acid on the zinc moves toward the copper electrode, as in the simple voltaic cell.  It does not reach the electrode, however, because, when it comes in contact with the copper sulphate, it changes places with the copper there, setting it free, but itself entering into the solution.  The copper freed from the copper sulphate solution travels to the copper electrode, and is deposited on it in a clean, bright layer.  Instead of a deposit of hydrogen there is a deposit of copper, and falling off in current is prevented.

The gravity cell is cheap, easy to construct, and of constant strength, and is in almost universal use in telegraphic work.  Practically all small railroad stations and local telegraph offices use these cells.

[Illustration:  FIG. 199.—­A dry cell.]

286.  Dry Cells.  The gravity cell, while cheap and effective, is inconvenient for general use, owing to the fact that it cannot be easily transported, and the dry cell has largely supplanted all others, because of the ease with which it can be taken from place to place.  This cell consists of a zinc cup, within which is a carbon rod; the space between the cup and rod is packed with a moist paste containing certain chemicals.  The moist paste takes the place of the liquids used in other cells.

[Illustration:  FIG. 200.—­A battery of three cells.]

287.  A Battery of Cells.  The electromotive force of one cell may not give a current strong enough to ring a door bell or to operate a telephone.  But by using a number of cells, called a battery, the current may be increased to almost any desired strength.  If three cells are arranged as in Figure 200, so that the copper of one cell is connected with the zinc of another cell, the electromotive force of the battery will be three times as great as the E.M.F. of a single cell.  If four cells are arranged in the same way, the E.M.F. of the battery is four times as great as the E.M.F. of a single cell; when five cells are combined, the resulting E.M.F. is five times as great.

CHAPTER XXXI

SOME USES OF ELECTRICITY

288.  Heat.  Any one who handles electric wires knows that they are more or less heated by the currents which flow through them.  If three cells are arranged as in Figure 200 and the connecting wire is coarse, the heating of the wire is scarcely noticeable; but if a shorter wire of the same kind is used, the heat produced is slightly greater; and if the coarse wire is replaced by a short, fine wire, the heating of the wire becomes very marked.  We are accustomed to say that a wire offers resistance to the flow of a current; that is, whenever a current meets resistance, heat is produced in much the same way as when mechanical motion meets an obstacle and spends its energy in friction.  The flow of electricity along a wire can be compared to the flow of water through pipes:  a small pipe offers a greater resistance to the flow of water than a large pipe; less water can be forced through a small pipe than through a large pipe, but the friction of the water against the sides of the small pipe is much greater than in the large one.