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Because R1 and R2 are expressed in the same units, the equation R1/R2 becomes a simple multiplication factor. This equation provides a numerical value for Cx and will be in the same units as C s (farad, microfarad, and so forth).

Similarly, the following resistance ratio exists between the four arms of the bridge, just as in the resistance bridge expression discussed earlier:

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Thus, both the unknown resistance and capacitance, R x and Cx, can be estimated in terms of known resistance R1, R2, Rs, and known capacitance C s.

In figure 1-7, for example, we know that R1 is 20 ohms, R2 is 40 ohms, Rs is 60 ohms, and C s is 10 microfarads. We can find the values of Cx and Rx by using the respective formulas as follows:


Q.19 When a bridge is used to measure resistance, what is the value of Rx if R1 equals 80 ohms, R2 equals 120 ohms, and R3 equals 280 ohms?

INDUCTANCE BRIDGE. - The value of the unknown inductance Lx may be determined by means of the simple bridge circuit shown in figure 1-8. Ratio arms R1 and R2 are accurately calibrated resistors. Ls is a standard inductor with a known inductance; Rs is the known resistance, and Rx represents the resistance of the unknown inductor.

Figure 1-8. - Inductance bridge.

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The ac signal is applied to the bridge, and variable resistors R1 and R2 are adjusted for a minimum or zero deflection of the meter, indicating a condition of balance. When the bridge is balanced, the following formulas may be used to find Lx.

(NOTE: The right side of this expression is NOT inverse as it was in the capacitance bridge.)



In figure 1-8, for example, the values of R1, R2, and Rs are 20, 40, and 60 ohms, respectively. The value of Ls is 10 millihenries. We can find the values of Rx and Lx by using their respective formulas as follows:


Thus, both the unknown resistance and inductance can be estimated in terms of the known values for R1, R2, Rs, and L s.

Q.20 When an unknown capacitance is tested with a bridge, what is the value of Cx if R1 equals 70 ohms, R2 equals 150 ohms, and Cs equals 550 microfarads? answer.gif (214 bytes)

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