A torch bulb, a battery holder and a few lengths of wire. That is the entire kit. In half an hour you can build two circuits that behave in completely different ways, and once you have watched a bulb dim or stay lit with your own eyes, the diagrams in textbooks stop being abstract. Everything here runs on a couple of AA cells, so there is nothing to be nervous about.

The parts you need

• Battery holder with two AA cells, giving about 3 volts. Match your bulbs to this. Never use a mains adaptor or a wall socket for these experiments.
• Torch bulbs in holders. Two or three is plenty. If you would rather use LEDs, choose ones with a built-in resistor, or add a resistor in series. An LED with nothing to limit the current will burn out in seconds.
• Connecting wire, ideally pre-cut with crocodile clips at each end. Bare wire works, but clips make rewiring quick and prevent the fiddly frustration that ends most first attempts.
• A switch, optional but handy. It also demonstrates neatly that a circuit has to be complete before anything happens.
• A multimeter, if you have one. An inexpensive one is perfectly adequate here.

What is actually happening in the wire

Three ideas cover almost everything. Voltage is the push the battery provides. Current is the flow of charge that results. Resistance is anything that opposes that flow, and a bulb filament is a deliberate resistor designed to get hot and glow.

For current to flow, there must be a complete loop from one battery terminal, through the components, and back to the other terminal. Break that loop anywhere — a switch, a loose clip, a blown filament — and the current stops everywhere in that loop. That single fact explains most of what follows.

Building a series circuit

In a series circuit the components sit one after another along a single path, so the current has only one route to take.

1. Clip a wire from the battery holder's positive terminal to one side of the first bulb holder.
2. Link the other side of that holder to one side of the second holder.
3. Run a final wire from the remaining side of the second holder back to the battery's negative terminal.

Connect the battery last. Both bulbs should glow, but notice how much dimmer they are than a single bulb on the same supply. The two filaments share the available voltage, so each receives roughly half, while the same current passes through both. Add a third bulb in series and everything dims again.

Building a parallel circuit

Now give the current a choice. Each bulb gets its own path between the same two battery terminals.

1. Wire from the positive terminal to the left side of the first holder, then a second wire from that same point to the left side of the second holder.
2. Do the same on the negative side: connect the right side of both holders back to the negative terminal.

Both bulbs now light at full brightness, each behaving as though it were the only one in the circuit. Unscrew one bulb and the other keeps glowing, because the current simply takes the route that is still open.

Series versus parallel: the differences you can see

Brightness

Series divides the voltage between components, so every bulb is dimmer than it would be alone. Parallel gives each branch the full supply voltage, so brightness stays the same however many branches you add. This is why household lighting circuits are wired in parallel: one lamp switching on does not dim the rest.

What happens when something fails

In series, one broken filament stops the current for the whole loop, so every bulb goes out. That is the old string-of-lights problem. In parallel, a failed bulb only kills its own branch. The rest carry on.

How fast the battery drains

Parallel circuits draw more total current, so the cells run down sooner. Series circuits draw less, but you pay for that in dimmer light. Neither arrangement is better; they simply trade different things.

Measuring rather than guessing

Set a multimeter to DC volts and hold the probes across each bulb in turn. In a series circuit the readings add up to roughly the battery voltage. In a parallel circuit every bulb reads much the same value. That is the series-sharing and parallel-sharing rule made visible in numbers.

Measuring current is different. You have to break the circuit and place the meter in the gap so the current flows through it, then reconnect the battery. Never try to measure current by touching the probes across a bulb or a battery — that creates a short circuit and can destroy the meter.

When nothing lights up

• Check the cells are the right way round and not flat. Try a fresh pair.
• Make sure every clip is biting bare metal, not the plastic insulation.
• Swap the bulbs one at a time. A single blown filament is the most common culprit.
• For LEDs, check polarity: the long leg goes to the positive side. Reverse it and nothing happens.
• Look for a stray wire bridging the battery terminals. Everything will go dark and the cell will get warm. Disconnect it straight away.

Staying safe

Batteries only. A pair of AA cells cannot give you a dangerous shock, which is exactly why this is the right place to learn. Mains electricity is a different matter entirely: if you want to understand the wiring in your home, that work belongs to a qualified electrician.

Do not short battery terminals deliberately, do not leave a circuit running unattended, and unclip the battery when you have finished. Bulbs and holders can get hot enough to soften plastic. Coin cells are a serious swallowing hazard, so keep them and any small parts well away from very young children.

Where to take it next

Change one thing at a time and note what happens. Swap a bulb for a motor and watch how much slower it runs in series. Put a switch in parallel with one bulb in a series loop and you can short that bulb out, plunging it into darkness while the other brightens. Try three bulbs in series, then rewire them in parallel and compare. Keeping a simple notebook of what you changed and what you observed is the whole method of experimental science, practised at kitchen-table scale.

Photo: 1457888 / Pixabay