  We develope a difference calculus analogous to the differential geometry by
translating the forms and exterior derivatives to similar expressions with
difference operators, and apply the results to fields theory on the lattice
[Ref. 1]. Our approach has the advantage with respect to other attempts [Ref.
2-6] that the Lorentz invariance is automatically preserved as it can be seen
explicitely in the Maxwell, Klein-Gordon and Dirac equations on the lattice.
