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
    • and are known and fixed
    • is known and adjustable
    • is unknown
  • Galvanometer

The Wheatstone bridge relies on the ratio of resistances between the known () and unknown () legs:

  • When ) 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 :
  • Very small changes in lead to a current flow across the bridge
  • can then be adjusted until the bridge is balanced, and the value of calculated

References

  1. Alfred Anaesthetic Department Primary Exam Tutorial Series
Last updated 2017-10-03

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