what is hay's bridge ? how it measures inductance?

Question

it is used for measurement of inductance.

Answer 1

One device used to accurately measure inductance (either AC or DC) in an existing circuit is called a HAY BRIDGE.

It is a four-arm alternating-current bridge used to measure inductance in terms of capacitance, resistance, and frequency; bridge balance depends on frequency.

The "meter" is connected between points 1 and 3 of the bridge.

In the upper pair of arms, there is a resistive element (RA) between points 1 and 2. There is an inductive element (LX) in series with a frequency element (FX) representing the actual circuit to be measured located between points 2 and 3.

In the lower pair of arms, there is a resistive element (RB) in series with a capacitive element (CB) located between points 1 and 4. The resistive element (RS) (found between points 3 and 4) is actually variable, and is used to "tune" the bridge and bring it to a balanced position.

The A and B branches (effectively connecting points 2-1-4) are referred to as the comparative circuit. The X branch (points 2-3) is referred to as the detector circuit. And finally, the S branch (points 3-4) is referred to as the control circuit.

Further information:
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.

If you want more on bridges ... see the link.

Answer 2

HAY BRIDGE Inductance measurements are seldom required in the course of troubleshooting. However, in some cases inductance measurements are useful and instruments are available for making this test. Many capacitance test sets can be used to measure inductance. Most manufacturers of capacitance test sets furnish inductance conversion charts if the test equipment scale is not calibrated to read the value of inductance directly. For the measurement of inductance, the following basic types of test equipment circuitry are used: (1) the bridge-circuit type, which is the most accurate, and (2) the reactance type, which is often an additional test circuit incorporated into another piece of test equipment to increase its utility. The measurement of capacitance using the capacitance-inductance-resistance bridge instrument was discussed. Since the measurement of capacitance and inductance are interrelated, the existing capacitance standards and loss controls of this test equipment are used whenever possible. A wider range of dissipation must be provided to accommodate the practical value of inductors. The 250DE+1325 , a typical rcl bridge and our reference in this discussion, uses two basic bridge circuits (Hay bridge and Maxwell bridge) to accommodate the extensive range in inductor loss factors. You should take time to review the bridges in NEETS, module 16, or other bridge-circuit descriptions before continuing.