1-23
then we multiply both sides of the expression by Rx to separate it:
For example, in figure 1-5, we know that R1 is 60 ohms, R2 is 100 ohms, and R3 is 200 ohms. To
find the value of R
x, we can use our formula as follows:
Use of ac Bridges
A wide variety of ac bridge circuits (such as the Wheatstone) may be used for the precision
measurement of ac resistance, capacitance, and inductance. Let’s look at ac bridges in terms of functions
they perform.
RESISTANCE BRIDGE.—An ac signal generator, as shown in figure 1-6, is used as the source of
voltage. Current from the generator passes through resistors R1 and R2, which are known as the ratio
arms, and through Rs and Rx. Again, Rx is known as resistance. Rs has a standard value and replaces R3 in
figure 1-6. When the voltage drops across R2 and Rs are equal, the voltage drops across R2 and Rx are
also equal; no difference of potential exists across the meter and no current flows through it. As we
discovered with the Wheatstone bridge, when no voltage appears across the meter, the following ratio is
true:
Figure 1-6.—Resistance bridge (ac).
For example, if in figure 1-6 we know that R1 is 20 ohms, R2 is 40 ohms, and Rs is 60 ohms, we can
find the value of Rx using our formula as follows:
