late circuit we must know something of how to measure currents.
An electric current is a stream of electrons. We measure it by finding
the rate at which electrons are traveling along through the circuit.

What do we mean by the word "rate?" You know what it means when a
speedometer says twenty miles an hour. If the car should keep going just
as it was doing at the instant you looked at the speedometer it would go
twenty miles in the next hour. Its rate is twenty miles an hour even
though it runs into a smash the next minute and never goes anywhere
again except to the junk heap.

It's the same when we talk of electric currents. We say there is a
current of such and such a number of electrons a second going by each
point in the circuit. We don't mean that the current isn't going to
change, for it may get larger or smaller, but we do mean that if the
stream of electrons keeps going just as it is there will be such and
such a number of electrons pass by in the next second.

In most of the electrical circuits with which you will deal you will
find that electrons must be passing along in the circuit at a most
amazing rate if there is to be any appreciable effect. When you turn on
the 40-watt light at your desk you start them going through the filament
of the lamp at the rate of about two and a half billion billion each
second. You have stood on the sidewalk in the city and watched the
people stream past you. Just suppose you could stand beside that narrow
little sidewalk which the filament offers to the electrons and count
them as they go by. We don't try to count them although we do to-day
know about how many go by in a second if the current is steady.

If some one asks you how old you are you don't say "About five hundred
million seconds"; you tell him in years. When some one asks how large a
current is flowing in a wire we don't tell him six billion billion
electrons each second; we tell him "one ampere." Just as we use years as
the units in which to count up time so we use amperes as the units in
which to count up streams of electrons. When a wire is carrying a
current of one ampere the electrons are streaming through it at the rate
of about 6,000,000,000,000,000,000 a second.

Don't try to remember this number but do remember that an ampere is a
unit in which we measure currents just as a year is a unit in which we
measure time. An ampere is a unit in which we measure streams of
electrons just as "miles per hour" is