The remarks that have been made on previous pages about connecting up batteries in series, multiple, and multiple series apply also to these Edison Primary Cells. Fig. 35 shows a battery of four of these cells connected in series. SECONDARY, OR STORAGE, BATTERIES The open and closed circuit batteries we have so far described are used to produce electricity by the action of the chemicals upon the elements contained in them. They are called primary batteries. [Illustration: Fig. 35] The batteries which we will now tell you of are called secondary, or storage, batteries, and do not of themselves make any primary current, but simply act as reservoirs, so to speak, to hold the energy of the electric current which is led into them from a dynamo or primary battery. At the proper time and under proper conditions these secondary batteries will give back a large percentage of the energy of the electric current which has been stored in them. This class of battery has been called by these three names: "secondary battery," "accumulator," and "storage battery"; but as the latter name is used almost exclusively in this country, we shall use it in the following description. TWO TYPES There are two distinct types of storage battery. One is called the "lead" or "acid" storage battery, and the other the "alkaline" or "nickel-iron" storage battery. Each of them simply acts as a reservoir to hold the energy of the electric current which is led into it, and each of them, under proper conditions, will give back that energy. As the lead storage battery is the oldest in point of discovery and invention, we will describe it first. THE LEAD STORAGE BATTERY A lead storage battery usually consists of a glass or hard-rubber jar containing lead plates and a solution consisting of water and sulphuric acid. A single unit is usually called a "cell." (Fig. 36.) There are always at least two lead plates in a storage-battery cell of this kind, although there may be any number above that. For the sake of making a clearer explanation to you, we will take as an illustration a cell containing only two plates.[3] [Illustration: Fig. 36] We have, then, a glass or hard-rubber jar containing two lead plates and a solution consisting of water and sulphuric acid. These plates are called the "elements," and one is called the positive and the other the negative element. The solution is called the "electrolyte." The positive element is a sheet of lead upon which is spread a paste made of red-lead. The negative element is a similar sheet of lead upon which is spread a paste made of litharge. Now, when these plates are thus prepared, they are put into the acid solution in the jar, and a wire attached to each plate is connected with the two wires from a dynamo or other source of electric current, just as a lamp would be connected.