One important function of the detector is to measure a particle's charge and momentum. For this reason, the inner parts of the detector, especially the tracking device, are in a strong magnetic field. The signs of the charged particles can easily be read from their paths, since positive and negative particles curve in opposite directions in the same magnetic field.

The momenta of particles can be calculated since the paths of particles with greater momentum bend less than those of lesser momentum. This is because a particle with greater momentum will spend less time in the magnetic field or have greater inertia than the particle with lesser momentum, and thus bends less in a magnetic field.