=Action of a Primary Cell.=--The fundamental fact on which the electro-chemical generation of current depends is, that if a plate of metal be placed in a liquid there is a difference of electrical condition produced between them of such sort that the metal either takes a lower or higher electrical potential than the liquid, according to the nature of the metal and the liquid. If two different metals be placed in one electrolytic liquid, then there is a difference of state produced between them, so that, if joined by wire outside the liquid, a current of electricity will traverse the wire. This current proceeds in the liquid from the metal which is most acted upon chemically to that which is least acted upon. Referring to fig. 41, the construction and action of a simple primary cell may be briefly described as follows: Place in a glass jar some water having a little sulphuric or other acid added to it. Place in it separately two clean strips, one of zinc, Z, and one of copper, C. This cell is capable of supplying a continuous flow of electricity through a wire whose ends are brought into connection with the two strips. When the current flows, the zinc strip is observed to waste away, its consumption in fact furnishing the energy or electromotive force required to drive the current through the cell and the connecting wire. The cell may therefore be regarded as a kind of chemical furnace in which the fuel is the zinc. [Illustration: FIG. 40.--Simple primary cell. It consists of two dissimilar metal plates (such as copper and zinc which are called the _elements_), immersed in the _electrolyte_ or exciting fluid contained in the glass jar.] =Ques. How are the positive and negative elements of a primary cell distinguished?= Ans. The plate attacked by the electrolyte is the negative element, and the one unattacked the positive element. =Chemical Changes; Polarization.=--The chemical changes which take place in a simple cell, consisting of zinc and copper elements in an electrolyte of dilute sulphuric acid, may be briefly described as follows: When the two elements are connected and the current commences to flow, the sulphuric acid acts on the surface of the zinc plate and forms sulphate of zinc. The formation of this new substance necessitates the liberation of some of the hydrogen contained in the sulphuric acid, and it will be found that bubbles of free hydrogen gas speedily appear on the surface of the negative element, that is, on the copper plate. While the zinc is being dissolved to form zinc sulphate, hydrogen gas is liberated from the sulphuric acid. [Illustration: FIG. 41.--Simple primary cell with circuit closed, showing direction of the current.] Some bubbles of the gas rise to the surface of the electrolyte and so escape into the air, _but much of it clings to the surface of the copper element which thus gradually becomes covered with a thin film of hydrogen_.