Batteries that are capable of being operated only for short periods, on account of polarization, are called open circuit batteries. Open circuit batteries are cheaper and more simple than closed circuit batteries. For ignition purposes, a battery is made that is a compromise between the closed and open circuit cells, this being a battery in which the polarization is only partially suppressed. As the demand for current on an ignition battery is small with comparatively long rests between contacts, the compromise battery answers the purpose and is fairly cheap. All primary batteries are in reality wet batteries, for the reason that it would be impossible to cause a chemical reaction and a current with a dry electrolyte. The action of dry and wet batteries is identical. There are many types of wet battery in use for various purposes, but few of them are adapted for gas engine ignition because of a tendency to polarize or because of the cost of maintenance. All wet batteries are not suitable for portable or automobile engines because of the slopping of the liquid electrolyte and the danger of breaking the containing jars. Their weight and bulk is also a drawback. If the electrolyte or the electrodes be made of impure material local currents will be generated. These currents decrease the life of the cell without producing any useful current in the ignition circuit. Due to the deteriorating effects of the local currents, batteries standing idle for several months will often be found to be completely discharged and worthless without having done any useful work. In the better grade of cells this loss is reduced to a minimum. A type of wet battery using a solution of caustic soda for an electrolyte, and having zinc and copper oxide electrodes, is extensively used for stationary ignition purposes, and is the most satisfactory type of wet cell for continuous work with this class of engine. The caustic soda battery is of the =CLOSED= circuit type, and is capable of furnishing a strong uniform current without danger of polarization. The hydrogen bubbles which cause polarization are oxidized or eliminated by the copper oxide electrode as soon as they are formed. The hydrogen combines with the oxygen of the copper oxide forming water. The copper oxide is gradually reduced to metallic copper by the reaction with the hydrogen, and in the course of time requires renewal. The copper oxide element is rather expensive and cannot be obtained as readily as the electrodes used in other cells. It will be noted that both electrodes are consumed in the caustic battery, the consumption of the zinc furnishing the current, and the reducing of the oxide furnishing the chemical energy for depolarizing the cell. (81) Dry Batteries.