flowed through; they won't tell the rate at which
the gas or water is flowing. They are like the odometer on the car which
gives the "trip mileage" or the "total mileage." We want a meter like
the speedometer which will indicate at each instant just how fast the
electrons are streaming through it.

There are several kinds of meters but I shall not try to tell you now of
more than one. The simplest to understand is called a "hot-wire meter."
You already know that an electron stream heats a wire. Suppose a piece
of fine wire is fastened at the two ends and that there are binding
posts also fastened to these ends of the wire so that the wire may be
made part of the circuit where we want to know the electron stream. Then
the same stream of electrons will flow through the fine wire as through
the other parts of the circuit. Because the wire is fine it acts like a
very narrow sidewalk for the stream of electrons and they have to bump
and jostle pretty hard to get through. That's why the wire gets heated.

You know that a heated wire expands. This wire expands. It grows longer
and because it is held firmly at the ends it must bow out at the center.
The bigger the rate of flow of electrons the hotter it gets; and the
hotter it gets the more it bows out. At the center we might fasten one
end--the short end--of a little lever. A small motion of this short end
of the lever will mean a large motion of the other end, just like a
"teeter board" when one end is longer than the other; the child on the
long end travels further than the child on the short end. The lever
magnifies the motion of the center of the hot wire part of our meter so
that we can see it easier.

[Illustration: Fig 10]

There are several ways to make such a meter. The one shown in Fig. 10 is
as easy to understand as any. We shape the long end of the lever like a
pointer. Then the hotter the wire the farther the pointer moves.

If we could put this meter in an electric circuit where we knew one
ampere was flowing we could put a numeral "1" opposite where the pointer
stood. Then if we could increase the current until there were two
amperes flowing through the meter we could mark that position of the
pointer "2" and so on. That's the way we would calibrate the meter.
After we had done so we would call it an "ammeter" because it measures
amperes. Years ago people would have called it an "amperemeter" but no
one who is up-to-date would call it so to-day.

[Illustr