*Leclanche Cell.* One of the most common forms of cell employed for bell-ringing, telephones, etc., is called the Leclanche cell, after its inventor, and consists of two elements, one of zinc and the other of carbon, immersed in a solution of _sal ammoniac_ or _ammonium chloride_. This cell has an E. M. F. of 1.4 volts, which is about half as much again as the voltaic cell. [Illustration: Fig. 55.—Carbon-Cylinder Cell, and Cylinder.] The most common form of Leclanche cell is illustrated in Figure 55. This type is usually known as a "carbon cylinder" cell because the positive element is a hollow carbon cylinder. The zinc is in the form of a rod passing through a porcelain bushing set in the center of the carbon cylinder. A battery of such cells can only be used successfully for open circuit work. The "open circuit" is used for bells, burglar alarms, telephone circuits, etc., or wherever the circuit is such that it is "open" most of the time and current is only drawn occasionally and then only for short periods. If the current is drawn for any appreciable length of time hydrogen gas will collect on the carbon cylinder and the cell will become _polarized_. When polarized it will not deliver much current. Many methods have been devised for overcoming this difficulty, but even the best of them are only partially successful. The usual method is to employ a chemical _depolarizing_ agent. Figure 56 shows a Leclanche cell provided with a _depolarizer_. The carbon is in the form of a plate placed in a _porous cup_ made of earthenware and filled with _manganese dioxide_. Chemists class _manganese dioxide_ as an _oxidizing_ agent, which means that it will furnish oxygen with comparative ease. Oxygen and hydrogen have a strong _chemical affinity_ or attraction for each other. [Illustration: Fig. 56.—A Leclanche Cell, showing the Porous Cup.] If the carbon plate is packed in manganese dioxide any hydrogen which tends to collect on the carbon and polarize the cell is immediately _seized_ by the oxygen of the manganese dioxide and united with it to form water. This form of Leclanche cell is called the disk type. It is capable of delivering a stronger current for a longer period of time than the carbon cylinder battery. The zinc is usually made in the form of a cylinder, and fits around the outside of the porous cup. *Dry Cells* are used extensively nowadays for all open circuit work on account of their convenience and high efficiency. The dry cell is not, as its name implies, "dry," but the exciting agent or electrolyte, instead of being a liquid, is a wet paste which cannot spill or run over. The top of the cell is poured full of molten pitch, thus effectively sealing it and making it possible to place the cell in any position.