A solenoidal induction coil with dynamically variable coil geometry is provided for inductively welding or heating continuous or discontinuous workpieces passing through the solenoidal induction coil in a process line. The coil geometry can change, for example, as the outer dimension of the workpiece passing through the solenoidal induction coil changes or as non-continuous workpieces pass through the solenoidal induction coil in an induction heating or welding process line.