Wheatstone Bridge

The Wheatstone bridge is an electrical device used to accurately measure very small changes in electrical resistance. The Wheatstone bridge is:

• Used in many other medical devices (e.g. invasive pressure monitoring)
• A device with infinite gain
• A null deflection galvanometer
• Not an amplifier
  As it does not increase current amplitude.

Mechanism

The Wheatstone bridge consists of:

• Battery
• Four resistors
  • $R_1$ and $R_3$ are known and fixed
  • $R_2$ is known and adjustable
  • $R_4$ is unknown
• Galvanometer

The Wheatstone bridge relies on the ratio of resistances between the known ($R_2 \over R_1$) and unknown ($R_4 \over R_3$) legs:

• When ${R_2 \over R_1} = {R_4 \over R_3}$) equal current flows down either limb and there is no current flow across the galvanometer
  At this point the bridge is said to be balanced.
  • The equation can then be re-arranged to solve for $R_4$:
    $R_4 = {R_2 \over R_1}.R_3$
• Very small changes in $R_4$ lead to a current flow across the bridge
• $R_2$ can then be adjusted until the bridge is balanced, and the value of $R_4$ calculated

References

1. Alfred Anaesthetic Department Primary Exam Tutorial Series