  We present in closed form some special travelling-wave solutions (on the real
line or on the circle) of a perturbed sine-Gordon equation. The perturbation of
the equation consists of a constant forcing term $\gamma$ and a linear
dissipative term, and the equation is used to describe the Josephson effect in
the theory of superconductors and other remarkable physical phenomena. We
determine all travelling-wave solutions with unit velocity (in dimensionless
units). For $|\gamma|$ not larger than 1 we find families of solutions that are
all (except the obvious constant one) manifestly unstable, whereas for
$|\gamma|>1$ we find families of stable solutions describing each an array of
evenly spaced kinks.
