What is meant by proximity effect in transmission line?

Assume the AC currents are going in opposite (SAME) directions in a twin pair transmission line. The magnetic fields so created have the effect of forcing the current flow towards the centre (OUTSIDE). The effect is greater with higher frequency. As the copper is not being effectively utilised, the resistance goes up. This effect is distinct from "skin effect". The effect is also greatest if the gap between wires is small. Unfortunately, as of April 2009 an on-line calculator is difficult to find.