A battery of emf 12V and internal resistance 2 ohm is connected to a 4 ohm resistor as shown in the figure.
a) Show that a voltmeter when placed across the cell and across the resistor, in turn, gives the same reading.
b) To record the voltage and the current in the circuit, why is voltmeter placed in parallel and ammeter in series in the circuit?
a) Emf, E = 12 V
Internal resistance, r = 2V
Now, using the formula,
E = V + Ir
When the voltmeter is connected across the cell,
I = 
V1 = 12 - 2(2) = 8 V
When the voltmeter is connected across the resistor,
V2 = IR
= 2 x 4 = 8 V
That is, V1 = V2
Hence proved.
b) Voltmeter has very high resistance to ensure that it is connection does not alter the flow of current in the circuit. Current chooses the low resistance path. Therefore, voltmeter is connected in parallel to the load across which potential difference is to be measured.
Ammeter measures the value of current flowing through the circuit. Ammeter has a very low value of resistance to ensure that all the current flows through it. Hence, it should be connected in series.
