K326 / K327 / 2027
Magnetism and electromagnetism overview

Topic 3 of 7

Finding field direction with a compass

A small compass is a bar magnet free to turn. Its north-seeking end gives the magnetic field direction at the compass's position.

Magnetic forces act without contact. A magnetic field describes the magnetic effect around a source; the field can turn a small magnet until it aligns with the local direction.

Read the north-seeking end

Identify the compass's marked N end and allow the needle to settle. The direction in which that end points is the field direction. Its S end points the other way.

The compass responds to the field where it is placed. It need not point towards the centre of a nearby bar magnet or towards geographic north. The field direction describes the needle's alignment; it is not a rule for the direction in which the centre of the whole compass must move.

Read the north-seeking end at each position

These are four separate settled-compass observations around one bar. The magnet's field is assumed to dominate the background field.

Four compass directions around an N-left, S-right bar magnetA horizontal bar at the centre has N on its left and S on its right. A small compass on the axis to the left of N has its marked N end pointing left, away from N. A compass above the centre points its N end right. A compass on the axis to the right of S points its N end left, towards S. A compass below the centre points its N end right. Each teal half-needle ends at the side labelled N. Its direction is tangent to the ideal local field, not necessarily geographic north or the direction towards the bar's centre. No field-direction arrow is being used as a force on the whole compass.NSLeft of NNAbove centreNRight of SNBelow centreN

The marked N end indicates the local field direction. Reversing the bar reverses these four ideal directions.

This bar has N on the left and S on the right. With other fields neglected, the compass N end points left on the axis beyond either end of the bar, and right above and below its centre.

Read four local directions

The same bar, four observation points

  • Left of the N pole: the compass N end points left, away from that pole.
  • Above the centre: the N end points right.
  • Right of the S pole: the N end points left, towards that pole.
  • Below the centre: the N end points right.

These are local directions around one source. If the magnet is reversed while the positions stay fixed, the ideal directions reverse too.

Map a field using a plotting compass

  1. Place a bar magnet on paper on a non-magnetic surface. Draw its outline and label its poles. Keep the magnet and paper fixed.
  2. Check the marked N end and that the compass turns freely. Place it at a starting point near the magnet and wait for it to settle.
  3. Mark the positions of the needle ends, identifying N. Draw a short arrow from its S-end mark towards its N-end mark.
  4. Move the compass so its S end occupies the previous N-end mark. Let it settle and mark the new N-end position.
  5. Repeat in small steps, then join the marks with a smooth curve and add arrows in the recorded direction.
  6. Start again at several other positions to build the field pattern.

A small compass better approximates the direction at one position. Record a settled needle rather than a moving or stuck one, and keep nearby magnets and magnetic objects away from the map.

The needle responds to the resultant field, including Earth's field. An ideal bar-magnet diagram neglects that background. Make observations where the bar's effect is strong enough to dominate; far from it, the background can change the observed direction.

What iron filings show

Iron filings on a sheet near a magnet become magnetised and align, revealing the rough shape of the pattern. The visible chains do not identify an arrow direction by themselves. Use the compass N-end convention to add the arrows.

Field lines are drawn representations. The filings help reveal their shape; they are not threads that already existed around the magnet.

Optional check A small compass settles at a marked position near a bar magnet. Its north-seeking end points right. What is the magnetic field direction at that position?
A small compass settles at a marked position near a bar magnet. Its north-seeking end points right. What is the magnetic field direction at that position?