DIY · Card 40 · Engineering · Electromagnetism
A nail, a length of wire and a battery. Wind the wire round the nail and touch it to the battery, and the nail becomes a magnet that picks up paper clips, until you let go.
01The brief
Make a magnet with an on and off switch, and see how strong you can make it.
60×
roughly how much an iron nail strengthens the field of the coil around it
1.5 T
about where iron saturates: past this, more current barely helps
1825
the year the first electromagnet lifted nine pounds of iron on one battery cell
An ordinary magnet is always on. It sticks to the fridge whether you want it to or not. An electromagnet is a magnet you control: on when current flows, off when it stops, and stronger or weaker depending on how you build it.
It is one of the most useful devices ever invented, inside every motor, doorbell, loudspeaker and hard drive. And it is built from the same three things as the homopolar motor: a battery, some copper wire and a piece of iron. Its field is the same kind that swings the needle of the homemade compass.
02Kit
A nail, a metre of wire, a battery and some paper clips to count.
Tick off what you have
0 of 4 ready
03Method
Wind a coil, connect the battery, pick up clips, switch off, then make it stronger.
Step 1 of 8 · Minute 0
You need a large iron nail, 8 to 10 cm long, about a metre of thin insulated copper wire, an AA battery, sticky tape and a pile of paper clips. A compass is useful for the encore.
It doesn’t pick anything up.
Why
The ends aren’t bare, or the battery is flat.
Fix
Scrape the ends until bright copper shows, press them firmly on the battery, and try a fresh one.
It is weaker than expected.
Why
The turns go in different directions and cancel out.
Fix
Rewind the coil, every turn going round the same way, tightly side by side.
The wire gets hot.
Why
A big current is flowing: the battery is nearly short-circuited.
Fix
Connect only for a few seconds at a time, and let it cool between tests.
04Physics
Every wire carrying a current is wrapped in a magnetic field, circling round it. One straight wire makes a weak field. Wind the wire into a coil and the circles from all the turns line up along the middle, adding together into one field, strongest inside the coil and shaped just like a bar magnet’s.
Put iron inside and something remarkable happens. Iron is full of tiny regions called domains, each a little magnet, normally pointing every which way. The coil’s field swings them into line, and their fields add to the coil’s, making it many times stronger. Build your own:
Current
5.0 A
Field in the core
0.32 T
Paper clips held
3
Battery flat in
12 min
The iron multiplies the coil’s field about 60 times. But the battery is working hard: it is close to a short circuit, so it gets warm and goes flat in minutes. Only switch on for a few seconds at a time.
Because the field comes from the current, it has a direction, and you can reverse it. Change which way the current goes round the coil and its north and south poles swap ends. Surround it with compasses and watch:
Switched on, the coil is a bar magnet with its north pole at the right end. Reverse the battery and the current flows the other way round the coil, and the poles swap.
Both widgets use typical values for a fresh AA battery, thin copper wire and an iron nail; the clip counts are a rough guide. Real batteries sag fast under this load, so your nail will weaken as you test.
05The big idea
A magnet
with a switch.
Electricity can be switched, measured and sent down a wire in an instant. Magnetism can push, pull and move things. The electromagnet joins them: it turns a signal in a wire into a force in the world.
That one idea runs the modern world. Motors turn current into motion, speakers turn it into sound, relays turn a small current into the switching of a large one, and generators run the whole thing backwards to make the electricity in the first place.
06The math
1 · The current
I = V ⁄ (r + R)
A battery of voltage V and internal resistance r drives current through wire of resistance R.
2 · The coil
B = μ₀ μ N I ⁄ l
N turns over a length l, carrying current I. μ₀ is a constant of nature; μ is how much the core helps.
3 · The pull
F = B² A ⁄ 2μ₀
The force a magnet pole of area A exerts grows with the square of the field.
A metre and a half of thin copper wire has a resistance of only about 0.1 Ω, less than the battery’s own 0.2 Ω, so an AA pushes about 5 A through it: nearly a short circuit. With 50 turns over 6 cm, the coil alone makes B = 1.26 × 10⁻⁶ × 50 × 5 ⁄ 0.06 ≈ 0.005 T, a hundred times Earth’s field but still too weak to lift a clip.
Slide in an iron nail, which helps by a factor of around 60, and the field becomes about 0.3 T. Because pull goes as B², that is thousands of times stronger than the bare coil. Double the turns and you double B and quadruple the pull, until the iron saturates near 1.5 T.
07Real life
From scrapyards to hospitals to the speaker in your phone, electromagnets turn current into force.
