This is an AC bridge. This Bridge uses mutual inductance to measure self-inductance over a wide range. Heaviside Campbell Equal Ratio Bridge employees a standard variable mutual inductance for the purpose. We connect the primary of a mutual inductometer in series with the supply. Now let us consider the secondary of the mutual inductometer has a self-inductance L2 and the series resistance R2. In the figure below, the secondary portion of the inductometer forms one arm of the bridge circuit. Say it is AB. We connect the coil of unknown inductance and resistance across the arm BC of the bridge. In the other two arms, we will use only resistance. This resistances are R4 and R3 respectively.
Now, we connect a detector between node A and C. We obtain the balance condition of the Heaviside Campbell Equal Ratio Bridge by adjusting the mutual inductance of the inductometer.
Theory of Heaviside Campbell Equal Ratio Bridge
Let us consider at balanced condition there will be a supply current i ampere. The bridge circuit divides the supply current into two components i1 and i2. The current i1 flows through arms BC and CD of the bridge. The current i2 flows through the arms BA and AD. Hence, the voltage drop across arm AD is i2R4 and the same for arm CD is i1R3. At balanced condition the potential of node A and C equal. For satisfying this condition we can write.
For the same reason, we can write also
Now, i = i1 + i2, so we can write
By dividing (ii) by (i) we get,
Now, by considering real and imaginary parts of the equation (iii) separately, we get
If we make,
Then, we can write
Modified Version of Heaviside Campbell Equal Ratio Bridge
Campbell further modified this bridge. He introduces one balancing coil in series with the coil under measurement in the arm BC. Also, He inserted one resistance box in series with the secondary of inductometer in arm AB. Then he obtained the balance of the bridge by varying the mutual inductance M and resistance r of the resistance box.
Here we have to find out the inductance and resistance of the coil under measurement i.e. L1 and R1.
In this modified version of the bridge, we have to take the reading twice. That means we have to obtain the balanced condition of the bridge twice.
Once we do this by inserting the coil under measurement in the circuit.
Then we do this by replacing the coil under measurement by a short circuit.
Suppose M1 and r1 are the value of mutual inductance and resistance of the resistance box for balancing the bridge circuit with the coil under measurement. On the other hand, M2 and r2 are the mutual inductance and resistance of the resistance box for balancing the bridge circuit with short-circuiting the coil under measurement. Then we can write,
By modifying the Heaviside Campbell Equal Ratio Bridge, Campbell was able to eliminate the lead resistance and inductance the coil under measurement.
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