Abstract
The helicity of a divergenceless vector field B(r) in a region D, defined as the integral integral DA.Bd3r, where B= Del xA, characterizes the topological configuration of the integral lines of B, being equal to zero if they are not linked. An expression for its value in terms of the linking number of pairs of lines is given. Although the paper is devoted to the cases in which B is the magnetic field, some other applications are indicated.