Solenoid

A solenoid is a coil in which the wire winding is located on a cylinder surface, so thin relative to the cylinder diameter. An ideal solenoid continues to have a very small diameter in relation to the distance of the wire windings from each other and thus a very high number of turns. A cylindrical coil for generating a ( spatially constant as possible ), the magnetic field is also sometimes referred to as a solenoid. Types of solenoids are described under air coil.

Magnetic field of solenoids

For the magnetic field strength H in the longitudinal axis of a cylindrical coil with the current I, the number of turns N, the length L and the radius r the following applies:

In this case, x is measured from the middle of the coil along the axis.

For a long solenoid with results from the fact that the magnetic field strength inside the coil on the axis of the approximately constant value

Has and drops to zero outside very quickly.

Inductance of solenoids

The inductance of a very long cylinder coil in air or vacuum is approximately

Here, the cross-sectional area (), and is the magnetic field constant.

For coils with ferromagnetic core, the formula is no longer applicable, since the outer part of the field is now relevant. Its average cross-section, instead of the coil cross -section can be used -, is, however, a closed magnetic circuit in the form of a high-permeability frame to which the coil is wound, instead of the coil length the mean circumference - that is the average magnetic path length. The Induktivitätsberechnung still requires then multiplied by the relative permeability of the core material.

For shorter air coils following approximate formula:

This formula has error of less than 1%.