A coil in the shape of an equilateral triangle of side $0.02 \mathrm{m}$ is suspended from a vertex such that it is hanging in a vertical plane between the pole pieces of permanent magnet producing a horizontal magnetic field $5\times {10}^{-2}$ tesla. Find the couple acting on the coil when a current of $0.1 A$ is passed through it and the magnetic field is parallel to its plane.

In the circuit shown in Figure, a voltmeter reads $30 V$, when it is connected across $400 \mathrm{\Omega }$ resistance. Calculate what the same voltmeter will read, when connected across the $300 \mathrm{\Omega }$ resistance.

A voltmeter of resistance $R_V$ and an ammeter of resistance $R_A$ are connected in a circuit to measure, a resistance R as shown in Figure. The ratio of the meter readings gives an apparent resistance $R\text{'}$. Show that $R$ and $R\text{'}$ are related by the relation

$\frac{1}{R}=\frac{1}{R\text{'}}=\frac{1}{R_V}$.