Marconi Instruments TF 2700 User manual

OPERATING INSTRUCTIONS
EB 2700
for
Universal Bridge
TF2700
Copyright @ 1962
MARCONI INSTRUMENTS LIMITED
STo ALBANS HERTFORDSHIRE ENGLAND
C. P. lk EB 2700
3/66/K 19 - 3/66

CONTENTS
Section Page
l GENERALINFORMATION ... ... ... 3
1.1 Features o.. ... ... ... ...3
l.l DataSummary ... ... ... ... 4
1.3 OptionalAccessories... ... ... ... 6
l OPERATION o.. ... ... ... ... 7
l.l GeneraI... ... ... ... ... 7
l.l.l BalancingtheBridge ... ... ...7
l.l. l ReadingtheResult ... ... . .. .. . 8
l.l BatteryCheck... ... ... ... 0..9
l.3 Connections ... o.. ... ... ...9
Fig. l.3 Controls and Operating Faci1ities ... . .. lO
l.4 Capacitance Measurements ... ... .'. Il
l.4.1 -using the Interna11 kc/sSource ... ... Il
l.4.l -usinganExterna1A.F. Source ... o.. 13
l. 4.3 E 1ectro1ytic Capacitors and use of Po1arizing Bias 13
l.4.4 CapacitancetoTrueEarth o.. ... ...14
l.5 InductanceMeasurements ... ... ... 15
l.5.1 -using theInterna11 kc/s Source ... ... 15
l.5.l -using anExterna1A.F. Source ... ... 16
l. 5.3 IncrementaI Inductance and use of Polarizing Bias 16
l.6 Resistance Measurements ... ... o.. lO
l.6.1 -usingtheInternallkc/sSource ... ... II
l. 6.l -using anExternal A. F. Source ... o.. II
l.6.3 -using the InternaI Battery ... ... ... II
l.6.4 -usinganExterna1D.C. Source o.. ... l3
l.7 Connectionof Externa1A.C. Source o. . . .. l5
l.8 LossBalance-QorD... ... ... ...l7
3 CIRCUITSUMMARY ... ... ... ... l8
3.1 BridgeCircuits ... ... ... ... l8
3. l Bridge Energizing Sources ... ... ... 19
3.3 Detector ... ... ... ... ... 19
4 MAINTENANCENOTES... ... ... ... 31
4.1 Replacement o£the Battery ... o.. ...31
4. l Replacement of Circuit Componente . .. ... 31
4.l. 1 AccesstoInterior .o. ... ... ... 31
4.l. l Adjustmento£DetectorSensitivity ... o.. 3l
4.l.3 Oscillator Frequency ... ... ... 3l
4.l.4 RangeResistorRI3... ... ... ... 3l
CIRCUITDIAGRAM ... ... insidebackcover
2

I GENERAL INFORMA TION
1.1 FEATURES
The TF 2700 Universal Bridge is a compact, self contained instrmnent
that brings new versatility to the rapid determination of a wide range of
impedance values. Facilities are provided far the application of external
a.c. and d.c. supplies far use in a variety of specialized measurement
procedures.
The internaI energizing sources far the bridge are a 1kc/ s oscillator
far a. c. measurements of capacitance, inductance and resistance; and the
9 V power supply battery far d.c. resistance measurements. The detector
has been designed to have a usable response from 20 c/s to 20 kc/s. This
allows an external source to be used to energize the bridge far measure-
menta aver a more representative range of frequencies. D. C. voltages
higher than that of the battery may alBo be applied externally {or improved
discrimination with re sistance measurement&.
Another facility enables a d. c. bias to be applied to the component
under test. Up to 500 V may be used far polarizing electrolytic capacitors
or far investigating the properties of non-linear resistors. Alternatively,
upto 40 mA may be passed through iron cored inductors by suitable external
connections or up to 200 mA when using the D.C. Choke Adaptor, TM 6113.
2700 (1b) 3

