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Boojho made the circuit given in the figure and observed that the bulb did not glow. On Paheli’s suggestion, he added one more cell in the circuit. The bulb now glows. Explain.
 
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Initially, the bulb did not glow because the required to make the bulb glow was insufficient. With the addition of another cell, the rises, indicating that the current flowing through the bulb reaches a certain level, and the bulb starts to shine as it receives enough energy to its filament.