Now let us see what takes place in the Edison alkaline storage battery. If an electric current from a dynamo or other source of electricity is caused to pass through the positive to the negative plate the oxygen present in the iron oxide passes to and remains with the nickel oxide. During all the time this is going on the battery is said to be "charging," and when all the oxygen has been removed from the iron oxide and is taken up by the nickel oxide, then the battery is said to be "charged," and the flow of current into the battery is stopped. A change has now taken place. The powder in the negative plate is no longer iron oxide, but has been reduced to metallic iron, because the oxygen has been removed. The powder in the positive plate is now raised to a higher or super oxide of nickel, because it has taken the oxygen that was in the iron. But the nickel oxide will readily give up its excess of oxygen, and the iron will receive it back freely if permitted. If the proper conditions are established, this transfer of oxygen will take place, but the iron cannot receive it without delivering energy. [Illustration: Fig. 38] The proper conditions are established by providing a conducting circuit between the two elements, in which lamps, motors, or other electrical devices are placed. As soon as this circuit is provided, the opportunity is given to the iron to receive the oxygen. This it does, and in so doing develops electrical energy. This energy is in the form of electric current which is then delivered by the battery on what is called the "discharge," and this current may be used for lighting lamps or for operating motors or other electrical devices. The battery is said to be discharging as long as the iron is receiving oxygen from the nickel oxide. As soon as it becomes iron oxide once more, the giving out of energy ceases and the battery is said to be "discharged," and must again be charged to obtain further work from it. Such a battery can be charged and discharged an indefinite number of times. This type of battery is very rugged, and its combinations are not self-destructive. It is very simple, as it provides chiefly for the movement of the oxygen back and forth; besides, it gives much more current for its weight than the lead type of storage battery. (Fig. 38 shows the plates of a standard Edison cell removed from container.) CONNECTING STORAGE BATTERIES On the discharge, one cell of a lead storage battery gives an average of about 2 volts, and a cell of alkaline storage battery about 1.2 volts, no matter what its size or the number of plates may be. When there are more than two plates in one cell, all the positives in that cell are connected together by metallic strips or bands, and all negatives in the cell are connected together in a similar way.