Unlike in a power transformer the two coils were "loosely coupled"; the secondary coil was separated physically from the primary coil, mounted on a rack which could be slid in or out of the primary coil, to vary the mutual inductance between the coils.
The two coils were "loosely coupled". This was unlike in a power transformer. The secondary coil was separated physically from the primary coil. The primary coil was mounted on a rack. This rack could be slid in or out of the primary coil. This was to vary the mutual inductance between the coils.