The voltaic battery, in its simplest form, is a trough containing an acid liquid in which pairs of plates of different metals are immersed. It is evident that if the action of the acid on each metal were precisely the same, equal quantities of each would be dissolved in the acid, and the equilibrium of chemical energies would not be affected. But, the action being different, this equilibrium is disturbed, and if the sum of these disturbances for a number of separate pairs of plates can be accumulated, it will become considerable. This is done by connecting the plates of the same metal in each cell by a metallic wire covered by some non-conducting substance. There are, therefore, two wires, one to the right hand, the other to the left, the loose extremities of which are called the poles of the battery. If we test these poles as we did the glass rod and stick of sealing-wax, we find that one pole is charged with positive and the other with negative electricity. In other words, the chemical energy, whose equilibrium was disturbed by the unequal action of the acid on the plates of different metals, has been transformed into electrical energy manifesting itself, as it always does, under the condition of two equal and opposite polarities. If we connect these two poles with one another the two electricities rush together and unite, and there is established what is called an electrical current circulating round the battery. As the chemical action of the acid on the metals is not momentary but continuous, the acid taking up molecule after molecule of the metal, so also the current is continuous. When we call it a current, the term is used for the sake of convenience, for as the current, as we shall presently see, will flow along the wire or other conducting substance for immense distances, as across the Atlantic, with a velocity of many thousands of miles per second, we can, no more than in the case of light, figure it to ourselves as an actual transfer of material particles swept along as by a river running with this enormous velocity, but necessarily as a transmission of some form of motion travelling by waves or tremors through the all-pervading ether in which the atoms of the conducting wire are floating. Be this as it may, the effect of these electric currents is very varied and very energetic. It can produce intense heat, for if, instead of uniting the two poles, we connect them by a thin platinum wire, it will, in a few seconds, become heated to redness. If the connecting wire is thicker, heat will equally be generated but less intense, thus maintaining the analogy to the current which rushes with more impetuosity through a narrow than through a wide channel. If the poles are tipped with a solid substance like carbon, whose particles remain solid under great heat, when they are brought nearly together intense light is produced and the carbon slowly burns away. This produces what is called the arc light, which gives such a strong illuminating