the nitric acid escapes from the porous pot and attacks the zinc directly.
One element of a secondary battery, or accumulator, consists of a so-called positive plate and a so-called negative plate immersed in dilute sulphuric acid. The positive plate consists of a leaden framework or grid filled up with electrically produced peroxide of lead. The negative plate consists of a corresponding leaden framework filled up with porous or spongy lead, also produced electrically. The positive plate corresponds to the carbon plus the nitric acid, the negative plate corresponds to the zinc in the primary battery. When the positive plate is connected by a conductor—say through a motor to the negative plate—an electric current passes from the one to the other, and the battery discharges. Instead, however, of the negative plate dissolving as in the case of a primary battery, the spongy lead becomes converted into sulphate without dissolving. The hydrogen gas which would appear at the surface of the positive plate is oxidised by the lead peroxide and reduces it. The electric pressure registered by a voltmeter arranged in the external circuit and connected to the positive and negative plates is a little over two volts when the accumulator is freshly charged. When the accumulator has given a certain amount of current for a certain length of time, a large proportion of the spongy lead on the negative plate becomes covered with sulphate of lead, and a large proportion of the lead peroxide on the positive plate becomes reduced. The electric pressure which the cell furnishes becomes diminished, and the accumulator is said to be discharged. The accumulator should under no circumstances be discharged after a voltmeter connected from plate to plate shows that the voltage has sunk to 1·75 volt per cell.
When a primary battery is discharged it cannot be used again except by renewing the materials, but an accumulator when it is discharged, by having furnished current for a certain prolonged period, can be charged—that is to say, restored to its original condition—by forcing the current through it the wrong
