railroad signaling, forklift, and standby power applications. Nickel–iron cells were made with capacities from 5 to 1250 Ah. Many of the original manufacturers no longer make nickel iron cells, but production by new companies has started in several countries. Plates The active material of the battery plates is contained in a number of filled tubes or pockets, securely mounted in a supporting and conducting frame or grid. The support is in good electrical contact with the tubes. The grid is a light skeleton frame, stamped from thin sheet steel, with reinforcing width at the top. The grids, as well as all other internal metal surfaces, are nickel-plated to prevent corrosion. The elements must remain covered with electrolyte; if they dry out, the negative plates oxidize and require a very long charge. The active material of the positive plates is a form of nickel hydrate. The tube retainers are made of thin steel ribbon, finely perforated and nickel-plated, about 4 in. long and 1/4 in. and 1/8in. in diameter. The ribbon is spirally wound, with lapped seams, and the tubes reinforced at about 1/2 in. intervals with small steel rings. Into these tubes nickel hydrate and pure flake nickel are loaded in thin, alternating layers (about 350 layers of each to a tube) and are tightly packed or rammed. The purpose of the flake nickel is to make good contact between the nickel hydrate and the tubes, and thereby provide conductivity. The tubes, when filled and closed, are then mounted vertically into the grids. The active material of the negative plates is iron oxide. The retainer pockets are made of thin, finely perforated nickel-plated steel, of rectangular shape, 1/2 in. wide, 3 in long and 1/8 in. maximum thickness. The iron oxide, in finely powdered form is tightly rammed into these pockets, after which they are mounted into the grids. After mounting they are pressed, forcing them into close contact with the grids. This corrugates the sides of the pockets to provide a spring contact of the pocket with the active material. Charge Charge/discharge involves the transfer of oxygen from one electrode to the other (from one group of plates to the other). Hence this type of cell is sometimes called an oxygenlift cell. In a charged cell the active material of the positive plates is superoxidized, and that of the negative plates is in a spongy or reduced state. If