Maxwell bridge

A Maxwell 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.

With reference to the picture, in a typical application R₁ and R₄ are known fixed entities, and R₂ and C₂ are known variable entities. R₂ and C₂ are adjusted until the bridge is balanced.

R₃ and L₃ can then be calculated based on the values of the other components:

$R_3 = {R_1 \cdot R_4 \over R_2 }$

$L_3 = R_1 \cdot R_4 \cdot C_2$

To avoid the difficulties associated with determining the precise value of a variable capacitance, sometimes a fixed-value capacitor will be installed and more than one resistor will be made variable.

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.

See also

• Wheatstone bridge