Now, carefully look at the arrangements in which the lamp glows. Compare these with those in which the lamp does not glow. Can you find the reason for the difference?
Answer
Yes. In the arrangements where the lamp glows, the connections form a complete, unbroken loop; in the others the loop is broken somewhere.
A closed loop: current leaves the + terminal, passes through the lamp, and returns to the – terminal. Break the loop anywhere and the lamp goes out.
The difference, stated as a rule:
Lamp glows — one terminal of the lamp is joined to one terminal of the cell, and the other terminal of the lamp to the other terminal of the cell.
Lamp does not glow — either a joint is left open (S.No. 2, 3), or both lamp terminals are joined to the same cell terminal (S.No. 4, 5).
An arrangement of the first kind is called an electrical circuit. It provides a complete path for electric current to flow through the lamp, and the lamp glows only when current passes through the circuit. By agreement, the direction of the current is taken to be from the positive terminal to the negative terminal of the cell.
Did you know? Sometimes a lamp does not glow even in a correct circuit. We then say the lamp has 'fused' — its filament has broken, so the path inside the bulb itself is open.