A Wheatstone bridge is a type of "bridge circuit" used to make measurements of resistance. The unknown resistance to be measured, Rx, is placed in the circuit with accurately known resistances R1, R2, and R3 (Fig. 19-76). One of these, R3, is a variable resistor which is adjusted so that when the switch is closed momentarily, the ammeter a shows zero current flow. The bridge is then said to be balanced.
(a) Determine Rx in terms of R1, R2,
(b) If a Wheatstone bridge is "balanced" when R1 = 590 Ω, R2 = 972 Ω, and R3 = 78.6 Ω, what is the value of the unknown resistance?

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B Rx R3! A Ammeter A `R2 RI D