What does a magnetic-field direction symbol mean in a diagram?
Quick answer
A dot inside a circle means the magnetic field points out of the page, toward you; a cross inside a circle means it points into the page, away from you. These symbols show direction, not field strength. To compare strength, look for a separate cue in the diagram, such as how closely spaced the field lines are. [S1]
The question, the details, the next step.
I keep seeing a dot or a cross inside a circle in magnetic-field diagrams. What am I supposed to read from it?
Treat each mark as a direction label for a field perpendicular to the flat page. A dot means the field points out of the page, toward the viewer. A cross means it points into the page, away from the viewer. [S1]
The marks answer “which way?” They do not, by themselves, tell you how strong the field is. A larger-looking dot or a row of more crosses should not be read as a strength scale unless the diagram explicitly defines one.
Can you show me how to use that in a simple page-direction example?
Imagine the page is a map and you are looking straight at it. At a point marked ⊙, the field points toward your eyes, coming out through the page. At a point marked ⊗, it points away from your eyes, going into the page. The circle marks the location in the drawing; the dot or cross gives the perpendicular direction.
For example, if a diagram labels point A with ⊙ and point B with ⊗, you can write: “At A, the field is out of the page; at B, it is into the page.” That description does not claim that either field is stronger. The dot-and-cross convention represents direction, not magnitude. [S1]
How do I know whether the field is stronger at one place than another?
Look for a separate strength cue rather than trying to infer strength from the dot or cross. In the cited wire-field illustration, field-line spacing is used separately: lines farther apart indicate a weaker field as distance increases. [S1]
A useful reading order is: first identify the location, then read the dot or cross for direction, and then inspect any separate field-line spacing, scale, or legend for strength. If the diagram gives no strength cue, the symbol alone does not establish which location has a stronger field.
What should I write in my notes so I don’t mix up direction and strength?
Use a two-part note and fill it in from the diagram:
“Direction at [point]: [out of the page / into the page], shown by [dot / cross]. Strength: [state only what a separate cue shows, such as ‘field lines are farther apart here’ / ‘not specified by this symbol’].”
Worked example: “Direction at A: out of the page, shown by a dot. Strength: not specified by the dot.” If comparing two locations, add field-line spacing or another stated cue separately. A dot or cross by itself cannot support a comparison of field strength.
A practical checklist
Find the point or region where the dot or cross is drawn.
Read a dot as out of the page, toward the viewer.
Read a cross as into the page, away from the viewer.
Keep direction separate from magnitude; do not treat symbol size or count as a strength scale.
Use a separate legend or field-line spacing to compare strength, if the diagram provides one.
If no strength cue is shown, note that the symbol alone does not establish field strength.
A few more useful answers
Does a dot mean the field is stronger than a cross?
No. A dot and a cross indicate opposite directions perpendicular to the page, not different field strengths. Look for a separate strength cue. [S1]
What does it mean if several dots appear together?
Each dot indicates a field directed out of the page at its marked location. The number of dots alone does not establish the field’s magnitude.
Can the symbols tell me the field direction in the plane of the page?
No. The dot-and-cross convention indicates direction perpendicular to the page; it does not give an in-page direction. [S1]
Sources
References used for this guide.
- [S1] 11.4 Magnetic Force on a Current-Carrying Conductor — University Physics Volume 2 openstax.org · Checked