Middle school electricity starts with one idea: a complete circuit. All this experiment takes is a single battery, a couple of wires and a small light bulb — enough to show students what the power source, the load, the switch and the wires each do, and to make the difference between a closed circuit, an open circuit and a short circuit crystal clear.

Students find circuit problems abstract mostly because they have never seen a real circuit light up. Get the bulb glowing with their own hands first; when they go back to circuit diagrams, every symbol suddenly has a real object behind it.

Safety first (read this before you start)

This experiment uses a single 1.5 V dry cell — a safe, low voltage that cannot shock anyone under normal handling. Even so, keep these rules in mind:

  • Never connect the two battery terminals directly with a wire (without the bulb in between). That creates a short circuit: the wire and the battery heat up fast, hot enough to burn fingers or make the battery leak.
  • This experiment uses dry cells only. Never, under any circumstances, run wiring experiments from a household wall outlet.
  • Do not open, heat or burn batteries; recycle used cells according to your local rules.

Materials and equipment

  • One 1.5 V dry cell (AA is fine), ideally in a battery holder;
  • Two insulated wires with about 1 cm of copper stripped bare at each end — alligator-clip leads are even easier;
  • One small light bulb (a 1.5–2.5 V flashlight bulb) with a matching bulb holder (without a holder you can press the wires directly against the bulb);
  • Optional: a small switch; a paper clip and a piece of pencil lead for the extensions later.

Step-by-step procedure

Be clear about the goal first: current should leave the battery's positive terminal, travel through a wire and the bulb, and return to the negative terminal — one unbroken loop.

  1. Look closely at the bulb: it has two contact points, the metal tip at the bottom and the threaded metal side (or a second pin). The filament is connected between these two points.
  2. Use the first wire to connect the battery's positive terminal (the end with the bump) to one contact point of the bulb.
  3. Use the second wire to connect the bulb's other contact point to the battery's negative terminal (the flat end).
  4. When both wires make good contact, the bulb should light. If it doesn't, check that every connection is clipped tight and that bare copper — not insulation — is touching the metal.
  5. Disconnect one wire for a moment, then reconnect it, and watch the bulb go out and light up again. That is the switch between an open circuit and a closed circuit — and it is exactly how a switch works.
Add a switch: if you have a small switch, wire it in series anywhere along the loop. Close the switch and the circuit is complete — the bulb lights; open it and the circuit is broken — the bulb goes out. A switch simply gives you control over whether the loop is complete.
Interactive simulation: this experiment has a drag-and-play simulator in our Interactive Lab. The lab interface is currently Chinese-only — the controls are simple sliders and switches, so it is still easy to explore.

Recording what you see

Write down exactly what happens — a simple table works well:

  • Both wires connected → bulb: on (closed circuit);
  • Either wire disconnected → bulb: off (open circuit);
  • One wire connected straight across the battery terminals, bypassing the bulb → the wire heats up noticeably (short circuit — disconnect immediately).

Then run these three comparisons — they make the idea of a loop stick much more firmly:

  • Swap the two wires (in other words, reverse the battery) → the bulb still lights. A filament bulb has no positive or negative side; reversing it changes nothing. (An LED is different — it won't light when reversed. If you have one handy, try it.)
  • Connect only the metal tip at the bottom of the bulb, without touching the threaded side → the bulb stays dark. Current must enter through one contact and leave through the other, passing through the filament in between — both contacts are essential.
  • Bring the switch contacts slowly closer without letting them touch → nothing; the instant the metal touches, the bulb is fully on. There is no "half-lit" transition. A circuit is either complete or it isn't — an on/off state, not a gradual one.
Bulb won't light? Troubleshoot in this order: ① Is the battery dead? Swap in a fresh one. ② Are the clips gripping bare copper rather than the insulation? (This is the most common culprit.) ③ Is the filament burned out? Hold the bulb up to the light — if the thin filament inside is broken in two, the bulb is dead. ④ Is the bulb screwed firmly into its holder? ⑤ Is the bulb's rated voltage too high? A 6 V bulb on a 1.5 V battery glows so dimly you can barely tell.

