The Wheatstone Bridge pressure transducer

A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance.  The electrical resistance in most medically relevant case is of a strain gauge, which is coupled to a measured parameter (eg. arterial or venous blood pressure). The coupling is usually via some sort of incompressible fluid-filled tubing. Typically, the circuit takes the shape of a cicular arrangement of resistors where the resistance of all but one is known. The remaining "unknown" resistor acts as the strain gauge. As pressure on the gauge changes, so does its resistance, and this causes a change in the current which flows though the Wheatstone bridge. The magnitude of this pressure can be inferred from the magnitude of the changes in current.

In summary:

• Wheatstone bridge = electrical circuit with one unknown resistor
• Used to measure an unknown electrical resistance.
• Resistance of the unknown resistor is determined by pressure
• Thus, the resistor becomes a pressure gauge
• This pressure gauge is coupled to the fluid-filled compartment of interest via some sort of incompressible fluid-filled tubing.

The Wheatstone Bridge pressure transducer in a little detail

The was a visual aid question - Question 6.1 from the second paper of 2009.

Here is a nice picture from Wikipedia:

The resistance of all the resistors is known, except for Resistor X.

V_G is a galvanometer which measures the current flowing between D and B.

Essentially, if the ratio of resistance in the R₁/R₂ limb is the same as the resistance of the R₃/R_x limb, there should be no current flowing through that galvanometer.

So, you can adjust the resistance of R₂ until the current drops to zero (which is when the resistance of R₂ is the same as the resistance of R_x).

Alternatively, you can calculate what the R_x resistance is using Kirchhoff's circuit laws, which is what ends up happening in the common hospital-grade blood pressure monitor.