secondary current wave of a closed iron circuit induction
coil or transformer, whose primary circuit receives alternating
current, is lagged from its theoretical position of 90 degrees behind
the primary wave an additional 90 degrees, so that the phases of the
two currents are directly opposed; or the secondary current working
lamps only in its circuit is one half a wave length behind a primary,
instead of a quarter wave length, as might have been expected.

But when it is understood that the iron core polarized in one
direction by the primary impulse does not begin to lose its magnetism
when that impulse simply weakens, but waits until an actual reversal
of current has taken place, it will be seen that the secondary
current, which can only be produced when magnetic lines are leaving
the core and cutting the secondary coil, or when the lines are being
evolved and passing into the core from the primary coil, will have a
beginning at the moment the primary reverses, will continue during the
flow of that impulse, and will end at substantially the same time with
the primary impulse, provided the work of the secondary current is not
expended in overcoming self-induction, which would introduce a further
lag. Moreover, the direction of the secondary current will be opposite
to that of the primary, because the magnetic circuits which are opened
up by the primary current in magnetizing the core, or which are closed
or collapsed by it in demagnetizing the core, will always cut the
secondary coil in the direction proper for this result. Transformers
of the straight core type with very soft iron in the cores and not too
high rates of alternation should approximate more nearly the
theoretical relation of primary and secondary waves, because the
magnetic changes in the core are capable of taking place almost
simultaneously with the changes of strength of the primary current.
This fact also has other important practical and theoretical bearings.

Let us assume a plain iron core, Fig. 7, magnetized as indicated, so
that its poles, N, S, complete their magnetic circuits by what is
called free field or lines in space around it. Let a coil of wire be
wound thereon as indicated. Now assume that the magnetism is to be
lost or cease, either suddenly or slowly. An electric potential will
be set up in the coil, and if it has a circuit, work or energy will be
produced or given out in that circuit, and in any other inductively
related to it. H