Maxwell's bridge
A Maxwell bridge (in long form, a Maxwell-Wien bridge) is a type of Wheatstone bridge used to measure an unknown inductance (usually of low Q value) in terms of calibrated resistance and capacitance. It is a real product bridge.
It uses the principle that the positive phase angle of an inductive impedance can be compensated by the negative phase angle of a capacitive impedance when put in the opposite arm and the circuit is at resonance; i.e., no potential difference across the detector and hence no current flowing through it. The unknown inductance then becomes known in terms of this capacitance.
With reference to the picture, in a typical application and are known fixed entities, and and are known variable entities. and are adjusted until the bridge is balanced.
and can then be calculated based on the values of the other components:
The additional complexity of using a Maxwell bridge over simpler bridge types is warranted in circumstances where either the mutual inductance between the load and the known bridge entities, or stray electromagnetic interference, distorts the measurement results. The capacitive reactance in the bridge will exactly oppose the inductive reactance of the load when the bridge is balanced, allowing the load's resistance and reactance to be reliably determined.
The Anderson Bridge
The Anderson Bridge is a very important and useful modification of the Maxwell-Wein bridge as
shown in the fig 6.1 (a)
The balance condition for this bridge can be easily obtained by converting the mesh impedance C,R3,R5 shown in the fig 6.1 (a)
to a equivalent star with the star point 0 as shown in fig 6.1 (b) by using star/delta transformation
As per delta to star transformation
And therefore
This method is capable of precise measurement of inductance and a widerange of values from a few µH
to several Henry.
to several Henry.