These are not really dry, for between the plates they are filled with a moist paste which is then sealed in with wax to keep it from drying out or from spilling. Instead of zinc and copper these batteries use zinc and carbon. No glass jar is needed, for the zinc is formed into a jar shape. In this is placed the paste and in the center of the paste a rod or bar of carbon. The paste doesn't contain sulphuric acid, but instead has in it a stuff called sal ammoniac; that is, ammonium chloride. The battery, however, acts very much like the one I described in my last letter. Ions of zinc leave the zinc and wander into the moist paste. These ions are positive, just as in the case of the gravity battery. The result is that the electrons which used to associate with a zinc ion to form a zinc atom are left in the zinc plate. That makes the zinc negative for it has more electrons than protons. The zinc ions take the place of the positive ions which are already in the paste. The positive ions which originally belonged with the paste, therefore, move along to the carbon rod and there get some electrons. Taking electrons away from the carbon leaves it with too many protons; that is, leaves it positive. In the little flash light batteries, therefore, you will always find that the round carbon rod, which sticks out of the center, is positive and the zinc casing is negative. The trouble with the battery like the one I used to make is that the zinc plate wastes away. Every time a zinc ion leaves it that means that the greater part of an atom is gone. Then when the two electrons which were left behind get a chance to start along a copper wire toward the positive plate of the battery there goes the rest of the atom. After a while there is no more zinc plate. It is easy to see what has happened. All the zinc has gone into solution or been "eaten away" as most people say. Dry batteries, however, don't stop working because the zinc gets used up, but because the active stuff in the paste, the ammonium chloride, is changed into something else. There's another kind of battery which you will need to use with your radio set; that is the storage battery. Storage batteries can be used over and over again if they are charged between times and will last for a long time if properly cared for. Then too, they can give a large current, that is, a big swift-moving stream of electrons. You will need that when you wish to heat the filament of the audion in your receiving set. The English call our storage batteries by the name "accumulators." I don't like that name at all, but I don't like our name for them nearly as well as I do the name "reversible batteries." Nobody uses this last name because it's too late to change. Nevertheless a storage battery is reversible, for it will work either way at an instant's notice.