3. Four identical cells cach ofe. m.f. 1.5 volts with internal resistance are connected to form a banery of 6.0V. The battery is then joined in series with an ammeter of negligible resistance and fixed external resistance of 12 ohms. The ammeter then reads 0.3A. (a) Draw diagram of the circuit. (b) Calculate () the internal resistance of the battery, and (1) the value of the resistor which must be joined in parallel with the 12-ohm resistor to increase the ammeter reading to 0.5A​