The study of various magnetic phenomena showed that wherever there is electric current, i.e. electric charges in motion, there is also a magnetic field of current.
Due to the fact that a magnetic field forms around the conductor when a current appears in the latter, it is often regarded as a source of magnetic field formation. In this sense, it is called the "magnetic field of the current."
Its direction depends on the direction of the electric current around which this field arises.
The fact of the existence of a magnetic field near a conductor with electric current can be detected in several ways. One of these methods is to use iron filings. Once in the magnetic field of the current, the pieces of iron that make up the iron filings become small magnetic arrows. Each of these arrows in a magnetic field is set so that its axis coincides with the orientation of the magnetic field in the place where it is located. With the help of many such arrows, you can see how the direction of the forces acting in a magnetic field changes during the transition from one point of the field to another. This arrangement in the magnetic field of the current of iron filings is called the magnetic spectrum.
We use magnetic filings and consider the direct current magnetic field. To do this, let the conductor pass through a sheet of cardboard, and sprinkle a thin layer of iron filings on the cardboard. We will notice that when passing current through the conductor, sawdust will be arranged around it in concentric circles.
The lines along which the magnetic field of the current locates the axis of the miniature magnetic arrows are called its lines of force. Using lines of force, it is very convenient to graphically display magnetic fields.
The line of force is drawn so that the tangent to it at any of its points indicates the direction of the force acting at this point to the north pole.
The chains formed in the magnetic field of the current using iron filings show what the shape of the lines of force in the magnetic field is.
The lines of force in a magnetic field are closed curves around a conductor. In particular, direct magnetic field power lines are concentric circles whose centers are located on the stream line.
To determine the direction of magnetic field lines in connection with the direction of the current, instead of sawdust, you need to use magnetic arrows. Surrounding them with a conductor with current, we determine the direction of the lines of force. When the current orientation in the conductor reverses, the magnetic arrows rotate 180 °, which indicates a corresponding change in the direction of the field lines of force.
The direction of the lines of force in a magnetic field is related to the direction of the current in the conductor by a simple rule proposed by the English scientist Maxwell: if the translational movement of the gimlet coincides with the direction of the current in the conductor, then the direction of rotation of the handle of the gimlet coincides with the direction of the lines of force in the magnetic field existing around conductor.
This rule is sometimes called the gimlet rule.
Now take a conductor bent around the circumference, and passing a current through it, we will again observe the location of the sawdust on the cardboard installed perpendicular to the plane of the circle. We will find that the magnetic lines of force are no longer regular circles, but even in this case all the lines will be closed, bypassing the conductor along which the current flows. Thus, magnetic lines of force always close around the current.
By placing small magnetic arrows at different points of this field, one can determine the direction of the field lines of force.
The gimlet rule applies, of course, to circular current, but in this case it is more convenient to interchange the direction of the current and magnetic field. Indeed, if the gimlet handle is rotated in current, then the movement of its tip will show which direction the magnetic field of circular current has.