Chapter 26, Problem 26.74P

The Wheatstone Bridge. The circuit shown in Fig. P26.74, called a Wheatstone bridge, is used to determine the value of an unknown resistor X by comparison with three resistors M, N, and P whose resistances can be varied. For each setting, the resistance of each resistor is precisely known. With switches S₁ and S₂ closed, these resistors are varied until the current in the galvanometer G is zero; the bridge is then said to be balanced. (a) Show that under this condition the unknown resistance is given by X = MP/N. (This method permits very high precision in comparing resistors.) (b) If galvanometer G shows zero deflection when M = 850.0 Ω, N = 15.00 Ω, and P = 33.48 Ω, what is the unknown resistance X?

Figure P26.74