~ '-
1.2 DATA SUMMARY
RANGES
Capacitance: O.5 pF to 1100 I1F in 8 range s of 110 pF to
1100 I1F full scale.
Inductance: O.2 I1H to 110 H in 8 range s of Il I1H to
110 H full scale.
Resistance: lO ni1 to Il M1 in 8 ranges of 1.1 a to
Il MrZ full scale.
Q lndication: Oto lO at l kc/s.
D Indication: Oto 0.1 or Oto lO at l kc/s.
ACCURACY
Basic Accuracy: :1:1%of reading :1:0.1"/0of range full scale.
Inductance: Additional -0.2"/0 of reading. +0 -0.3 I1H.
First Ranges only:
Capacitance. 100 I1F 201 f d .:1:1 Fto 1100 I1F: :I: IOo rea lng. jJ...
Inductance. 0.2jJ.H +2"/0-10"/0of reading. +0 -O. 3jJ.H dependent
to lljJ.H: upon Q.
Resistance. lO ~ 201 f di :1:1 -..n
l :I: IO o rea ng. LLA'.
to l. a:
Residuals to be COI approx. O.l pF.
subtracted: Lo. approx. 0.2 jJ.H.
Ro. approx. 2~.
BRIDGE SOURCE
InternaI: l kc/s :1:50;0oscillator far a.c. measurements
of C. L, and R. 9 V battery for d.c. mea-
surements of R between 1.2 a and 50 ~.
4

E xte rnal: A. F. tram 20 cl s to 20 kcl s may be used
far C. L. and R measurements where an
alternative frequency is more approp~iate.
Additional bridge errar is normally lesa
than O.25%.
D.C. may be used far R measurements in
piace of internaI battery to improve the
d. c. sensitivity at range extremes. (For
low d.c. R values a sensitive galvanometer
is alBo required.)
FACILITIES: A polarizing voltage may be applied to the
component under test. Non-linear elements
may be tested by applying a variable d. c.
bias or a. f. signal.
POWER SUPPLY: InternaI 9 V battery. world standard type;
usual current drain approx. 7 mA.
DIMENSIONS &. WEIGHT: Height Width Depth Weight
8 in llj in 8 in 8j lb
(23.5 cm) (29 cm) (23.5 cm) (3.8 kg)
5

I.] OPTIONAL ACCESSORIES
D. C. Choke Adaptor. TiPe TM 6113
This enables d. c. currenta up to ZOOmA from an external supp1y to
be passed through inductors under test at 1 kcl s in the range 100 mH to
100 H. It is fitted with spade-ended leads far connecting to the bridge
termina1s. Errors introduced by the Adaptor do not genera11y exceed 3%
and may be eliminated by simple substitution methods.
Isolating Transformer, Type TM 71Z0
This is intended far connection between the bridge and an externa1
a.f. energizing source in arder to keep the bridge isolated from earth.
6

2 OPERAnON
2.1 GENERAL
The operational controls of this bridge are arranged to provide an
easy to use, direct reading measurement system. Fig. 2.3 summarizes
the control functions. (See page lO.)
l.l.l BALANCING THE BRIDGE
NormaI balancing procedure entails adjusting the main BALANCE
controls, in conjunction with the LOSS BALANCE control for reactive
components, in arder to bring the meter to as near zero de£lection as pos-
sible. The SENSITIVITY control allows the detector sensitivity to be red-
uced for a clearer meter indication of approaching balance with components
of unknown value.
The meter de£lection will always be to the right of zero for a. c.
measurements; but ford.c. resistance measurements it will swing through
zero as the balance point is passed so that when the de£lection is to the left
of zero the value of the unknown is higher than the setting of the bridge and
vice versa.
Failure to obtain a complete balance null during a. c. measurements
may be for one or more of the foliowing reasons:
R Too much stray capacitance or inductance.
C and L Incorrect adjustment of the LOSS BALANCE partic-
ularlyat low Q or high D sett_ings. This may be due
to the limited resolution of the contrai at Q lese than
l. Alternatively it may mean that the loss of the
test component is too complex to be simply resolved
by either series or paralielloss balance. This could
mean that it is frequency conscious so that the balance
point for the harmonic content of the a. f. source is
different tram that of the fundamental. Any external
a. f. signal used to energize the bridge should there-
fore be free tram harmonics and mains hum; it may
be necessary to use an external frequency selective
detector, connected between the DET +VE and -VE
(chassis) terminals.
C, L and R Pick up of interference, e. g., mains hum, by the
test component or its connections, see Section l.3.
Check for pick up by inserting an open circuit jack
plug in the EXT A. C. socket when the meter indic-
ation should fali to zerG.
7

