Every magnet is surrounded by a magnetic field — invisible, untouchable, yet doing real work. This experiment "develops" the field step by step with three things that are easy to find (paper clips, iron filings and a small compass): first locate where the magnetism is strongest, then let the filings trace out the shape of the field lines, and finally interrogate each point's direction with the compass needle.
Safety first (read this section before you start)
- Neodymium magnets (the silvery, extra-strong kind) snap together with great force and can easily pinch fingers. Grip them by the sides and separate them slowly; never let two slam together — strong magnets can shatter and throw off sharp chips.
- Never let young children handle small magnets. Swallowing two or more is a medical emergency: they attract each other through the intestinal wall.
- Keep magnets away from phones, computer hard drives, and the magnetic stripes on bank and transit cards, and away from anyone with a cardiac pacemaker.
- Iron filings are very fine: do not blow on them with your mouth, and do not rub your eyes. Seal the filings inside a clear zip-top bag before use, and wash your hands afterward.
- No power sources or heating are involved, but lay a sheet of white paper on the table anyway to make the filings easy to collect.
Materials and equipment
- Two bar magnets (black whiteboard magnets or fridge-magnet strips also work; regular shapes are easier to observe);
- A small compass (common in stationery shops; a phone's compass app can serve as a reference but must not be held close to the magnet);
- A small packet of fine iron filings, or snippets cut from an old steel-wool pad;
- White paper, a clear zip-top bag, clear tape;
- About twenty paper clips, for finding the poles.
Step 1: find where the magnetism is strongest
Scatter the paper clips across the table, hold the bar magnet by its middle, and sweep it slowly, lying flat, over the pile. Lift it and count: which part picked up the most clips?
You'll find the two ends are packed with clips while the middle holds almost none. These two strongest ends are the magnet's poles. Hang the magnet horizontally from a thread and let it settle: the end that points north is the north pole (N), and the end pointing south is the south pole (S).
Step 2: let the iron filings trace the field lines
Sprinkle the filings evenly inside the clear zip-top bag, press it flat and seal it, then tape the bag onto the white paper. Place the bar magnet underneath the paper and tap the paper gently with a finger — tap, don't shake.
With each tap the filings jump and resettle, quickly forming curve after curve that leaves one end of the magnet, sweeps around in a wide arc, and returns to the other end. Near the poles the curves are dense and crisp; along the magnet's midsection they are sparse. These patterns are the shape of the magnetic field lines.
Step 3: ask the compass for directions
The filings drew the shape of the lines but said nothing about their direction. That job belongs to the compass: place it directly above the magnet, directly below, and just off each pole in turn. Each time, wait for the needle to come fully to rest, then note which way the red N end points. Move the compass one small step in the direction its N end indicates, watch where it swings next, and keep walking step by step — the path you trace out is a field line.
What you should observe
Check your observations against these points and write them into your lab notes:
- The paper clips hang almost entirely from the magnet's two ends and won't stick to the middle: the poles are at the ends, where magnetism is strongest.
- The filings' curves set out from one pole, loop around, and return to the other pole, densest near the poles.
- With the compass above the magnet, the N end points one way; below the magnet, the opposite way. Moving along a curve, the needle always lies tangent to it.
- Flip the magnet over and the filings' pattern stays the same, but every compass reading reverses.
- With two magnets, unlike poles facing attract, and the filings between them join into a solid bridge; like poles facing repel, leaving a nearly filing-free blank band in the gap.
- Slide the magnet farther away and the pattern fades fast: the farther the distance, the weaker the field.
The science
The magnets never touch each other — what acts across the gap is the magnetic field: an invisible physical entity surrounding every magnet, through which magnetic forces are transmitted.
To picture the field, physics introduces magnetic field lines: a family of directed curves whose tangent at every point matches the field direction at that point. The convention for field direction is: the direction a resting compass needle's N end points is the field direction at that point. By this convention, the field lines outside a magnet must run from the N pole around to the S pole, while inside the magnet they run from S to N — so field lines are closed loops. They never cross, because a compass at any single point can only point one way; and where the lines crowd together the field is stronger, exactly matching the dense filings near the poles.
Why do the filings line up by themselves? Because each filing is magnetized by the field into a tiny compass needle, swings around to the local field direction, and links nose-to-tail with its neighbors. Tapping the paper briefly frees the filings from friction so they can rotate into the lowest-energy position. One point deserves emphasis: field lines are a human drawing aid — no such lines actually exist in space; the filings merely display their shape.
The rule between poles is like poles repel, unlike poles attract. The compass needle's N end points north because the Earth itself is a giant magnet, and the Earth's magnetic south pole lies near the geographic North Pole — unlike poles attract, so N points north.
Notes for teachers and parents
- Time and grouping: 25 minutes, in pairs; filings sprinkled on top of the paper, magnet underneath.
- Say this before hands touch anything: keep filings away from eyes; keep magnets away from phones and magnetic cards.
The step most often skipped. Before sprinkling, have students predict the shape of the field lines and sketch it in their notebooks, then compare with the real pattern. Use the compass at plenty of positions so they discover for themselves that "the needles always line up nose to tail."
How to tell they truly understand. A student who can state that field lines leave the N pole and return to the S pole, and can identify a pole from a compass reading, has met the goal.
Take it further, and common misconceptions
- Make your own compass: stroke a sewing needle along a magnet thirty times, always in the same direction, to magnetize it; push it through a small piece of foam and float it on water — it will swing itself to point north–south.
- What happens if you snap a magnet in half? You get two smaller magnets, each with its own N and S poles. You can never obtain an isolated single pole — a sharp contrast with electric charge.
- Misconception 1: "Outside the magnet, field lines run from S to N." Backwards — outside they run N to S, inside S to N.
- Misconception 2: "Magnets attract all metals." They attract only a few magnetic materials such as iron, cobalt and nickel; copper, aluminum, gold and silver won't budge — test it on the spot with coins.
- Misconception 3: "The field exists only where the filings drew lines." The field fills all the space around the magnet; the lines are just the few curves we choose to draw.
- One step further lies the true junction of electricity and magnetism: a current-carrying wire produces a magnetic field of its own — the starting point of every motor and generator.
When a student can point at the filing patterns and explain "where this line starts, which way it runs, and where the field is strongest," this experiment has been done right.