The leakage flux can be practically eliminated by winding the primary and secondary, one over the other, uniformly around a laminated iron ring of uniform x-section. But such an arrangement is not commercially practicable except in very small sizes, owing to the cost of threading a large number of turns through the ring.
The various methods used for reducing leakage flux are:
1. By Increasing Height of Window :
The area of the window is determined by the x-sectional area of primary and secondary windings. For a given area of the window, the reluctance of the leakage flux path is increased by increasing the height and reducing the width of the window.
Usually, the ratio of height to width is kept upto 4 because if the window is made very narrow, an excessive proportion of the width is occupied by the insulation.
2. By Arranging Primary and Secondary Windings Concentrically :
Each winding is equally divided between the two limbs, the general practice being to place lv winding next to the core and hv winding on the outside, as illustrated in fig.
With concentric windings, the leakage flux is approximately a half of that with the winding on separate limbs.
Furthermore, the leakage flux is linked with only a part of the primary or secondary winding, so that the leakage flux is considerably reduced.
3. By Sandwiching Primary and Secondary Windings:
The aim of sandwiching is again to arrange the primary and secondary windings as close to each other as possible, so that leakage flux is reduced.
The leakage flux can be further reduced by using more and more sandwiched sections for both primaries and secondary windings, but this arrangement has the disadvantage that adjacent sections must be
well insulated from each other.
So with a larger number of sections, an excessive amount of space is taken by insulation.
4. By Adopting Shell Type Construction:
In shell-type transformers, the low voltage and high voltage windings are sandwiched, as illustrated in fig. 
This arrangement of using sandwiched coils results in more closeness of the primary and secondary coils and so the leakage flux is considerably reduced.
In shell-type construction, since iron surrounds both the windings, so the fluxes created by primary and secondary windings tend to follow the least reluctance iron path around them rather than the high reluctance air path. Thus leakage flux is further reduced.