A cell might be likened to a furnace in which the zinc is the fuel which is burned to furnish the energy. We know that if the zinc is burned or oxidized in the open air it will give out energy in the form of _heat_. When it is burned or oxidized slowly in acid in the presence of another metal it gives out its energy in the form of _electricity_. The acid might be likened to the fire, and the copper to a hand which dips into the cell to pick up the current and takes no part chemically. If a wire is connected to each of the plates and the free ends of the wires touched to the tip of the tongue it will produce a peculiar salty taste in the mouth indicating the presence of a current of electricity. If the wires are connected to an electric bell, the bell will ring, or, instead, the current may be used to run a small motor. If the cell is made of two zinc plates or two copper plates, the bell will not ring, because no electricity will be produced. In order to produce a current, the electrodes must be made of two different materials upon which the acid acts differently. Current may be obtained from a cell made with a zinc and carbon plate or from one with zinc and iron. Therefore, in order to make a battery it is necessary to have a metal which may be consumed, a chemical to consume or oxidize it, and an inactive element which is merely present to collect the electricity. When the wires connected to the two plates are joined together, a current of electricity will flow from the copper plate through the wire to the zinc. The copper is known as the _positive_ pole and the zinc as the _negative_. A simple voltaic cell may be easily made by cutting out a strip of zinc and a strip of copper, each 3 1/2 inches long, and one inch wide. A small hole bored through the upper end of the strips will permit them to be mounted on a wooden strip with a screw as shown in Figure 53. The connecting wires are placed under the heads of the screws. Care should be exercised to arrange the screws used for mounting the electrodes to the wooden strip so that they do not come exactly opposite, and there is no danger of the points touching and forming a short circuit. [Illustration: Fig. 53.—The Elements of Simple Voltaic Cell.] [Illustration: Fig. 54.—A Home-Made Voltaic Cell.] An ordinary tumbler or jelly glass will make a good battery jar. The exciting liquid should be composed of One part of sulphuric acid Ten parts of water One of the disadvantages of the voltaic cell is that it becomes _polarized_, that is, small bubbles of hydrogen which are liberated by the chemical action collect on the copper plate and cause the strength of the battery to fall off rapidly. There are a great number of _elements_, as the zinc and copper are called, and an even greater number of different solutions or _excitants_ which can be employed in place of sulphuric acid to make a cell, forming an almost endless number of possible combinations.