Yup! Like Mike said.
Also, and I learned this the hard way, if you want your field to INSTANTLY stop effecting your objects, you must key the magnitude up to the frame just before you want it to stop, then advance one or more frames, and key the magnitude at 0.
Otherwise, and you may or may not want this, it will gradually decrease magnitude from your first key, to the next.
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS