In the cells considered so far, it has been implied that the electrodes were in contact with ions in solution. The dry cell, shown on the right, is especially convenient because the solutions are replaced by a moist paste in a sealed container. The zinc casing is the anode, with the half-reaction

The cathode is a central carbon rod surrounded by a paste of manganese dioxide (MnO₂), ammonium chloride (NH₄Cl), and water. The paste and the zinc casing are separated only by a porous paper barrier. The cathode reaction is a complex one, but can be represented as

A dry cell delivers about 1.5 volts (0.76V + 0.75V). If the cell is used continuously, the current slowly decreases as ammonia gas builds up around the carbon rod and insulates it. If the cell is allowed to rest, this ammonia diffuses toward the anode and combines with zinc ions to form a complex ion, Zn(NH₃)₄²⁺ . The cell then is able to deliver a stronger current again. This is why flashlight batteries appear to run down with steady use, but to recover after standing idle for a time.