2..1.2. READING THE RESULT
The main BALANCE controls of the bridge comprise a ten position
switch with a variable interpoIating contraI mounted concentrically. The
switch provides increments of O.l between each position. The variable
contraI covers one incremento with slight overIap; its calibration of 00
to 100 giving the second decimaI piace withintermediate divisions of 0.002..
The overall calibration is therefore Oto aver 1.1; examples of BALANCE
contraI readings are given in Fig. 2..l.
( (
c.}o.s.. (o)00604
QJ( (
(c),- (o)0-.0.
Fig. 2..l BaIance ContraI Indication
The table of range full scale values is linked to the BALANCE controls
by the RANGE MULTIPLIER switch. The correct impedance value is obt-
ained by multiplying the BALANCE contraI reading by the fun scale value
indicated by the FUNCTION and RANGE MULTIPLIER switches. Fig. 2.2
shows the bridge set to a value of 66.6 mH.
...
~
""K
Fig. 2.2
Test Component Value - 66.6 mH
8

2.2 BATTERY CHECK
Since the bridge is battery powered its measurement sensitivity will
be dependent on battery voltage. This may be monitored by connecting a
voltmeteror anammeter between the DET+VE and -VE (chassis) terminals
with the bridge set to the lO M.'"2,R INT D. C. range. The voltage measured
should be at least 6 V or the current at least 40 mA; if not, the battery
should be replaced, far details see Section 4. l.
NOTE: In arder to preserve the battery the instrument should be
switched OFF after each measurement. When an external
bias is being used the precautions given in the appropriate
Operation section mustbe observed before altering the positions
of the FUNCTION and RANGE MULTIPLIER switches.
2.3 CONNECTIONS
Components to be measured are connected across the two large term-
inals marked HI and DET +VE. The connecting leads should be kept as
short as possible in arder to avoid introducing stray capacitance and mains
hum pick up which will obscure the balance indication.
When measuring large items or componente it is important to avoid
the introduction of external interference signals via the DET +YE terminaI
or else a poor or false balance will be obtained. li the component has an
isolated screen it is probably best to connect this to the -VE (chassis)
terminaI; but in the event of this screen having too large a capacity (say
aver 200 pF) to the contents on test, it may be alternatively connected to
the HI terminaI. The screen in this case must be well isolated or spaced
from the bridge case to avoid re-introducing the stray capacity.
Two P40 jack plugs are provided with the Bridge far the connection
of external a. c. and d. c. supplies. An alternative a. f. energizing source
should normally be connected to a jack plug inserted into the EXT A. C.
socket. The d. c. supply far a polarizing bias should be similarly connect-
ed to the BIAS socket.
For some measurements an alternative detector may be connected
between the DET +VE and -VE (chassis) terminals in arder to obtain a
more sensitive balance indication. Full details of these connections are
given in the section appropriate to the desired type of measurement.
9

~
i~
i
Fig. Z.3 Controls and Operational Facilities
IO

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.2..4 CAPACITANCE MEASUREMFNTS
Capacitance values of 0.5 pF to 1100 IIF may be measured at I kcl s
from the internai oscilla.tor or at frequencies of 20 cl s to 20 kcl s from an
external source where an alternative frequency is more appropriate. Fig.
2.4 shows the basic bridge configuration used far these measurements.
In addition to an external a. c. source, an external d. c. supply may alBo be
connected far the pola.rization of electrolytic capacitors.
MTECTW
c.
Fig. 2.4
Basic Capac;itance Bridge
'"~
2.4. l MEASUREMENTS USING THE INTERNAL l kc/ s SOURCE
.. (I) Connect the component to the test terminale.
(2) Select the range full scale, appropriate to the expected value of
capacitance, with the FUNCTION and RANGE MU.LTIPLIER switches.
(3) Set the D-Q switch to D x .01 far normal capacitors and to D x l far
electrolytics.
(4) Set the LOSS BALANCE contrai to about l, and set the SENSITIVITY
contrai to give a meter deflection of lese than full scale.
(5) Bala.nce the bridge with the main BALANCE and LOSS BALANCE
controls, increasing the sensitivity to keep the meter defl~ction above
zero until the SENSITIVITY contrai is fully clockwise.
The capacitance of the component under test is given by rnltltiplying
the setting of the main BALANCE contrai with the range full scale, as des-
cribed in Section 2.1; with values below lO pF it is advisable to check tre
C of the bridge by bala.ncing it with the test terminale open circuited.
o
Since the calibration of the LOSS BALANCE contrai is only nominai its set-
ting far balance must only be taken as a guide to the quality of the test
component; see Section 2.8 far further details.
12

