The bobbin in transformer terminology is the spindle shape on which a coil of wire is wound.  This equation computes the inductance of a coil of wire on a bobbin.

A transformer is for stepping up or stepping down an AC power source by using the principle of magnetic induction between the transformer's two wire windings.   There are a number of calculations in designing or building a transformer.

In general inductors use a ferrite core.  Reducing the Inductance leakage will lead to increase of capacity.