1.--By _chemical action_. 2.--By _friction_. _By Chemical Action._--_Chemical action_ is the chief source of free electricity, the representative of which is the galvanic, or Voltaic, battery. The electricity so generated is also termed dynamical electricity, due to there being a constant electric current, so long as the poles of the battery producing it are kept closed; the electricity being thus in a _dynamic_ or moving state. By chemical action is signified that which occurs when two or more substances so act upon one another as to produce a third substance differing altogether from the original ones in its properties, or when one substance is brought under such conditions that it forms two or more bodies differing from the original ones in their properties. _Definition and Properties of a Voltaic Cell._--The _Voltaic_ cell consists of an insulating jar, containing a liquid, in which are placed two plates or pieces of dissimilar metals; the liquid must be composed of two or more chemical elements, one of which at least tends to combine with one or other of the metals, or _with both in different degrees_. By a Voltaic _battery_ is meant a number of cells above one; this term, however, is often applied to a single cell when working by itself. A "_simple_ Voltaic cell," "element," or "couple," consists of two metals placed in a conducting liquid. If two metals--for instance, zinc and copper--are placed in water slightly acidulated, without touching each other, no effect is apparent; but if they be made to touch, bubbles of hydrogen gas are formed over the copper plate, and continue forming these until the plates are separated. After being in contact for some time, the copper plate will be found unaltered in weight, but the zinc plate will have lost weight, and the portion so lost will be found in the liquid in the form of sulphate of zinc. The same effects are also produced by connecting the two plates by means of some conducting substance, instead of placing them in contact. Zinc is invariably employed as one of the metal plates, on account of the ease with which it dissolves in dilute acids; and the greatest results are obtained when the second metal plate is not acted upon at all by the liquid, for then the whole effect due to the oxidation of the zinc plate is obtained; but when the second plate is also chemically acted upon, then only the effect due to the difference between the two chemical actions is obtained, for, as will be explained further on, they each act in directly opposite directions. _Voltaic Current._--The Voltaic current makes its appearance under the general laws of electrical action.