2.4.2 MEASURE:MENTS USING AN EXTERNAL A. F. SOURCE
An alternative frequency may sometimes be used to advantage far -.
high value capacitors, especially electrolytics as these are often manufac-
tured to have a specific 50 to 120 c/s value. Aiso at 10wer frequencies the
effect of lead inductance is much lesa significant, and the LOSS BALANCE
contrai setting becomes proportionately less criticaI.
The measurement procedure is similar to that described in Section
2.4.1 and details of connections are given in Section 2.7. When using
frequencies other than l kc/ s multiply the D and Q scales bya factor OI
f/1000, where f is the frequency in CIBo
2.4.3 ELECTROLYTIC CAPACITORS AND USE OF POLARIZING BIAS
Due to their construction electrolytic capacitors often have relatively
large series and paral1elloss components. The series 10ss is usually
greater and so the D setting of the LOSS BALANCE controls should be used,
this being essential when a polarizing voltage is applied.
The LOSS BALANCE adjustment may be found to be rather criticaI
therefore the sensitivity should be reduced in arder to obtain a more
satisfactory balance. Too high an accuracy has 1ittle meaning far very
high loss components and no more sensitivity is desirable than is justified
by the component quality.
Application of Polarizing D. C. Bias
-
Polarization of normal quality e1ectrolytic capacitors does not have
any greateffectuponthe capacitance value unless appliedfor a considerable
time. A polarizing d. c. supply may be applied to the capacitor under test,
however, via the BIAS socket, as shown in Fig. 2.5.
"',TE"
~~~~~==~E::~~-o"T +
..~ or y ". --,
""'" , PASSC. --
.:-'- s-
Fig. 2. 5 Application of
D. C. Bias to Electrolytics ,2700 (la) 13

.A suitably limited voltage of up to 500 V d. c. should be connected to
the BIAS jack plug, tip positive, sleeve negative. Since tms bias is fed to
Cx via the RANGE resistor, a limiting resistor RL' must be added in series
with the supply to prevent the t W rating of the RANGE resistor from being
exceededin the event of Cx becoming short circuited. Recommended values
for RL are:-
(a) 5 ~ per 100 V when Cx is greater than IIJ.F.
(b) 25 ~ per 100 V when Cx is less than IIJ.F.
The by-pass capacitor, CI, is necessary in arder to ensure an a.c.
path for the bridge energizing signal. lt should be at least the full scale
value of the range in use or llJ.F, whichever is the larger, and of approp-
riate voltage rating.
The stray capacitance between the bias supply and bridge must be
kept to a minimurn, preferably less than 100 pF. lf the supply is earthed,
it is probably only necessary to keep the bridge about 6 in (15 cm) from
the nearest earthed metalwork, and to avoid handling the case wmlst balan-
cing the bridge.
When an external 3,.c. source is al so being used it will be internally
connected to the positive side of the bias supply. If, however, measure-
ments of an earthed capacitance are to be made it may be more convenient
if the two jack plugs are inter-changed.
WARNING:
(l) The bridge case is connected to the positive side of the bias
supply via the BALANCE resistors (O to l~OOa), and the EXT
A. C. socket. Note also that the negative side of the supply is
connected to the Hl terminaI.
(2) The bias supply must be switched OFF before changing the
bridge configuration or switching the bridge off.
2.4.4 MEASUREMENT OF CAPACITANCE TO TRUE EARTH
This measurement may be carried out provided the Hl terminaI is
connected to the earthed terminaI of the capacitor under test. Fig. 2.6
shows the connections. If the test capacitor is some distance from the
bridge a screened lead should be used with the screen connected to true
earth. The capacitance of the lead must then be subtracted from the
result!l.
lt is important to minimize the stray capacitance of the bridge case
to earth for this measurement. Keep the bridge at least 6 in (15 cm) from
earthed metalwork and avoid handIing the case while balancing it.
14

MTEC~
elo
55
"001
-..
Fig. 2.6 Measurement of Fig. 2.7
Capacitance to True Earth Basic Inductance Bridge
2.S INDUCTANCE MEASUREMENTS .
Inductance value s of O. 2 I1H to 110 H ma y be mea sured at l kc/ s from
the internaI oscillator or at frequencies of 20 c/s to 20 kc/s, as appropriate
to the inductor, from an external source. Fig. 2.7 shows the basic bridge
