Fig.19 shows a metal ring symmetrically situated relative to the air gap in the iron ring. Let the magnetising winding be concentrated over a short length of the core. The flux through the metal ring may be considered as useful flux and that which
returned by such paths as 1, 2 and 3 is the leakage flux. The useful flux passing across the gap tends to buldge outwards (this is because the lines of force repel each other when passing through a non-magnetic material) as shown roughly in Fig. 19,
thereby reducing the effective area of the gap and reducing the flux density in the gap. This effect is referred to as fringing and the longer the air gap, the greater is the fringing
It is seen that the effect ofleakage flux is to increase the total flux through the exciting winding.
If ɸt = total flux produced (produced in the exciting winding), and
ɸ = useful flux available in the air-gap,
[Note. Magnetic leakage can be minimised by placing the exciting coils or winding as close as possible to the air gap or to the points in the magnetic circuit where the flux is to be utilised.]