The question of whether an article should be omitted seems to be quite complicated and subject to numerous conventions.
Moreover, it appears that there are acceptable variations: e.g., both
The electric dipole moment has a magnitude p=qd.
The electric dipole moment has magnitude p=qd.
may be correct or, at least, acceptable. (The latter variant was suggested to me, in a similar case, by a professional editor.)
Unfortunately, style manuals, such as The Chicago Manual of Style and New Oxford Style Manual, offer little guidance on this topic.
Does a comprehensive, authoritative, and/or detailed reference for the zero article exist?