Magnetic field geometries

The magnetic field is the easiest tool for determining the momentum of a particle through its trajectory and, the more accurate the knowledge of the magnetic field, the more accurate the particle momentum determination. This simple fact explains why more and more particle detectors have been equipped with electro-magnets, either conventional (where the iron is magnetized by resistive coils) or superconducting (most of the magnetic field is produced by the superconducting coil, at a temperature around 4 K). As the size of the detectors increases, with the size of the colliders, more and more of these magnets are superconducting, mainly for reducing the running cost. This paper will recall first the various magnetic field configurations which can be used. Then, a summary of the progress done in the construction of detector magnets since the 70's to the next collider to be put in operation in 2007, the Large Hadron Collider LHC, will be done through several examples. Finally, a look at what can be the limitations of such magnets for the future projects will be done as conclusions.