where J is the charge-current 3-form. Within dielectric media there exist charges bound up in otherwise neutral atoms. These charges are not free to move around very much, but distortions to the distribution of charge within the atom can allow dipole (or more generally multipole) moments to form, with which is associated a dipole field. Separating bound and free charges in the charge-current three form J = J b o u n d + J f r e e , the bound source is associated with a particular solution called the polarization field P satisfying