which way the electron
stream happens to be going in the coil. The atomic magnets in the iron
turn around each time the current reverses and they are always,
therefore, lined up so that the plunger is attracted. If the plunger has
much inertia or if the oscillations of the current are reasonably
frequent the plunger will not move back and forth with each reversal of
the current but will take an average position. The stronger the a-c
(alternating current) the farther inside the coil will be this position
of the plunger. The position of the plunger becomes then a measure of
the strength of the alternating current.

Instruments for measuring alternating e. m. f.'s and currents, read in
volts and in amperes. So far I haven't stopped to tell what we mean by
one ampere of alternating current. You know from Letter 7 what we mean
by an ampere of d-c (direct current). It wasn't necessary to explain
before because I told you only of hot-wire instruments and they will
read the same for either d-c or a-c.

When there is an alternating current in a wire the electrons start, rush
ahead, stop, rush back, stop, and do it all over again and again. That
heats the wire in which it happens. If an alternating stream of
electrons, which are doing this sort of thing, heats a wire just exactly
as much as would a d-c of one ampere, then we say that the a-c has an
"effective value" of one ampere. Of course part of the time of each
cycle the stream is larger than an ampere but for part it is less. If
the average heating effect is the same the a-c is said to be one ampere.

In the same way, if a steady e. m. f. (a d-c e. m. f.) of one volt will
heat a wire to which it is applied a certain amount and if an
alternating e. m. f. will have the same heating effect in the same time,
then the a-c e. m. f. is said to be one volt.

Another electromagnetic instrument which we have discussed but of which
more should be said is the iron-cored transformer. We consider first
what happens in one of the coils of the transformer.

The inductance of a coil is very much higher if it has an iron core. The
reason is that then the coil acts as if it had an enormously larger
number of turns. All the atomic loops of the core add their effects to
the loops of the coil. When the current starts it must line up a lot of
these atomic loops. When the current stops and these loops turn back
into some of their old self-satisfied groupings, they affect the
electrons in the c