There is a second important effect of the acid solution or electrolyte in a cell. If pure sulphuric acid were used, the first action or production of an electrical charge on the zinc plate would be the same, but when the plates were joined by the wire the current would soon cease. The reason for this lies in the fact that the sulphate of zinc, which is the compound produced by the acid plus the zinc, being insoluble in pure undiluted sulphuric acid, remains on the surface of the zinc plate. The coating of sulphate of zinc thus formed also operates as a protective agent, and no further electrical charge can be induced until it is removed. The addition of water to the acid has the effect of allowing the sulphate of zinc to dissolve, and the zinc plate is left free for further action. =Ques. What governs the rate of current flow of a primary cell?= Ans. The size of the elements and their proximity. =Effects of Polarization.=--The film of hydrogen bubbles affects the strength of the current of the cell in two ways: 1. It weakens the current by the increased _resistance_ which it offers to the flow, for bubbles of gas are bad conductors; 2. It weakens the current by setting up an opposing _electromotive force_. Hydrogen is almost as oxidizable a substance as zinc, especially when freshly deposited (in the “nascent” state), and is electro-positive; hence, the hydrogen itself produces a difference of potential, which would tend to start a current in the opposite direction to the true zinc-to-copper current. It is therefore an important matter to abolish this polarization, otherwise the currents furnished by batteries would not be constant. =Methods of Depolarizing.=--One of the chief aims in the arrangement of the numerous cells which have been devised is to avoid polarization. The following are the methods usually employed: 1. Chemical methods; _a_. Oxidation of the hydrogen by potassium bichromate and by nitric acid. _b_. Substitution of the hydrogen by some other substance which does not give a counter electromotive force of polarization; for instance, in the Daniell cell by replacement of the copper in copper sulphate by the hydrogen, the copper being deposited on the positive pole. 2. Electro-chemical means; It is possible by employing double cells, to secure such action that some solid metal, such as copper, shall be liberated instead of hydrogen bubbles, at the point where the current leaves the liquid. This electro-chemical exchange obviates polarization. 3. Mechanical methods. _a_. Agitation of the liquid or of the positive electrode, in order to prevent the accumulation of hydrogen thereon. _b_. Corrugating or roughing the positive electrode, as in the Smee cell. This causes the hydrogen gas to form in large bubbles which rise to the surface more rapidly than the small bubbles which form on a smooth electrode.