ss syringe having a short length of
rubber tubing at one end, and a large rubber bulb at the other. The
rubber tube is inserted in the cell and enough electrolyte drawn up
into the syringe to float the hydrometer so as to be able to obtain a
reading. This subject will be treated more fully in a later chapter.

Changes at Negative Plate. The charging current changes lead sulphate
into spongy lead, and acid is formed. The acid is mixed with the
diluted electrolyte outside of the plates. As the charging proceeds
the active material shrinks or contracts, and the weight of the plate
actually decreases on account of the difference between the weight and
volume of the lead sulphate and spongy lead. If the cell has had only
a normal discharge and the charge is begun soon after the discharge
ended, the charge will proceed quickly and without an excessive rise
in temperature. If, however, the cell has been discharged too far, or
has been in a discharged condition for some time, the lead sulphate
will not be in a finely divided state as it should be, but will be
hard and tough and will have formed an insulating coating over the
active material, causing the charging voltage to be high, and the
charge will proceed slowly. When most of the lead sulphate has been
reduced to spongy lead, the charging current will be greater than is
needed to carry on the chemical actions, and will simply decompose the
water into hydrogen and oxygen, and the cell "gasses." Spongy lead is
rather tough and coherent, it, and the bubbles of gas which form in
the pores of the negative plate near the end of the charge force their
way to the surface without dislodging any of the active material.

Changes at the Positive Plate. When a cell has been discharged, a
portion of the lead peroxide has been changed to lead sulphate, which
has lodged in the pores of the active material and on its surface.
During charge, the lead combines with oxygen from the water to form
lead peroxide, and acid is formed. This acid diffuses into the
electrolyte as fast as the amount of sulphate will permit. If the
discharge has been carried so far that a considerable amount of
sulphate has formed in the pores and on the surface of the plate, the
action proceeds very slowly, and unless a moderate charging current is
used, gassing begins before the charge is complete, simply because the
sulphate cannot absorb the current. The gas bubbles which originate in
the interior of the plate forc