The box plates are used almost entirely where long life is the main consideration, but the greater weight of grid, with consequent greater cost, has caused them for most purposes to be superseded by the Exide type.
The grids of the Exide type are castings of antimony-lead alloy, made as light as possible consistent with necessary mechanical and elec trical conductivity. The box grids are com posite, consisting of perforated lead sheets, upon which are cast intersecting ribs, or bars, of antimony-lead alloy, which gave the necessary strength; and each plate consists of two parts riveted together, with the active material en closed between them.
in place so as to be both retained and protected, while Fig. 8 shows the finished plate, the active material filling all the interior spaces. This type of plate is used in sizes from about three square inches surface up to 700, in thickness from three-thirty-seconds to one-fourth inch, and for every variety of purpose for which lead batteries are employed.
Box Negative.— For some purposes, how ever, it is desirable to have a plate which will give the maximum possible life, consequent in creased weight being a secondary consideration. To meet this requirement the °box)) negative While any oxide of lead may be used for the active material of negative plates, a long process of elimination has finally resulted in the universal adoption of litharge PbO, for this purpose, applied as a putty-like mass, made by mixing the litharge with dilute sulphuric acid. An initial charge, or converts the litharge into metallic sponge lead, giving the plate a characteristic light slate color.
Plante Negative.—There are still a few Plante negatives used, in this country only, such a one being shown in Fig. 10, but their weight and cost are against them, and they are to-day almost a thing of the past.
Positive Plates.— The positive plate offers a more complicated problem, chiefly because lead peroxide, under the conditions which exist in a storage battery, does not possess much sustained mechanical strength, but gradually becomes soft, and in time is loosened from the plate, falls to the bottom of the cell and thence forward plays no useful part in the life of the battery.
To maintain the capacity over an extended period of charges arid discharges three distinct methods are employed.
in its design than the negative, in order to withstand the greater corrosion and mechanical stresses to which it is subject. While various oxides, or mixtures of such, are used by differ Pasted Positive.—In the pasted type of plate, when new, only a part of the lead peroxide is available for entering into the chemical re actions of the cell, as the acid penetrates only partially into the interior of the mass. As the surface wears away the action penetrates far ther in, and thus the inner part of the mass acts as a reserve and maintains the capacity for a number of cycles, roughly stated at 300 to 500. By making the plate very thin, say of the order of one-eighth inch, the reserve of active material is cut down to a minimum, hence giving a plate of maximum capacity per unit of weight, but of relatively shorter life.
Positive plates of the pasted type are uni versally employed where lightness, or maximum capacity for given weight, is the primary con sideration, and the correct balance between thick, heavy plates of long life and thin, light ones of shorter life, to meet special conditions, is a continual problem to the designer. The general trend of modern practice, however, is toward much thinner plates than were thought practicable a few years ago, a thickness of three-thirty-seconds inch being quite common.
The Exide type of pasted plate, Fig. 11, which has become standard in the United States, and largely so in Europe, is very similar to the negative plate, except as to its active material. The grid, Fig. 12, is, however, more substantial ent manufacturers as positive active material, the usual material is red lead, PhD., mixed to a putty-like mass with weak sulphuric acid, or ammonium sulphate solution. The initial charge, or formation, converts this into lead peroxide, a material which when dry has about the consistency of plaster of Paris and a very characteristic dark chocolate brown color.