  A free test particle in 5-dimensional Kaluza-Klein spacetime will show its
electricity in the reduced 4-dimensional spacetime when it moves along the
fifth dimension. In the light of this observation, we study the coupling of a
5-dimensional dust field with the Kaluza-Klein gravity. It turns out that the
dust field can curve the 5-dimensional spacetime in such a way that it provides
exactly the source of the electromagnetic field in the 4-dimensional spacetime
after the dimensional reduction.
