Halliday, Resnick & Walker — Circuits: Problem 7

7 In Fig. 27-22, a voltmeter of resistance $R_V = 300\ \Omega$ and an ammeter of resistance $R_A = 3.00\ \Omega$ are being used to measure a resistance $R$ in a circuit that also contains a resistance $R_0 = 100\ \Omega$ and an ideal battery with an emf of $\mathcal{E} = 18.0\ \text{V}$. Resistance $R$ is given by $R = V/i$, where $V$ is the potential across $R$ and $i$ is the ammeter reading. The voltmeter reading is $V'$, which is $V$ plus the potential difference across the ammeter. Thus, the ratio of the two meter readings is not $R$ but only an apparent resistance $R' = V'/i$. If $R = 85.0\ \Omega$, what are (a) the ammeter reading, (b) the voltmeter reading, and (c) $R'$? (d) If $R_A$ is decreased, does the difference between $R'$ and $R$ increase, decrease, or remain the same?