Dry batteries are by far, the most convenient and economical form of primary battery to use, for there is no fluid to slop and leak, the first cost is low, the output is large for the weight, and last but not least, the cell can be thrown away when exhausted without great monetary loss. This does away with the expense and annoyance of changing wet cells, a factor that will be appreciated by those that are far from a source of chemical supplies. Since the advent of the automobile the use of dry cells has extended so that they may be obtained in almost any country town or village. While the cell is not dry, strictly speaking, the solution is held in such a way that it cannot slop around in the cell nor leak out of the seal. The only fault of a dry cell is its tendency to deteriorate with age because of the constant contact of the electrolyte with the electrodes. The negative electrode of the dry cell (zinc) is in the form of a cup which serves as a containing vessel for the electrolyte and the depolarizer. The electrolyte is usually composed of a solution of ammonium chloride, with a small percentage of zinc sulphate, this fluid being held by some absorbent material such as blotting paper, or paper pulp. The electrolyte is applied to the electrodes by means of the saturated blotting paper, which is also used to line the zinc container, thus providing insulation between the electrodes. A rod of solid carbon which forms the positive electrode is placed in the center of the container, and the space between the rod and the zinc is packed solidly with granulated carbon, the blotting paper lining preventing contact of the zinc with the carbon. Pulverized manganese dioxide is mixed with the granulated carbon for a depolarizer. [Illustration: Brookes Four Cylinder Gasoline Engine Direct Connected to Dynamo. ] After the zinc container is filled with the electrolyte and pulverized carbon, the top of the container is closed hermetically by means of sealing wax. Granulated carbon is used for it presents a large surface to the electrolyte, reduces the internal resistance of the cell, and therefore increases the current output of the battery. As soon as the battery starts generating current, polarization begins, with the liberation of hydrogen gas. If the cell is discharged at a high rate, the manganese dioxide will be unable to absorb all of the gas, and consequently pressure will be erected within the cell. The greater the rate of discharge, the greater will be the amount of hydrogen set free, and the higher the pressure. If a short circuit exists for any length of time, the pressure of the excess hydrogen will speedily ruin it, as the cell will puff up, or even burst under the pressure. If the rate of discharge be kept so low that all of the gas will be absorbed by the manganese, as soon as generated, the cell will furnish a steady current until the elements of the cell or the electrolyte are exhausted.