lesson
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Hold two strong magnets close together, and you can feel an invisible force pushing or pulling before they even touch.
That invisible region of force around any magnet is called a magnetic field.
The Shape of a Magnetic Field
We represent this invisible force using magnetic field lines, which always point outward from the north pole (N) and curve around into the south pole (S).
In 1600, English physician William Gilbert first mapped these curved pathways using a tiny pivoted needle, proving that Earth itself behaves like a giant magnet.
📊A clean, responsive interactive diagram (width 100%, max-width 500px, height 320px) showing a horizontal bar magnet labeled 'N' (red end on left) and 'S' (blue end on right). Symmetrical smooth field lines emerge from N, loop widely through the surrounding space, and enter S. Directional arrows along each line point away from N and toward S. Lines are closest together right at the poles to indicate higher field strength. Hovering over any part of the field highlights the line and shows a tooltip: 'Field direction: N to S'. Clean modern styling with #1e2945 text, #ffffff card background, light border #e6e6e6.
Where field lines are packed closely together near the poles, the magnetic field is strongest; where they spread apart, the field is weaker.
How can we reveal these invisible lines right on our lab bench?
Two Ways to Map Fields
The fastest method uses iron filings — tiny slivers of iron scattered across a sheet of paper placed over a magnet. Each filing temporarily turns into a microscopic magnet and aligns with the field lines, instantly showing the overall shape.
The precise method uses a plotting compass, which is a miniature magnetic needle balanced on a pivot that points along the exact direction of the local field.
📊An animated step-by-step walkthrough showing compass plotting around a bar magnet (width 100%, max-width 500px, height 340px). Step 1: Place compass near North pole; needle points away. Mark a dot at the needle's tip. Step 2: Move compass so its tail sits on that dot, mark new tip dot. Step 3: Repeat across the page until reaching the South pole. Step 4: Draw a smooth line connecting dots with an arrow pointing toward South. Includes 'Next Step' and 'Reset' buttons. Clear color coding: Compass North needle tip is red, South tail is white.
What happens if we replace the solid metal bar with a coiled wire carrying electric current?