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. [Illustration: 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.