3-3
The comparing circuit contains branches A and B and has provisions for changing the ratios of the
branches with respect to each other, which enables various measuring ranges to be obtained. Comparison
of figures 3-1 and 3-2 shows that either or both branches of the comparing circuit do not necessarily
contain resistors alone. Branch B of the Hay bridge, containing CB and RB in series connection, provides a
striking contrast with the parallel connection of CB and RB of the Maxwell bridge.
The measuring circuit in figure 3-2 also contains two branches. The resistance, capacitance, or
inductance to be measured is connected to branch X of the bridge-measuring circuit. The subscript X is
also used in figure 3-1 to designate the circuit parameters involved in computing the values of various
electronic parts. Branch S contains the variable control used to bring the bridge into a balanced condition.
A potentiometer is used for this purpose in most bridge equipment, because it offers a wide range of
smoothly variable current changes within the measuring circuit.
The third arm of the bridge is the detector circuit. The detector circuit may use a galvanometer for
sensitive measurements that require high accuracy. In the case of bridges using ac as the power source,
the galvanometer must be adapted for use in an ac circuit. In many practical bridge circuits using ac to
operate the bridge, an electron-ray indicating tube is used to indicate the balanced condition by opening
and closing the shadow area of the tube. Headsets are also used for audible balance detection, but this
method reduces the accuracy obtainable with the bridge.
Switches are used in bridge circuits to control the application of operating power to the bridge and to
complete the detector circuit. Frequently, the two switching functions are combined into a single key,
called a bridge key, so that the operating power is applied to the bridge prior to the detector circuit. This
sequence reduces the effects of inductance and capacitance during the process of measurement.
The most unfavorable condition for making a measurement occurs when the resistance, capacitance,
or inductance to be measured is completely unknown. In these cases, the galvanometer cannot be
protected by setting the bridge arms for approximate balance. To reduce the possibility of damage to the
galvanometer, you should use an adjustable shunt circuit across the meter terminals. As the bridge is
brought closer to the balanced condition, the resistance of the shunt can be increased; when the bridge is
in balance, the meter shunt can be removed to obtain maximum detector sensitivity.
Bridges designed specifically for capacitance measurements provide a dc source of potential for
electrolytic capacitors. The electrolytic capacitors often require the application of dc polarizing voltages
in order for them to exhibit the same capacitance values and dissipation factors that would be obtained in
actual circuit operation. The dc power supply and meter circuits used for this purpose are connected so
that there is no interference with the normal operation of the capacitance-measuring bridge circuit. The
dissipation factor of the capacitor may be obtained while the capacitor is polarized. In figure 3-2, the
signal voltage in the A and B branches of the bridge will be divided in proportion to the resistance ratios
of its component members, RA and RB, for the range of values selected. The same signal voltage is
impressed across the branches S and X of the bridge. The variable control, RS, is rotated to change the
current flowing through the S and X branches of the bridge. When the voltage drop across branch S is
equal to the voltage drop across branch A, the voltage drop across branch X is equal to the voltage drop
across branch B. At this time the potentials across the detector circuit are the same, resulting in no current
flow through the detector circuit and an indication of zero-current flow. The bridge is balanced at these
settings of its operating controls, and they cannot be placed at any other setting and still maintain this
balanced condition.
The ability of the bridge circuit to detect a balanced condition is not impaired by the length or the
leads connecting the bridge to the electronic part to be measured. However, the accuracy of the
measurement is not always acceptable, because the connecting leads exhibit capacitive and inductive
