a unit in which we measure the
speed of trains or automobiles.

If you wanted to find the weight of something you would take a scale and
weigh it, wouldn't you? You might take that spring balance which hangs
out in the kitchen. But if the spring balance said the thing weighed
five pounds how would you know if it was right? Of course you might take
what ever it was down town and weigh it on some other scales but how
would you know those scales gave correct weight?

The only way to find out would be to try the scales with weights which
you were sure were right and see if the readings on the scale correspond
to the known weights. Then you could trust it to tell you the weight of
something else. That's the way scales are tested. In fact that's the way
that the makers know how to mark them in the first place. They put on
known weights and marked the lines and figures which you see. What they
did was called "calibrating" the scale. You could make a scale for
yourself if you wished, but if it was to be reliable you would have to
find the places for the markings by applying known weights, that is, by
calibration.

How would you know that the weights you used to calibrate your scale
were really what you thought them to be? You would have to find some
place where they had a weight that everybody would agree was correct and
then compare your weight with that. You might, for example, send your
pound weight to the Bureau of Standards in Washington and for a small
payment have the Bureau compare it with the pound which it keeps as a
standard.

That is easy where one is interested in a pound. But it is a little
different when one is interested in an ampere. You can't make an ampere
out of a piece of platinum as you can a standard pound weight. An ampere
is a stream of electrons at about the rate of six billion billion a
second. No one could ever count anywhere near that many, and yet
everybody who is concerned with electricity wants to be able to measure
currents in amperes. How is it done?

First there is made an instrument which will have something in it to
move when electrons are flowing through the instrument. We want a meter
for the flow of electrons. In the basement we have a meter for the flow
of gas and another for the flow of water. Each of these has some part
which will move when the water or the gas passes through. But they are
both arranged with little gear wheels so as to keep track of all the
water or gas which has