The positive plate of this accumulator is of the Planté type, but it is not simply a casting of pure lead, but is made by a building-up process which allows of the use of a lead-antimony mixture for the grids. This gives greater strength, and the grids themselves are unaffected by the chemical changes which take place during the charging and discharging of the cell. The active material, that is the material which undergoes chemical change, is pure lead tape coiled up into rosettes, which are so designed that the acid can circulate through the plates. These rosettes are driven into the perforations of the grid by a hydraulic press, and during the process of forming they expand and thus become very firmly fixed. The negative plate has a frame made in two parts, which are riveted together after the insertion of the active material, which is thus contained in a number of small cages. The plate is covered outside with a finely perforated sheet of lead, which prevents the active material from falling out. It is of the utmost importance that the positive and negative plates should be kept apart when in the cell, and in the Chloride accumulator this is ensured by the use of a patent separator made of a thin sheet of wood the size of the plates. Before being used the wood undergoes a special treatment to remove all substances which might be harmful, and it then remains unchanged either in appearance or composition. Other insulating substances, such as glass rods or ebonite forks, can be used as separators, but it is claimed that the wood separator is not only more satisfactory, but that in some unexplained way it actually helps to keep up the capacity of the cell. The plates are placed in glass, or lead-lined wood or metal boxes, and are suspended from above the dilute sulphuric acid with which the cells are filled. A space is left below the plates for the sediment which accumulates during the working of the cell. In all but the smallest cells several pairs of plates are used, all the positive plates being connected together and all the negative plates. This gives the same effect as two very large plates, on the principle of connecting in parallel, spoken of in Chapter IV. A single cell, of whatever size, gives current at about two volts, and to get higher voltages many cells are connected in series, as with primary cells. The capacity is generally measured in ampere-hours. For instance, an accumulator that will give a current of eight amperes for one hour, or of four amperes for two hours, or one ampere for eight hours, is said to have a capacity of eight ampere-hours.