where are the four components of the current density. (Here it is a matter of convention to write instead of , i.e. to use capital letters, and to write instead of . However, the vector rsp. tensor components and the above-mentioned forms have different physical dimensions. Moreover, by decision of an international commission of the International Union of Pure and Applied Physics, the magnetic polarization vector is called formula_80 since several decades, and by some publishers , i.e. the same name is used for different quantities.)