and the like are usually electromagnetic instruments.
Ammeters and voltmeters are alike in their design. Both are sensitive
current-measuring instruments. In the case of the voltmeter, as you
know, we have a large resistance in series with the current-measuring
part for the reason of which I told in Letter 8. In the case of ammeters
we sometimes let all the current go through the current-measuring part
but generally we let only a certain fraction of it do so. To pass the
rest of the current we connect a small resistance in parallel with the
measuring part. In both types of instruments the resistances are
sometimes hidden away under the cover. Both instruments must, of course,
be calibrated as I have explained before.

In the electromagnetic instruments there are several ways of making the
current-measuring part. The simplest is to let the current, or part of
it, flow through a coil which is pivoted between the _N_ and
_S_ poles of a strong permanent magnet. A spring keeps the coil in
its zero position and if the current makes the coil turn it must do so
against this spring. The stronger the current in the coil the greater
the interaction of the loops of the coil and those of the iron atoms and
hence the further the coil will turn. A pointer attached to the coil
indicates how far; and the number of volts or amperes is read off from
the calibrated scale.

Such instruments measure direct-currents, that is, steady streams of
electrons in one direction. To measure an alternating current or voltage
we can use a hot-wire instrument or one of several different types of
electromagnetic instruments. Perhaps the simplest of these is the
so-called "plunger type." The alternating current flows in a coil; and a
piece of soft iron is so pivoted that it can be attracted and moved into
the coil. Soft iron does not make a good permanent magnet. If you put a
piece of it inside a coil which is carrying a steady current it becomes
a magnet but about as soon as you interrupt the current the atomic loops
of the iron stop pulling together. Almost immediately they turn into all
sorts of positions and form little self-satisfied groups which don't
take any interest in the outside world. (That isn't true of steel, where
the atomic loops are harder to turn and to line up, but are much more
likely to stay in their new positions.)

Because the plunger in an alternating-current ammeter is soft iron its
loops line up with those of the coil no matter