= Galvanometer null detection
In a <Wheatstone bridge>, the <galvanometer> connects the two comparison junctions. Its purpose is to locate a <null measurement>, rather than to measure the branch <electric current> through the unknown <resistor>. Using <Wheatstone bridge detector loading> and <Thévenin's theorem>, a settled <moving-coil galvanometer> with winding <electrical resistance> $R_g$ obeys
$$
I_g=\frac{V_{\mathrm{oc}}}{R_{\mathrm{th}}+R_g},\qquad \theta=S_I\frac{V_{\mathrm{oc}}}{R_{\mathrm{th}}+R_g}.
$$
Here $V_{\mathrm{oc}}$ is the bridge's <open-circuit voltage> and $R_{\mathrm{th}}$ its <Thévenin resistance>. For a functioning passive detector with finite positive <electrical resistances>, exact $I_g=0$ implies $V_{\mathrm{oc}}=0$, preserving <Wheatstone bridge balance>. Away from balance, the detector loads the junctions; its <electrical resistance> cannot be ignored.
For the illustrative $R_g=100\,\Omega$, $S_I=3000\,\mathrm{rad/A}$ and a chosen $R_{\mathrm{th}}=1900\,\Omega$, a $1.00\,\mathrm{mV}$ bridge <open-circuit voltage> produces $0.500\,\mathrm{\mu A}$ and $0.0859^\circ$. That lies inside the stipulated $0.10^\circ$ zero tolerance. The corresponding resolvable <open-circuit voltage> scale is about $1.16\,\mathrm{mV}$, not arbitrarily small. Actual limits also depend on the zero stability and calibration.
Use the detector checks in the <slide-wire resistance bridge experiment>: perturb the comparison to either side of the proposed balance and require opposite signs, while confirming the supply is on. A broken connection, a jammed pointer or an inactive supply can also give a zero reading. Conversely, a healthy bridge detector reads zero while the bridge arms still carry <electric current>. A zero deflection alone proves neither a broken conducting path nor infinite sensitivity.
Back to article page
