As you know, the magnetic needle, made in a magnetic field, turns. So, at its ends there are the magnetic forces, forming a pair of forces (rice. 22.3, and). When the arrow is set fixedly, these forces should be directed along the same straight line, along which is located an arrow (rice. 22.3, b). This means, with the help of magnetic needles to determine the location of the lines, along which magnetic forces act on magnetic needles.

The magnetic field in schema is conditionally represented magnetic lines of force, which recently became known as lines of the magnetic field. Line of the magnetic field induction is called a line, at each point where small magnetic needles are arranged in tangent.

In practice, picture of location of magnetic induction lines in the plane are easy to obtain using steel filings, since each particle of sawdust, once in the magnetic field, magnetized and becomes very small magnetic arrow, which is along the lines of the induction field. In figure. 22.4 shows the magnetic field of a rectilinear current in the plane, perpendicular to the conductor, obtained using sawdust and several magnetic needles. Induction line is considered directed in the wrong direction, at which point the North pole arrow, t. e. clockwise, when viewed from above (the direction of the current).

Through each point of space passes only one line of induction, therefore, the induction line never intersect each other.

From the pic. 22.4 see, what lines of magnetic field are closed, t. e. have no beginning, no end and always cover the conductor with current. This is a very important property of lines of magnetic field induction. Remember, the lines of the electric field begin and end on electric charges (or in infinity). Field, line induction which are always closed, called vortex. Unlike the potential field of electric charges, the magnetic field is a vortex.

From everything stated above we can conclude, that magnetic field and electric current always exist together. In nature there is never a magnetic field without an electric current and electric current without a magnetic field.