_The Dry Battery._--The cleanliness, convenience, high efficiency, and comparatively low internal resistance of the dry cell has brought it into great favor in the last few years. It is now extensively used in preference to the Leclanché and other open-circuit batteries having liquid electrolyte for light work, such as bells, gas-lighting, burglar alarms, ignition on motor-boats, automobiles, etc. The dry cell is also used in great numbers for pocket flash-lamps, and in other ways where it would be impossible to employ batteries containing liquids. A dry cell consists of zinc, carbon, and the electrolyte, which is a mixture so made that it is in the form of a gelatinous or semi-solid mass, so that it will not run or slop over. A piece of sheet zinc is formed into a long tube, and a round, flat piece of zinc is soldered at one end, thus making a cup open at one end. This forms the cell itself, and at the same time becomes one of the elements. The other element is a piece of battery carbon which is long enough to project out of the top of the cell about half an inch or more. While the cell is being filled with the electrolyte the carbon is held up by a support so that it does not touch the zinc at the bottom of the cup. Of course, the zinc cup and the carbon are provided with proper binding-posts or other attachments, so that conducting wires can be connected. The electrolyte is packed into the cup and around the carbon in such a way that the cup is entirely filled within about half an inch from the top, and then some melted tar or pitch is poured over the top of the electrolyte. This seals the cell and binds the contents solidly together. Just before the sealing compound hardens, one or two holes are made in it so that the gases may escape. The composition of the electrolyte itself is not exactly alike in all dry cells, as the various manufacturers follow their own particular formulas. However, as you may be curious to know something about it, we would state that one formula embraces flour, water, plaster of Paris, granulated carbon, zinc chloride, ammonium chloride, and manganese binoxide. You will remember that the Leclanché and the dry batteries are purely open-circuit cells, and that they can be used to advantage for electric bells, annunciators, burglar alarms, gas ignition, etc., where _the current of electricity is not doing_ continuous work, but only for a few seconds at a time. Consequently, the batteries have a little rest in between, if only for a few seconds. Now, if we were to attempt to use open-circuit batteries for electric lights or motors, where the electricity must work constantly every second, the batteries would "polarize"--that is to say, they would only work a few minutes and then stop, because the chemicals used in them are of that kind that they will only allow the battery to do a little work at a time.