configuration used far these measurements. The application of an external
d.c. bias al so enables measurements suchas incrementalinductance of d.c.
polarized smoothing chokes to be made.
2.5. l MEASUREMENTS USING THE INTERNAL l kc/ s SOURCE
(l) Connect the component to the test terminals.
(2) Select the range full scale, appropriate to the expected value of
inductance, with the FUNCTION and RANGE MULTIPLIER switches.
(3) Set the D-Q switch to :-
(i) Q far air cored coils,
(ii) D x .01 far high Q filter coils,
(iii) D x l far laminated iron cored inductors.
(4) Set the LOSS BALANCE contrai to about l, and set the SENSITIVITY
contrai to give a meter deflection of less than full scale.
(5) Balance the bridge with the main BALANCE and LOSS BALANCE
controls, increasing the sensitivitytokeep the meter deflection above
zero until the SENSITIVITY contrai is as fully clockwise as possible
far the most discriminating balance.
15

~ The LOSS BALANCE setting is very criticaI for a Q less than
l or a D greater than 0.5 and it is necessary to persevere until the
settings which give the lowest meter deflection are obtained. This is
especially important at low Q due to interaction of the controls.
Alternative settings should be tried.
The inductance value ofthe component under test is given by multiply-
ing the setting of the main BALANCE control with the range full scale, as
described in Section l.l. Since the calibration of the LOSS BALANCE
control is only nominaI its setting for bala.nce must only be taokenas a guide
to the test component's quality; see Section l. 8 for further details.
Air cored coils shou1d be spaced clear of metalwork by one or two
diameters to avoid alteration of the inductance by induced currents.
Self capacitance of high value inductors can affect the measured
inductance value, though this is more noticeable at higher frequencies.
The connecting leads shou1d therefore be kept spaced apart.
.With magnetic cored inductors it may be necessary to position them
so that pick up of external magnetic fields is low enough not to affect the
balance indication. The amount of pick up may be checked by inserting an
open circuited jack plug into the EXT A. C. socket; this will switch off
the oscilla.tor, so that only pick up will be detected. The residual meter
indication shou1d be very low. Magnetic cores al so have non-linear char-
acteristics so that a true balance is difficult to obtain. U se only sufficient
sensitivity forthe accuracy required, and consider the use of d. c. bias or
an alternative frequency.
l. 5. l MEASUREMENTS USING AN EXTERNAL A. F." SOURCE
It may be more desirable to measure the inductance of some comp-
onents at frequencies other than l kc/ s. Measurements on smoothing
chokes at mains frequency, or on components of ultrasonic equipment at
lO kc/ s can provide more realistic results.
The measurement procedure is simila.r to that described in Section
l. 5. l, and details of connections are given in Section l. 7. When using
frequencies other than l kc/ s multiply the D and Q scales by a factor of
f/lOOO, where f is the frequency in c/ s.
l. 5. 3 INCREMENTAL INDUCTANCE AND USE OF POLARIZING BIAS
Due to the non-linear B/H relationship of iron cored inductors it is
often required to know the incrementaI inductance of a component. This i
the inductance value measured with a small a. c. signal, while a d. c. bias
16

is passed through the component to take the operating point to the desired
position on the B/H curve.
Measurements of this type may be carried out with this bridge using
several alternative methods~-
lA
v*
soov
Fig. Z.8 Connection of
D. C. Bias, Method l '"IO
(l) The external d. c. bias may be applied via the BIAS socket as shown .
in Fig. Z.8. A suitably limited voltage of up to 500 V d. c. should be
connected to the BIAS jack plug, tip positive, sleeve negative. Since
the bias current alBo flows through the range resistor, the oscillator
transformer and the external a. c. supply, if being used, the permis-
sible current is limited by the t W rating of the range resistors.
TABLE l lists the maximum current far any range.
Theby-pass capacitor CI shouldbe ora suitable value topass the a.c.
signal and of appropriate voltage rating. .
TABLE l
Range 100 H lO H l H 100 mH
full scale
