In the simplest form of cell, as zinc, copper, and dilute sulphuric acid, no attempt has been made to prevent the evil of polarization, hence, it will quickly polarize when the current is closed for any length of time, and may be classified as an open circuit cell. When polarization is remedied by chemical means, the chemical added is one that has a strong affinity for hydrogen and will combine with it, thus preventing the covering of the negative plate with the hydrogen gas. [Illustration: FIGS. 42 and 43.--Carbon cell and carbon cylinder. Carbon possesses a natural power to prevent a limited amount of polarization by absorbing the hydrogen gas coming from the zinc rod; hence it is used in various shapes for open circuit cells, which gives rise to as many different names, such as _Samson_, _Hercules_, _Law_, _National_, _Standard_, etc. In all these types of cell, sal-ammoniac and zinc are used, and by corrugating the carbon, fluting it, or making concentric cylinders, special merits are obtained in each case. The carbon element is usually made in the form of a porous cup, filled with oxide of manganese to prevent polarization, and then sealed. The zinc rod is inserted through a porcelain insulator. About 4 to 6 ounces of sal-ammoniac are generally used for cells of ordinary size. The salt is placed in the jar, water poured in until it is about two-thirds full, and then stirred till all the salt is dissolved. When the carbon cylinder is inserted, the solution should be within 1-1/2 inches of the top of the jar. The electromotive force is from 1.0 to 1.4 volts for the different forms of carbon cell.] =Ques. What is a depolarizer?= Ans. A substance employed in some types of cell to combine with the hydrogen which would otherwise be set free at the positive electrode and cause polarization. The chemical used for this purpose may be either in a _solid_ or _liquid_ form, which gives rise to several types of cell, such as cells with a single fluid, containing both the acid and the depolarizer, cells with a single exciting fluid and a solid depolarizer, and cells with two separate fluids. In the two fluid cell, the zinc is immersed in the liquid (frequently dilute sulphuric acid) to be decomposed by the action upon it, and the negative plate is surrounded by the liquid depolarizer, which will be decomposed by the hydrogen gas it arrests, thereby preventing polarization. In _open circuit cells_ polarization does not have much opportunity to occur, since the circuit is closed for such a short period of time; hence, these cells are always ready to deliver a strong current when used intermittently. In _closed circuit cells_ polarization is prevented by chemical action, so that the current will be constant and steady till the energy of the chemicals is expended. =Ques. What is a depolarizer bag?=