Either way round

The idea

An incandescent lamp glows whichever of its terminals is connected to the positive terminal of the cell, as long as the circuit is complete and current flows through the filament.

Swap the wires

A cell has a + and a – terminal. An incandescent lamp has a metal case and a metal tip. Suppose the + wire is on the case.

What do you think happens if you swap the wires, so + goes to the tip instead?

• The lamp glows just the same
• The lamp stays dark
• The lamp glows dimmer

Does the order matter?

The lamp is already joined. Use the swap button. Watch where the + and – tags end up. Then try the fused lamp.

No front or back

An incandescent lamp glows whichever of its terminals is connected to the positive terminal of the cell, as long as the circuit is complete and current flows through the filament.

A lamp that does not glow in a complete circuit is usually fused: its filament is broken. A broken filament stops the current.

• + wire on | Lamp
• the metal case | glows
• the metal tip | glows

Notes

An incandescent lamp glows either way round, as long as the circuit is complete and current flows through the filament. A fused lamp has a broken filament and stays dark.

Check yourself

The + wire is joined to the lamp's tip and the – wire to its case. The circuit is complete. Does the lamp glow?

A lamp does not glow although the circuit looks complete. What is the likely reason?

An incandescent lamp glows as long as...

A fused lamp is put in the circuit and the wires are swapped. What happens?

• Yes — correct. Yes! An incandescent lamp glows either way round.
• No, the wires are the wrong way round. For an incandescent lamp the order does not matter.
• Only a little. It glows normally. The order of the wires does not change that.
• The + wire is on the wrong terminal. Either terminal works for an incandescent lamp.
• The filament is broken (the lamp is fused) — correct. Yes! A broken filament stops the current.
• The wires are too short. Short wires still carry current.
• + is joined to the metal case. It glows with + on the tip, too.
• the circuit is complete and current flows through the filament — correct. Yes! That is all it needs.
• + is joined to the metal tip. It glows with + on the case, too.
• It glows because the wires are swapped. Swapping wires cannot mend a broken filament.
• It stays dark, because a broken filament stops the current — correct. Yes! The break stops the current either way round.
• It glows a little. No current can pass through a broken filament.