283.  Electrical Terms.  The plates or strips used in making an electric cell are called electrodes; the zinc is called the negative electrode (-), and the carbon the positive electrode (+); the current is considered to flow through the wire from the + to the-electrode.  As a rule, each electrode has attached to it a binding post to which wires can be quickly fastened.

The power that causes the current is called the electromotive force, and the value of the electromotive force, generally written E.M.F., of a cell depends upon the materials used.

When the cell consists of copper, zinc, and dilute sulphuric acid, the electromotive force has a definite value which is always the same no matter what the size or shape of the cell.  But the E.M.F. has a decidedly different value in a cell composed of iron, copper, and chromic acid.  Each combination of material has its own specific electromotive force.

284.  The Disadvantage of a Simple Cell.  When the poles of a simple voltaic cell are connected by a wire, the current thus produced slowly diminishes in strength and, after a short time, becomes feeble.  Examination of the cell shows that the copper plate is covered with hydrogen bubbles.  If, however, these bubbles are completely brushed away by means of a rod or stick, the current strength increases, but as the bubbles again gather on the + electrode the current strength diminishes, and when the bubbles form a thick film on the copper plate, the current is too weak to be of any practical value.  The film of bubbles weakens the current because it practically substitutes a hydrogen plate for a copper plate, and we saw in Section 282 that a change in any one of the materials of which a cell is composed changes the current.

This weakening of the current can be reduced mechanically by brushing away the bubbles as soon as they are formed; or chemically, by surrounding the copper plate with a substance which will combine with the free hydrogen and prevent it from passing onward to the copper plate.

[Illustration:  FIG 198.  The gravity cell.]

In practically all cells, the chemical method is used in preference to the mechanical one.  The numerous types of cells in daily use differ chiefly in the devices employed for preventing the formation of hydrogen bubbles, or for disposing of them when formed.  One of the best-known cells in which weakening of the current is prevented by chemical means is the so-called gravity cell.

285.  The Gravity Cell.  A large, irregular copper electrode is placed in the bottom of a jar (Fig. 198), and completely covered with a saturated solution of copper sulphate.  Then a large, irregular zinc electrode is suspended from the top of the jar, and is completely covered with dilute sulphuric acid which does not mix with the copper sulphate, but floats on the top of it like oil on water.  The hydrogen formed by the chemical action of the dilute sulphuric