[填空题]
A Wheatstone bridge is a type of “br6t/wovhjk+qgg -31 za3f5bfdidge circu6.s vnr gzc:n 4i+eqyjn g.w0.it” used to make measurements of resistance.g q nv6yizeg r4. jn.scn+:.0w The unknown resistance to be measured, $R_x$, is placed in the circuit with accurately known resistances $R_1$ ,$R_2$, and $R_3$(Fig. Below). One of these, $R_3$, is a variable resistor which is adjusted so that when the switch is closed momentarily, the ammeter Ann Meter shows zero current flow.
(a) Determine $R_x$ in terms of $R_1$ ,$R_2$, and $R_3$.
(b) If a Wheatstone bridge is “balanced” when $R_1\,=\,630\Omega$,$R_2\,=\,972\Omega$, and $R_3\,=\,42.6\Omega$, what is the value of the unknown resistance? $\Omega$
