Letters of a Radio-Engineer to His Son

by John Mills

I am not going to tell you very much about the storage battery but you ought to know a little about it if you are to own and run one with your radio set. When it is all charged and ready to work, the negative plate is a lot of soft spongy lead held in place by a frame of harder lead. The positive plate is a lead frame with small squares which are filled with lead peroxide, as it is called. This is a substance with molecules formed of one lead atom and two oxygen atoms. Why the chemists call it lead peroxide instead of just lead oxide I’ll tell you some other time, but not in these letters.

30Between the two plates is a wood separator to keep pieces of lead from falling down between and touching both plates. You know what would happen if a piece of metal touched both plates. There would be a short circuit, that is, a sort of a short cut across lots by which some of the electrons from the negative plate could get to the positive plate without going along the wires which we want them to travel. That’s why there are separators.

The two plates are in a jar of sulphuric acid solution. The sulphuric acid has molecules which split up in solution, as you remember, into hydrogen ions and the ions which we called “sulphate.” In my gravity battery the sulphate ions used to coax the zinc ions away into the solution. In the storage battery on the other hand the sulphate ions can get to most of the lead atoms because the lead is so spongy. When they do, they form lead sulphate right where the lead atoms are. They don’t really need whole lead atoms, because they have two more electrons than they deserve, so there are two extra electrons for every molecule of lead sulphate which is formed. That’s why the spongy lead plate is negative.

The lead sulphate won’t dissolve, so it stays there on the plate as a whitish coating. Now see what that means. What are the hydrogen ions going to do? As long as there was sulphuric acid in the water there was plenty of sulphate ions for them to associate with as often as they met; and they would meet pretty often. But if the sulphate ions get tied up 31with the lead of the plate there will be too many hydrogen ions left in the solution. Now what are the hydrogen ions to do? They are going to get as far away from each other as they can, for they are nothing but protons; and protons don’t like to associate. They only stayed around in the first place because there was always plenty of sulphate ions with whom they liked to play.

When the hydrogen ions try to get away from each other they go to the other plate of the battery, and there they will get some electrons, if they have to steal in their turn.

I won’t try to tell you all that happens at the other plate. The hydrogen ions get the electrons which they need, but they get something more. They get some of the oxygen away from the plate and so form molecules of water. You remember that water molecules are made of two atoms of hydrogen and one of oxygen. Meanwhile, the lead atoms, which have lost their oxygen companions, combine with some of the sulphate ions which are in that neighborhood. During the mix-up electrons are carried away from the plate and that leaves it positive.

The result of all this is a little lead sulphate on each plate, a negative plate where the spongy lead was, and a positive plate where the lead peroxide was.

Notice very carefully that I said “a little lead sulphate on each plate.” The sort of thing I have been describing doesn’t go on very long. If it did the 32battery would run down inside itself and then when we came to start our automobile we would have to get out and crank.

How long does it go on? Answer another question first. So far we haven’t connected any wire between the two plates of the battery, and so none of the electrons on the negative plate have any way of getting around to the positive plate where electrons are badly needed. Every time a negative sulphate ion combines with the spongy lead of the negative plate there are two more electrons added to that plate. You know how well electrons like each other. Do they let the sulphate ions keep giving that plate more electrons? There is the other question; and the answer is that they do not. Every electron that is added to that plate makes it just so much harder for another sulphate ion to get near enough to do business at all. That’s why after a few extra electrons have accumulated on the spongy lead plate the actions which I was describing come to a stop.

Do they ever begin again? They do just as soon as there is any reduction in the number of electrons which are hopping around in the negative plate trying to keep out of each other’s way. When we connect a wire between the plates we let some of these extra electrons of the negative plate pass along to the positive plate where they will be welcome. And the moment a couple of them start off on that errand along comes another sulphate ion in the solution and lands two more electrons on the plate. That’s how the battery keeps on discharging.