Project 15 — The Electromagnet Crane

Text written by Claude Fable 5 (claude-fable-5).

Project 15 — The Electromagnet Crane

The one idea: electricity flowing in a coil makes magnetism — a magnet you can switch on and off with a button.

This opens the third arc (Projects 15–19). The first arc was about making a loop; the second gave the circuit an eye. This arc is about the series’ first genuinely new force — and the punchline, five projects from now, is that it runs in both directions. The parent-side companion for the whole arc is the electromagnetism explainer.

Power note — read this one. A coil of copper is nearly a short circuit (~1 Ω), so this project deliberately uses a 2×AA (3 V) pack, a pushbutton, and a press-to-lift habit. The 4×AA pack would push twice the current for no extra fun. The wire and cells getting warm is normal; press for seconds, not minutes. (This is Project 1’s “always put something in the loop” rule met from the other side: today the coil IS the something, only barely.)

What you need

• Enameled magnet wire, 0.2–0.4 mm, a few metres — the copper-coloured wire whose insulation is a varnish you can’t see
• A big iron nail or bolt — the fatter and more boring, the better
• 2×AA holder, a pushbutton, alligator clips
• Sandpaper, paperclips, steel washers
• Crane rig: chopstick, string, tape, a chair to tape it to

Build it

1. Wind. Leave 10 cm of wire free, then let the child wind neat-ish turns down the nail and back up — aim for 100 or more. (Neatness helps less than count; this is a job a six-year-old can own completely.) Tape the ends so it doesn’t unravel; leave 10 cm free at that end too.
2. Sand. The varnish is an insulator — clips on unsanded wire do nothing, which is worth letting happen once. Sand the last centimetre of each end until it’s bright copper all round.
3. Loop. Battery → pushbutton → coil → battery. Alligator clips everywhere.
4. Press. The nail grabs paperclips. Release. They fall. That’s the whole magic: cranes in scrapyards lift cars with this exact trick — grab with the button held, carry, release to drop.
5. Crane it. Nail on a string on a chopstick, cargo on the floor, button in hand: load the washers into a cup without touching them.

Then two experiments, child predicting first:

• Half the turns (unwind 50): fewer paperclips lift. Turns are strength.
• The compass. Hold the (unpressed) coil near a compass — nothing. Press — the needle swings. Flip the battery in its holder and press again — it swings the other way. Electricity has a direction (the child knows this from the motor), and now the magnetism has one too.

What to say to the child

“A magnet that’s only a magnet while you hold the button! Real crane drivers in scrapyards have exactly this, big enough to pick up a whole car — button down, grab the car; carry it over; button up, drop. And look — the compass believes our nail is a real magnet. When you flip the battery, the compass swings the other way, because the electricity is running around the coil the other way. You made magnetism out of electricity, and you can even steer it.”

There’s a story worth telling here: in 1820 Hans Christian Ørsted noticed a compass needle twitch during a lecture, next to a wire he happened to switch on. Nobody had ever connected electricity and magnetism before. The child just ran the actual experiment, with better wire.

For you — the physics

• The field of a solenoid is $B = \mu_0 n I$ inside: with 100 turns over 5 cm ($n = 2000\ \text{m}^{-1}$) and ~1.5 A, that’s ~4 mT of air-core field — a fridge magnet is ~5 mT, so bare coil ≈ barely anything, which the child can verify by pulling the nail out.
• The nail is the amplifier. Iron’s domains align with the coil’s field and add their own; the effective multiplication for an open nail is ~×10 to ×100 (geometry-limited, not the material’s μᵣ ~ thousands — the return path through air is the bottleneck; the explainer does this properly with magnetic circuits). Poles end up strong enough to hold a chain of clips.
• Why current is expensive here: the coil is ~1 Ω of copper, so the AAs work near their internal-resistance limit (~1.5–2 A, sagging). Strength goes as $NI$ (ampere-turns) — hence the two knobs, and why “more turns of thinner wire” is the engineer’s usual answer (more $N$, and the added resistance self-limits $I$). Heating is $I^2R$ — the press-to-lift duty cycle is thermal management the child performs without knowing it.
• The clip that won’t quite let go: after release, a paperclip often clings weakly — remanence. Iron’s domains don’t all snap back; the nail has learned a little permanent magnetism (hysteresis). A few sharp taps shakes the domains loose. Soft iron is used in electromagnets precisely because it forgets quickly; Project 16’s neodymium magnets are the opposite material choice — they never forget.

Try next

• Core audition: same coil around a pencil, an aluminium-foil roll, a steel bolt, a stainless spoon. Only the ferromagnetics amplify — a materials-science result from the toy box.
• Fishing game: paperclip fish with paper bodies, crane above, most fish in two minutes wins. Release skill matters — which is the point.
• Keep the coil. Seriously — don’t unwind it. Project 17 reveals a factory-made version of it hiding inside a little blue box.

→ Next: Project 16 — the magic that works through the table.

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