(a)[3]
Describe the structure of a metal wire strain gauge. If you wish, you may include a diagram.
(b)[1]
A strain gauge $S$ is included in the circuit of Fig. 9.1. The operational amplifier (op-amp) is ideal. The circuit output potential $V_{\text{OUT}}$ is described by $V_{\text{OUT}} = \frac{R_F}{R} \times (V_2 - V_1)$. (i) State the name given to the ratio $\frac{R_F}{R}$.
(b(ii))[3]
The resistance of the strain gauge $S$ is $125\,\Omega$ when it is not strained. Calculate the magnitude of $V_1$ so that, when strain gauge $S$ is unstrained, the output $V_{\text{OUT}}$ is zero.
(b(iii))[2]
During one test, the resistance of $S$ rises to $128\,\Omega$. $V_1$ remains unchanged. The ratio $\frac{R_F}{R}$ is $12$. Calculate the magnitude of $V_{\text{OUT}}$.