ation: Fig 11]

If we had a very carefully made ammeter we would send it to the Bureau
of Standards to be calibrated. At the Bureau they have a number of
meters which they know are correct in their readings. They would put one
of their meters and ours into the same circuit so that both carry the
same stream of electrons as in Fig. 11. Then whatever the reading was on
their meter could be marked opposite the pointer on ours.

Now I want to tell you how the physicists at the Bureau know what is an
ampere. Several years ago there was a meeting or congress of physicists
and electrical engineers from all over the world who discussed what they
thought should be the unit in which to measure current. They decided
just what they would call an ampere and then all the countries from
which they came passed laws saying that an ampere should be what these
scientists had recommended. To-day, therefore, an ampere is defined by
law.

To tell when an ampere of current is flowing requires the use of two
silver plates and a solution of silver nitrate. Silver nitrate has
molecules made up of one atom of silver combined with a group of atoms
called "nitrate." You remember that the molecule of copper sulphate,
discussed in our third letter, was formed by a copper atom and a group
called sulphate. Nitrate is another group something like sulphate for it
has oxygen atoms in it, but it has three instead of four, and instead of
a sulphur atom there is an atom of nitrogen.

When silver nitrate molecules go into solution they break up into ions
just as copper sulphate does. One ion is a silver atom which has lost
one electron. This electron was stolen from it by the nitrate part of
the molecule when they dissociated. The nitrate ion, therefore, is
formed by a nitrogen atom, three oxygen atoms, and one extra electron.

If we put two plates of silver into such a solution nothing will happen
until we connect a battery to the plates. Then the battery takes
electrons away from one plate and gives electrons to the other. Some of
the atoms in the plate which the battery is robbing of electrons are
just like the silver ions which are moving around in the solution.
That's why they can go out into the solution and play with the nitrate
ions each of which has an extra electron which it stole from some silver
atom. But the moment silver ions leave their plate we have more silver
ions in the solution than we do sulphate ions.

The only thing that can happe