Abstract: One of two coils of approximately the same diameter, forming the primary and secondary coils of a transformer for tube welding or other induction heating equipment, is mounted for movement relative to the other, preferably about a pivotal axis parallel to the axes of the coils, to enable its axis to be shifted towards and away from the axis of the other coil and the coils are so arranged that loops or turns of each coil, which are connected together in series or parallel, can pass between the loops or turns of the other coil to facilitate changing the coupling factor and thus adjusting the power delivered to the heating coil.