oil. Where first they opposed the motion of these
electrons, now they insist on its being continued for a moment longer.
I'll prove that by describing two simple experiments; and then we'll
have the basis for understanding the effect of an iron core in a
transformer.

[Illustration: Fig 33]

Look again at Fig. 33 of Letter 9 which I am reproducing for
convenience. We considered only what would happen in coil _cd_ if a
current was started in coil _ab_. Suppose instead of placing the
coils as shown in that figure they are placed as in Fig. 108. Because
they are at right angles there will be no effect in _cd_ when the
current is started in _ab_. Let the current flow steadily through
_ab_ and then suddenly turn the coils so that they are again
parallel as shown by the dotted positions. We get the same temporary
current in _cd_ as we would if we should place the coils parallel
and then start the current in _ab_.

[Illustration: Fig 108]

The other experiment is this: Starting with the coils lined up as in the
dotted position of Fig. 108 and the current steadily flowing in
_ab_, we suddenly turn them into positions at right angles to each
other. There is the same momentary current in _cd_ as if we had
left them lined up and had opened the switch in the circuit of
_ab_.

[Illustration: Fig 109]

Now we know that the atomic loops of iron behave in the same general way
as do loops of wire which are carrying currents. Let us replace the coil
_ab_ by a magnet as shown in Fig. 109. First we start with the
magnet at right angles to the coil _cd_. Suddenly we turn it into
the dotted position of that figure. There is the same momentary current
in _cd_ as if we were still using the coil _ab_ instead of a
magnet. If now we turn the magnet back to a position at right angles to
_cd_, we observe the opposite direction of current in _cd_.
These effects are more noticeable the more rapidly we turn the magnet.
The same is true of turning the coil.

The experiment of turning the magnet illustrates just what happens in
the case of a transformer with, an iron core except that instead of
turning the entire magnet the little atomic loops do the turning inside
the core. In the secondary of an iron-cored transformer the induced
current is the sum of two currents both in the same direction at each
instant. One current is caused by the starting or stopping of the
current in the primary. The other current is due to the turning of the
atomic