Electricity may be generated by chemical agencies. When any two different metals, such as zinc and copper, are placed in an acid or solution and wires are attached to them and connected, a current of electricity flows through the wire. This arrangement of metals in a liquid is called a cell. When the wires are not connected, bubbles of hydrogen collect around the zinc plate, but the moment the wires are connected, the hydrogen gas begins to appear on the copper plate.
Commercial zinc contains a great many impurities, such as iron and carbon. Little circuits are set up between the zinc and carbon impurities; hence the bubbles which appear on the zinc when it is immersed in the acid. This bubbling may be avoided by amalgamating the zinc, i.e., by covering it with mercury so that the zinc is used up only when the current is flowing.