Scrapyard cranes
A scrapyard crane lifts tonnes of steel with a giant electromagnet: a disc of coils round an iron core. Switch on, lower it onto a pile of scrap and lift; switch off over the truck and it all drops. Exactly your nail and paper clips.
MRI scanners
A hospital MRI scanner surrounds the patient with a huge coil of superconducting wire, chilled until it has no resistance at all. It makes a field of 1.5 to 3 tesla, tens of thousands of times stronger than Earth’s, to map the inside of the body.
Maglev trains
Magnetic levitation trains are held above their track and pushed along it by electromagnets switched on and off in sequence. With no wheels touching the rail, the Shanghai maglev reaches speeds of over 400 km/h.
Doorbells and relays
Press a doorbell and current flows through a little electromagnet, which yanks an iron arm against a bell. Relays work the same way, using a small current through a coil to flip a switch that controls a much bigger one.
Loudspeakers
Behind every speaker cone is a coil of wire in the field of a permanent magnet. The music arrives as a rapidly changing current; the coil becomes an electromagnet that pushes and pulls against the magnet hundreds of times a second, and the cone moves the air.
Electric motors
Every electric motor, in fans, drills, cars and phones, spins because electromagnets push against magnets, switched so they are always pushing in the right direction. The simplest motor of all is on its own card.
1825
In 1820 the Danish physicist Hans Christian Ørsted noticed that a compass needle twitched whenever he switched on a current in a nearby wire. Electricity, it turned out, makes magnetism. Within five years an English shoemaker turned lecturer, William Sturgeon, had put the discovery to work.
He bent an iron bar into a horseshoe, varnished it so the bare wire wouldn’t short against it, and wound copper round it. Connected to a single battery cell, his seven-ounce magnet lifted nine pounds of iron, about twenty times its own weight. It won him a silver medal, and began a line of invention that leads straight to every motor in the world.
08Is and isn’t
Any nail will do.
It has to be iron or ordinary steel. Aluminium, brass and most stainless steel won’t carry magnetism, and a coil wound on them is no stronger than the coil alone.
Electricity and magnetism are different things.
They are two sides of one force. Every current makes a magnetic field, and every changing magnetic field makes a current. The nail electromagnet is the first half of that discovery.
More batteries always make it stronger.
Up to a point. Once the iron saturates, extra current adds very little field, but it heats the wire and drains the batteries faster.
Once magnetised, the nail stays a magnet.
Soft iron loses nearly all its magnetism the instant the current stops, which is what makes it useful. Hard steel holds on to some, which is how permanent magnets are made.
09Go further
A
Turns test
Count the clips with 20, 40, 60 and 80 turns. Plot clips against turns. Is it a straight line, or does it level off?
B
Core swap
Try a steel screw, a pencil, an aluminium rod and an iron bolt as the core. Which makes the strongest magnet? Which keeps some magnetism when switched off?
C
Pole finder
Use a compass to find which end is north. Then reverse the battery. Which way does the current go round the coil when the point is north?
D
Magnetic switch
Make a simple doorbell: an electromagnet that pulls a strip of steel against a spoon when you press a button.
E
Through the table
Can your electromagnet move a paper clip through a sheet of card, or a thin table top? How thick can the barrier be?
10Questions
Wind about a metre of thin insulated copper wire tightly round a large iron nail, all in the same direction, leaving the two ends free. Strip the ends and touch them to the two ends of an AA battery. While the current flows, the nail picks up paper clips; disconnect it and they drop.
An electric current always makes a magnetic field around it. Wind the wire into a coil and the fields of all the turns add up into one strong field along the middle. An iron core inside lines up its own tiny magnetic domains with that field and makes it many times stronger.
Add more turns of wire, push more current through it with an extra battery in series, and use a core of soft iron. Each helps until the iron saturates, when all its domains already point the same way. Beyond that, more current only heats the wire.
Because a coil of thick copper wire has very little resistance, so the battery is almost short-circuited. A large current flows, several amps, and both the wire and the battery warm up. Only connect it for a few seconds at a time, or the battery will go flat in minutes.
Soft iron’s magnetic domains are easy to line up and just as easy to jumble again. Without the coil’s field holding them in line, they fall back to pointing every which way. Hard steel holds on to its alignment, which is why a steel screw can stay slightly magnetic.
Safety · Use only AA or AAA batteries, never mains electricity or rechargeable lithium packs. Connect for a few seconds at a time: the wire and battery get hot. Keep the nail’s point away from eyes.
All DIY cards →Keep wandering
A few more pieces in the same spirit — math, design, and slow attention.
How to Make a Simple Electric Motor (Homopolar Motor)
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How to Build a Compass With a Needle, a Magnet and Water
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