THEOREM 1
Each current-carrying conductor is surrounded by circular field lines.
If a compass needle is held in this field line region ( fig. 2) and direct current flows through the copper rod, the needle will always align itself tangentially to the circular lines. Very important here is the fact that when the direction of the current is reversed, the compass needle also changes direction by rotating 180°. From this it can be seen that the field lines can be assigned a direction which depends on the direction of the current generating them.
To make this assignment easier to remember, the "right-hand rule" was defined in which, after forming a fist with the thumb spread out, the individual fingers indicate the direction assignment: If the thumb points in the direction of the current flow, the remaining fingers point in the direction of the field lines.