The batteries we have been describing will do the ordinary work for which they are intended for sometimes a year without requiring any attention, but if we try to make them do work for which they were not intended, they would only last a few days. If we should want to operate electric lights or motors continuously from a battery we must, therefore, use CLOSED-CIRCUIT BATTERIES There is a great variety of ways in which closed-circuit batteries are made, but, as the main principles are very much alike, we will only describe two general kinds, those with and those without a porous cup.[2] In the first place, we must state that closed-circuit batteries proper usually consist of a glass jar and two elements--carbon and zinc. Sometimes a porous cup is used; for what reason you will soon learn. The chemicals that are used are usually different from those used in the open-circuit batteries and are much stronger. These chemicals are usually sulphuric acid and bichromate of potash (or chromic acid), which are mixed with water. We will now examine two of the types of closed-circuit batteries, taking first the one without the porous cup, of which the Grenet is a good example. [Illustration: Fig. 30] This battery, as you see, consists of a glass jar, in which are placed two plates of carbon and one of zinc. (Fig. 30.) The latter is between the two carbon plates and is movable up and down, so that it may be drawn up out of the solution when it is not desired to use the battery. When the zinc is in the solution there is a steady and continuous current of electricity developed, which can be taken away by wires from the connections on top of the battery. If the zinc were left in the solution when the battery was not in use, the acid would act upon it almost as much as though the electricity were not being used, and thus the zinc would be eaten away and the acid would be neutralized, so that no more action could be had when we wanted more electricity. Now, in the Grenet battery we can light a lamp or run a motor for several hours continuously, but at the end of that time the solution would become black and it would do no more work. Then we must throw out that solution and put in fresh, and the battery will do the same work again, and so on. If you should only want to light your lamp or run your motor for a few minutes, you could pull the zinc up from the solution and put it down again when you wanted the electricity once more. The carbon element in the battery is not consumed by the acid, although the zinc is. [Illustration: Fig. 31] Now you will see the use of the porous cup. We will take as an illustration of this type an ordinary battery in which a porous cup is used. (Fig. 31.)