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Substitution Methods
In many ac bridges, the unknown
is
connected in
series or in parallel with the main adjustable com-
ponent, and balances are made before and after the
unknown i s connected. The magnitude of the unknown
then equals the change made in the adjustable compo-
nent, since the total impedance of the unknown arm
remains constant. The chief advantage of this substi-
tution technique i s that its accuracy depends only on
the calibration of the adjustable arm and not on the
other bridge arms (as long a s they are constant). The
substitution principle can a l s o be used to advantage
with any bridge if the balances are made with an exter-
nal, calibrated, adjustable component.
5.1.5 BRIDGES WITH ACTIVE ELEMENTS.
If a potentiometer-amplifier combination is con-
nected a s a bridge element, fixed capacitance and
conductance
standards can be used, with current
adjusted by variation of voltage rather than of imped-
ance magnitude.
The principle i s used in the GR
Type 1633 Incremental-Inductance Bridge, which can
accurately measure nonlinear elements.
5.1.6 THE TRANSFORMER RATIO-ARM BRIDGE.
Transformer ratio arms, introduced almost a
century ago, have recently come into considerable
favor because of certain outstanding advantages. Ratio
accuracies of a few parts per million are possible,
even for transformer-ratios of up to 1000 t o 1, and the
ratio is virtually unaffected by age, temperature, and
voltage.
Figure 5-6 shows a transformer bridge in ele-
mentary form.
The balance condition for capacitance
i s
C~
N~
Figure
5 -6.
A capacitance
bridge with transformer ratio
arms.
Figure 5-6 also explains the exceptional ability
of the transformer bridge to make three-terminal meas-
urements without the use of a guard circuit or auxiliary
balance.
Capacitances from the H terminals appear
across the low-impedance transformer winding, while
those from the L terminals are across the detector,
where they do no enter the balance expression.
These
capacitances are thus excluded from the measurement
of direct capacitance, C,, between H and L terminals.
Because this type of bridge can tolerate relatively
large capacitances from both s i d e s of the unknown to
the guard point, long cables with guard shields can be
used for remote measurement, and circuit capacitances
can often be measured in situ.
Conventional bridges can also be adapted for
three-terminal measurements (although they generally
cannot tolerate a s low an impedance to guard). On the
GR Types 1650 and 1608 Impedance Bridges, any stray
capacitance is in parallel with a standard capacitor of
a t least 0.1 p F and usually has negligible effect.
A
Wagner-type guard circuit (GR Type 716-P4) is avail-
able for u s e with the GR Type 716 Capacitance Bridge.
On the Type 1605 Impedance Comparator an electronic
amplifier provides a guard point.
5.1.7 LIMIT BRIDGES AND COMPARATORS.
In limit bridges, the unbalance voltage of the
bridge actuates a meter, which indicates the degree
of deviation of one impedance from another.
The GR
Type 1652 Resistance Limit Bridge, which includes
an adjustable standard resistor, can limit-test resistors
over a wide range. The Type 1605 Impedance Compar-
ator indicates the magnitude and phase differences
between the unknown and an external standard.
On
this instrument, the availability of several sensitive
ranges enables the user to measure small differences
very accurately. For instance, the nominal *3% accu-
racy of the comparator is translated into an actual
measurement accuracy of +0.009% on the *0.3% full-
scale range if suitable standards are used.
5.1.8 THE AUTOMATIC BRIDGE.
The ultimate in convenience i s a bridge that
balances itself. The GR Type 1680 Automatic Capaci-
tance Bridge
fully automates the balance procedure
-
selecting range, balancing, and indicating both
capacitance and loss in digital in-line form.
The implications of such automatic measurement
are far-reaching.
The conversion of bridge-measured
data into digital and binary-coded form (the Type 1680
has a binary-coded decimal output) gives the bridge
a c c e s s t o the whole modern arsenal of data-processing
equipment
-
printers, tape-punchers,
sorters, etc.
Speed is one obvious byproduct of automatic equip-
P R I N C I P L E S O F O P E R A T I O N
5-3
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