  In this paper, Kaluza-Klein theory is revisited and its implications are
elaborated. We show that electromagnetic 4-potential can be considered as a
shearing-like deformation of a 5-dimensional (5D) manifold along the fifth
(5th) axis. The charge-to-mass ratio has a physical meaning of the ratio
between the movement along the direction of the 5th axis and the movement in
the 4D space-time. In order to have a 5D matter which is consistent with the
construction of the 5D manifold, a notion of particle-thread is suggested.
Examinations on the compatibility of reference frames reveal a covariance
breaking of the 5th dimension. The field equations which extend Einstein's
field equations give the total energy-momentum tensor as a sum of that of
matter, electromagnetic field, and the interaction between electric current and
electromagnetic potential. Finally, the experimental implications are
calculated for the weak potential case.
