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Owens Bridge

This bridge is used to measure the inductance of a coil in terms of resistance and capacitance. C1 and C2 are standard capacitors. R1 and R3 are variable resistances. R4 is a fixed resistance. The coil whose self-inductance L is to be determined is connected in the arm AD in series with R3.

Let the impedances in the four arms of the bridge be Z1, Z2, Z3, and Z4 at the balance points.
    Z1 = R1 + 1/jC1ω; Z2 = 1/jC2ω; Z3 = R3 + JLω; Z4 = R4

When the bridge is balanced, we have
    Z1/Z2 = Z3/Z4
Or    Z1Z4 = Z2Z3
    (R1 + I/jC1ω)R4 = I/jC2ω - (R3 + jLω)
    R1R4 + R4/jC1ω = R3/jC2ω + L/C2

Equating the imaginary Parts, we have

    R4/R1 = R3/C2 or R3/R4 = C2/C1                            … (1)

Equating the real parts, R1R4 = L/C2
. :            L = R1R4C2                             … (2)

Thus we have two balances conditions which are independent of one another. The condition (1) is satisfied by varying R3 and (2) by varying R1 till the sound is minimum in both the cases.

Owens Bridge

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