The plates should then be assembled in groups of three, as shown in Figure 40. The positive plate is placed in the centre with a separator on either side. Two negative plates are then placed on the outside. The lugs on the negative plates should come opposite to each other. A square lead block having a hole bored through the centre may be placed between the two negative lugs. The lugs are then clamped together with a binding post and a screw. The plates are held in a compact bundle by two heavy rubber bands passing around them. Each group of plates is then placed in its proper jar and the jar filled full of a mixture composed of: 1. Four parts of water, and 2. One part of sulphuric acid. The plates are now ready for forming. The cells are connected in series by leading a wire from the negative of one to the positive of another and so on. The terminals of the battery are then connected to a steady source of direct current of at least ten volts. The positive pole of the battery should be connected to the positive of the current source and the negative to the negative. The source of current may be (1) the 110 volt D. C. supply in series with a lamp bank as described in Chapter IV; (2) the 110 volt A. C. supply after it has passed through a rectifier; (3) another battery, or (4) a shunt wound dynamo. [Illustration: FIG. 42.—An empty Storage Cell Grid and also a Pasted Plate both of which are on the market for experimenters who wish to build their own Cells.] The current passed through the storage cells during the forming process should be about one ampere for cells of the size described above. As soon as the positive plates of the storage cells have changed to a dark chocolate-brown color and the negatives to a gray-slate, disconnect the storage battery from the source of current and proceed to use it just as you would any ordinary battery. Use it until it is exhausted and then connect to the charging current again, taking care to make certain that the positive pole of the battery is connected to the positive pole of the current source. After the cells have been recharged and discharged in this manner about ten times they will be completely "formed" and ready for permanent service. Complete directions for recharging storage cells and instructions for their care and maintenance will be found further on. The only objection to the storage cells just described is that the paste is liable to fall out of the plates in time. The plates or "grids" as they are called used in commercial storage cells are cast in elaborate moulds which make it possible to overcome this difficulty. Such grids cannot however be made by the experimenter. Jars, pasted plates and empty grids may be purchased from well known firms dealing in apparatus for the experimenter, and with their aid it is possible to construct a very substantial and durable storage cell at home. The empty grids or fully formed plates may be purchased in the following sizes: