Q.A voltmeter has a resistance of 30 \Omega. What will be its reading, when it is connected across a cell of emf 2 V having internal resistance 10 \Omega?

Converting a galvanometer into a voltmeter -- a potential-difference-measuring instrument connected in PARALLEL with the circuit element being measured -- needs its voltage range raised, its effective resistance INCREASED (opposite to the ammeter conversion; an ideal voltmeter has infinite resistance, so it draws negligible current and does not disturb the potential difference it measures), and its coil protected from an excessive applied voltage. All three are achieved by connecting a HIGH resistance X in SERIES with the galvanometer.

If V is the voltage to be measured at full-scale deflection, it splits across the coil and the added series resistance, $V=I_gX+I_gG$, giving $X=(V/I_g)-G$ as the resistance that must be added. A larger series resistance gives a larger measurable voltage range; the voltmeter's own net resistance, $R_V=G+X=G\,n_V$ (where $n_V=V/(I_gG)$ is the range-multiplying factor), is correspondingly large, closer to the ideal infinite-resistance voltmeter.

The SAME base galvanometer can be converted into an ammeter (parallel, low shunt) or a voltmeter (series, high resistance) purely by choosing which of these two opposite modifications to apply, which is why an ammeter and a voltmeter -- despite measuring entirely different quantities and connecting into a circuit in opposite ways -- are built from the identical underlying instrument.