We want our magnet to project, as it were, its force across the greatest length of air gap.
An electric pressure is induced in the secondary circuit, but of not enough intensity to cause a spark to jump across the air gap.
Why is a spark produced at the air gap at break and not at make of the primary current?
In the electric bell, we saw that an air gap at the push button stopped the flow of electricity.
A copper disc, when inserted within the air gap of the ring magnet, was brought into rapid rotation.
It is simply that you thereby reduce the reluctance offered by the air gap to the flow of the magnetic lines.
Decreasing this air gap will cause the cutout to open late, and increasing it will cause the cutout to open early.
An air gap in a magnetic circuit always tends to hasten demagnetizing.
In the above example we have for simplicity assumed that the flux in passing across the air gap does not spread out at all.
It requires a vastly higher voltage to make a spark jump across an air gap than it does to lead it across thus.