on the surface of the plate than in the inner portions because there
is a constant supply of acid available at the surface, whereas the
formation of sulphate in the interior of the plate requires that acid
diffuse into the pores of the active materials to replace that already
used up in the formation of sulphate. In the negative plate, however,
the sulphate tends to form more uniformly throughout the mass of the
lead, because the spongy lead is more porous than the lead peroxide,
and because the acid is not diluted by the formation of water as in
the positive plate.

Changes at the Positive Plate. In a fully charged positive plate we
have lead peroxide as the active material. This is composed of lead
and oxygen. From this fact it is plainly evident that during discharge
there is a greater chemical activity at this plate than at the
negative plate, since we must find something to combine with the
oxygen in order that the lead may form lead sulphate with the acid.
In an ideal cell, therefore, the material which undergoes the greater
change should be more porous than the material which does not involve
as great a chemical reaction. In reality, however, the peroxide is not
as porous as the spongy lead, and does not hold together as well.

The final products of the discharge of a positive plate are lead
sulphate and water. The lead peroxide must first be reduced to lead,
which then combines with the sulphate from the acid to form lead
sulphate, while the oxygen from the peroxide combines with the
hydrogen of the acid to form water. There is, therefore, a greater
activity at this plate than at the lead plate, and the formation of
the water dilutes the acid in and around the plate so that the
tendency is for the chemical actions to be retarded.

The sulphate which forms on discharge causes the active material to
bulge out because it occupies more space than the peroxide. This
causes the lead peroxide at the surface to begin falling, to the
bottom of the jar in fine dust-like particles, since the peroxide here
holds together very poorly.

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CHAPTER 6.
WHAT TAKES PLACE DURING CHARGE.
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Voltage. Starting with a battery which has been discharged until its
voltage has decreased to 1.7 per cell, we pass a current through it
and cause the voltage to rise steadily. Fig. 24 shows the changes in
voltage during charge. Ordinar