-5
Special Mark off Scale
6-29
.99 ma
30-100
.98 ma
The readings from 6-100 mv are always within two per cent,
and in most cases well under one percent.
accuracy is less.
Prom 0-5 mv., the
The settings are as follows:
Dial Reading
1
2
3
4
5
Actual millivolts
.82
2.8
1.7
4.13
4.8
Ajust the current through the potentiometer according to the
23
table above as soon as the approximate range is determined,
using the HPot. Adj.) control, which is P-l.
Directions for removing the chassis from the cabinet*
In case service work should be found necessary, the chassis
should be removed according to the following directions 3
1*
Remove right hand screw from bakelite jack cover on
rear of chassis, and loosen left hand screw.
2.
Remove crackle finish screws froirv top and sides.
Do not loosen nickle plated screws along the bottom.
3.
Unsolder ground and input connections to case at in­
switch.
4.
Tip case on left end.
Slide right end of chassis
forward slightly, so that the chassis will clear the flange
on the case.
5.
Watch out for jack strip and cover.
Set case flat on table.
With a large screwdriver the
case may be sprung sufficiently to draw out the chassis.
Summary.
In use, then, the amplifier is allowed to warm
up for a few minutes, after which the well-shielded Input con­
nections are made, and the m plifier zero is adjusted.
The
current or EMF to be measured is turned on and the meter read­
ing noted.
The input switch is then switched to "Potentio­
meter" and the dial turned until the meter gives the same
reading as before.
The dial then indicates directly the mill­
ivolts across the input resistance.
The current is obtained
by dividing this value 2 x 10 7, the value of the input
24
resistance.
For very small currents, the value is 3 x lO**^1
amp per div. of M-l.
Readings are quickly and easily made,
and, because of the potentiometer feature, calibration charts
and curves are unnecessary.
The instrument is reliable and,
as long as the internal controls are not tampered with, as
fo&l-proof as an instrument of its sensitivity could be.
The current range is from 3 x lCT^ amperes to 3 x 10”*^
amperes, although smaller currents can be detected.
voltage range is from 0-100 millivolts.
The
While normally volt­
ages will be indicated directly on the potentiometer, it is
well to note that a half division on M-l indicates 1/3 milli­
volt.
BIBLIOGRAPHY
25
SUMMARY OF REFERENCES
1* Brooks, H.B., “Sensitivity of a galvanometer as a function
of its resistance'*. Bur. Standards Jour. Research 4 :297
2
Penick: “Direct Current Amplifier Circuits for use with the
Electrometer Tube. Rev. Sci. Inst. 8:196-198 (1937)
This reference provides a summary of various circuits
devised for use with the electrometer tube. An extensive
bibliography is provided.
3
Roberts, Shepard, "A Feedback Micromicroammeter" Rev. Sci.
Inst. 10: 181-183 (1939)
An a.c. operated device of excellent stability,
providing a sensitivity of 10-12 amperes. Requires large
grid resistors.
4. Gabus, G.H., and Poole: "A Portable Phototube Unit using
an RCA 954 Tube” Rev. Sci. Inst.8 : 196-198 (1937)
A battery operated outfit using a grid resistor of 1011
ohms and providing a sensitivity of l(P-3 amperes. Quickly and
easily set up, especially suited for photocell work.
5*
G.P* Harnwell and S.N. Van Voorhis, “Balanced Electro­
meter Tube and Amplifier Circuits for small Direct
Currei ts” Rev. Sci. Inst. 5:244 (1934)
6 . Page, Leigh and Norman I. Adams, Principles of Electricity,
p 180.
Van Nostrand Company, 1931.
APPENDIX
26
LIST OP PARTS
P-1,3 4, General Radio potentiometer type
214-A
100 ohms
P-2
type
314-A
600 ohms
P-3
type
314-A
2000 ohms
p-5,: ?-6 ,
type 301-A 20,000 ohms
P-7
type 214-A
P -8
International Resistance Co potentiometer
type CS
200 ohms
10, 000 ohms
R-5
Wirewound resistor, 50 ohms, 1 watt.
R-6
Carbon resistor, 20 megohms, two required.
R-7
Wirewound adjustable resistor, 10,000 ohms, 50 watt.
R-8 , R-9, Carbon resistors, 0.1 megohm, 1 watt
(wirewound resistors are to be preferred)
R-10
Carbon resistor, 0*25 megohm, 1/2 watt, for sensitivity
control.
R-ll
it
R-13
R-14,
R-15
Wirewound resistor, 200 ohms, 10 watts.
ii
0 .1 0
M
M
M
n
H
Wirewound resistor, 1000 ohm, 1 watt.
Two wirewound resistorsm 50 ohms, 1 watt, as shown in
Pig. 5.
Two wirewound resistors, 2 ohms, 2 watts for insertion
in filament supply of VT-1 and VT-2. See text.
Sw.
Cutler Hammer rotary line switch.
Sw-1
Cutler Hammer rotary switch, s.p.s.t.
Sw-2
Sw-3. Centralab Isolantite insulated Transmitter
Switch, single deck, #2542
Sw-4
Yaxley four pole double throw rotary switch, contacts
wired in parallel for s.p.d.t.
C-l, !-2. Electrolytic condensers, 8 mfd, 450 volts.
27
Parts list, cont*
Choke, Filter, 12 henry, 60 m.a.
Transformer, Powere, with 2.5 and 6.3 filament secondaries.
Dry Cell, Burgess Little Six, 4FH
Closed circuit jacks, as indicated on Fig. 5.
Two circuit jack, as shown in Fig. 5
Miscellaneous hardware, wire, and cabinet.
Tube complement:
2
type
1603 or 6C6--VT-1 and VT-2
1
type
874
1
type
53
VT-4
1
type
80
VT-5
--VT-3
Three required.
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