Problem 30.7: Wheatstone bridge

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The animation shows a "Wheatstone bridge," which is used to measure unknown resistors. Restart. Assume an ideal battery (no internal resistance) and ideal meters (resistance is given in ohms, electric potential is given in volts, and current is given in milliamperes). Begin the animation (with unknown resistor A) to read the current on the ammeter. In a Wheatstone bridge you adjust the variable resistor until the ammeter reads 0 and then you can calculate the value of the unknown resistor.

  1. What is the unknown resistor in this case?
  2. Develop an algebraic expression for the unknown resistance as a function of R1, R2, and the variable resistor.
  3. Use your expression to quickly calculate unknown resistor B and unknown resistor C.

Problem authored by Anne J. Cox.