Shim (magnetism)

A shim (English " shim " wedge compensation piece ) is a device for homogenization of magnetic fields. The term comes from English and means originally washer or wedge. From the purely mechanical means by which one has once set the position and parallelism of the pole pieces of an electromagnet, the term "electrical current shims " derived for current-carrying coils, which have assumed the same function. They do this by generating magnetic fields that are much weaker than the field to be homogenized with which they are superimposed.

The auxiliary coils are to designed so that their fields in the sample with simple spherical harmonics can be described as interference fields, the cause of spatially removed are from the sample, the shape of such Kugelfächenfunktionen have low order in it also ( approximately).

When shimming the inhomogeneities of the magnetic field can be eliminated by additional coils ( active shimming ) or magnetized iron parts ( passive) are suitably attached. The correction resulting fields superimposed on the field of the main coil and homogenize it.

Application

For proper operation of a magnetic resonance tomograph, primarily but with significantly stronger instruments for scientific research shims are used ..