Range l M} 100 141 lO 141 l 141
Re sistor
I 0.5 mA Z. Z mA 7 mA ZZ mA
max
Range lO mH l mH 100 ~H lO ~H
full scale
Range 100(1 10(1 l a 0.1 a
Re sistor
I 70 mA ZOOmA ZOOmA ZOOmA
max
17

Excessive capacitance between the bias supply and the bridge case
should be avoided, aS it may affect the LOSS BALANCE setting.
Isolation of one or the other from true earth is advisable.
CAUTION: To avoid high transient voltages short circuit Lx before
switching off the bias supply.
lIMIT" "
lOtO...
-~CJ
IIMX ~J20'"
40... - 5OOY
.
IIA'.
"-Y I
-~
. m"
Fig. 2.9 Connection to D. C. Bias, Method 2
(2) The method shown in Fig. 2. 9 allows up to 40 mA to be passed
through the test component on any range.
The inclusion of an isolating capacitor, CI, in series with the a. c.
source ensures that alI the bias current flows through the test comp-
onent. The bias current also flows through the 'BALANCE resistors
so thatalteration oftheir value during balancing will vary the current.
A large value limiting resistor, RL' included in series with. the supply
will keep these variations to a minimum; it should be mounted close
to the DET +VE terminaI to avoid interference pick up. The wattage
rating of the BALANCE resistors limits the maximum permissible
bias current on any range to 40 mA.
Since the detector is not frequency selective it is very important to
keep the stray frequencies introduced to the bridge to an absolute
minimum, so that the balance indication is not obscured. The bias
supply, therefore, should be hum free; the bridge and the leadstcon-
nected to it positioned to minimize pick up. The hum leve l may be
checked by inserting an open circuited jack plug into the EXT A. C.
socket; the meter indication should be very low.
CAUTION: To avoid high transient voltages short circuit Lx before
switching off the bias supply.
18

..
.,. .
... .,.. 'AS'"
"-',
S«N~ O$.~=3-". ~ ti
-,. ...,
- ..,.
Fig. 2.10 Coxmection a£ D. C. Bias, Method 3
(3) The method shown in Fig. 2.10 allows any required current to be
passed through the component under test, Lx, limited only by Rs' .Because the bias supply is coxmected directly across Lx its source
resistance will appear as an increased shunt loss. The series
resistor, Rs. must therefore, be included to keep the shunt loss
within the range covered by the LOSS BALANCE control, otherwise
it may not be possible to balance the bridge. The value of Rs should
be at least 1000fl per l H of test component value.
Isolating capacitor, CI, connected to a jack plug in the BIAS socket
prevents the bias current flowing through any other of the bridge arms.
The bias supply, o£ 500 V maximum, should be hwn free and a cap-
acitor, C.' should be included to by-pass any pick up. Check £or
excessive hwn and noise pick up, by inserting an open circuit jack
plug into the EXT A.C. socket, the meter indication at the sensitivity
in use should be negligibly small.
CAUTION: To avoid high transient voltages short circuit Lx before
switching off the bias supply.
(4) This method, showninFig. 2.11, uses the D.C. ChokeAdaptor,
TM 6113, available as an optional accessory. This Adaptor is
designed for use at l kc/ s only and enables currents up to 200 mA
d. c. to be passed through inductors in the range 100 mH to 100 H.
It employs two tuned circuits to isolate the bridge from the d.c.
source. These provide a high impedance to the a. c. signal and are
capable of carrying the bias current without change to their tuning.
Full operational procedure is given in the separate handbook supplied
with the adaptor. See alBo the note at the end of section 3.3.
2700 (la) 19

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, --- - ,
I I
1-
Il..
"""
I
I
I
I
:
I
I .-
' ~
"'H
Fig. 2. Il U se of D. C. Choke Adaptor
2.6 RESISTANCEMEASUREMENTS
Resistance may be measured at 1 kc/ s from the internaI oscillator,
at frequencies of 20 c/s to 20 kc/s from an external source, or at d.c. from
the internaI battery or an external d. c. supply. Fig. 2.12 shows the basic
bridge configuration for measurements with (a) a. c. and (b) d. c. For
resistance measurements the LOSS BALANCE arm is replaced by a 100 (2
resistor, andfor d.c. measurements the meter only is .used as the detector.
D£TECTIM
I. I. l.
~~ !IT!-
I_M +T D.c.S_T
IV'Wt
J-1"
"...
(a) A.c. (b) D.C.
Fig. 2.12 Basic Resistance Bridges
20