The science: why the bulb lights up

A working circuit needs three things: a power source (the battery, which provides voltage), a load (the bulb, which converts electrical energy into light and heat) and wires (which connect everything and give the current a path). Usually a switch is added to control the loop.

There is a voltage between the battery's two terminals — think of it as creating a height difference between the two ends of a water pipe. When the wires and the bulb close the loop, free electrons drift in one direction under that voltage, forming an electric current. As the current passes through the bulb's very thin filament, the filament's relatively high resistance makes it heat up — so hot that it glows. That's the light.

Break the loop anywhere (an open circuit) and current stops flowing — the bulb goes dark. But if current bypasses the load and flows straight between the battery terminals (a short circuit), the resistance is tiny, the current is huge, and the wire heats up dangerously fast. That is exactly why you must never short a battery's terminals.

Four things students get wrong most often

  • "Current flows out of the battery and gets used up in the bulb." No. Current is the same everywhere in the loop — exactly as much flows out of the bulb as flows in; none of it goes missing. What gets used up is electrical energy (converted into light and heat), not current. This is the number-one misconception in all of school electricity, and many later problems fall apart because of it.
  • "A short circuit means the wire is broken." The opposite. An open circuit is a broken loop with no current; a short circuit is current bypassing the load and flowing directly between the source terminals — tiny resistance, enormous current, wires heating up and even catching fire. The two terms sound alike and mean exactly opposite things.
  • "The switch only works if it's placed before the bulb." Not so. A series circuit has only one path, so a switch breaks the loop equally well anywhere along it. Household wiring puts switches on the live wire for safety (so the lamp socket is dead when switched off), not because current "reaches the switch first."
  • "The battery stores current." A battery provides voltage — the ability to push charge. Current only appears once the circuit is closed. With no complete loop, no amount of voltage produces any current. That is the real reason the bulb stays dark in an open circuit.

Tips for teachers and parents

  • Timing and groups: one 40-minute period, students in pairs; have each partner build the circuit once, so nobody does it all for the group.
  • Say it before hands touch equipment: never short the battery terminals directly (they heat up); hold leads by the insulated part when clipping onto terminals.

The step most people skip. Have students draw the current path on paper before they connect anything. When a circuit doesn't light, don't hand them the answer — have them trace their own drawing section by section until they find the break. This step is worth more than getting the bulb lit.

How to tell they really understand. If a student can point to the real components and name all four parts, and explain why breaking the loop anywhere puts the light out, they've met the standard. If they can also distinguish an open circuit from a short circuit, that's excellent work.

Take it further

Once the basic circuit works, try these variations to deepen the understanding:

  • Test conductors and insulators: leave a gap in the circuit and bridge it in turn with a paper clip, a pencil lead, an eraser and a plastic ruler. Whatever lights the bulb is a conductor; whatever doesn't is an insulator.
  • Put two bulbs in series: wire two identical bulbs one after the other in the same loop and notice they glow dimmer than a single bulb — the resistance along the path has increased.
  • Add a second battery in series: connect two cells end to end (positive of one to negative of the other) for 3 V, and the bulb gets noticeably brighter. Don't exceed the bulb's rated voltage by much, or the filament will burn out.
  • Build a "continuity tester": wire the battery, bulb and two leads into a loop with a gap, then touch the bare lead ends to things around the house: a stainless-steel spoon, pencil lead, a coin, paper, a dry stick, a damp cloth… Record what lights the bulb. Students discover that all metals conduct — and that even a damp cloth makes the bulb glow faintly. That is exactly why you must never touch outlets or appliances with wet hands.
  • Draw the circuit diagram. Redraw the circuit you just built using symbols: long and short parallel lines for the battery, a circle with a cross for the bulb, a break with an angled line for the switch, straight lines for the wires. The habit of "diagram first, wires second" pays off enormously when series and parallel circuits arrive.

When students can use the words "source, load, wires, switch" and "closed, open, short circuit" to explain everything they observed, this experiment is truly done. And the next time they look at a circuit diagram, behind every symbol will be the bulb they lit themselves.