We mustn't let it get too much discharged for the lead sulphate is not soluble, as I just told you, and it will coat up that plate until there isn't much chance of getting the process to reverse. That's why we are so careful not to let the discharge process go on too long before we reverse it and charge. That's why, when the car battery has been used pretty hard to start the car, I like to run quite a while to let the generator charge the battery again. When the battery charges, the process reverses and we get spongy lead on the negative plate and lead peroxide on the positive plate. You've learned enough for one day. Write me your questions and I'll answer and then go on in my next letter to tell how the audion works. You know about conduction of electricity in wires; that is, about the electron stream, and about batteries which can cause the stream. Now you are ready for the most wonderful little device known to science: the audion. LETTER 5 GETTING ELECTRONS FROM A HEATED WIRE DEAR SON: I was pleased to get your letter and its questions. Yes, a proton is a speck of electricity of the kind we call positive and an electron is of the kind we call negative. You might remember this simple law; "Like kinds of electricity repel, and unlike attract." The word ion[2] is used to describe any atom, or part of a molecule which can travel by itself and has more or less than its proper number of electrons. By proper number of electrons I mean proper for the number of protons which it has. If an ion has more electrons than protons it is negative; if the inequality is the other way around it is positive. An atom or molecule has neither more nor less protons than electrons. It is neutral or "uncharged," as we say. No, not every substance which will dissolve will dissociate or split up into positive and negative ions. The salt which you eat will, but the sugar will not. If you want a name for those substances which will dissociate in solution, call them "electrolytes." To make a battery we must always use an electrolyte. Yes, it is hard to think of a smooth piece of metal or a wire as full of holes. Even in the densest solids like lead the atoms are quite far apart and there are large spaces between the nuclei and the planetary electrons of each atom. I hope this clears up the questions in your mind for I want to get along to the vacuum tube. By a vacuum we mean a space which has very few atoms or molecules in it, just as few as we can possibly get, with the best methods of pumping and exhausting. For the present let's suppose that we can get all the gas molecules, that is, all the air, out of a little glass bulb. The audion is a glass bulb like an electric light bulb which has in it a thread, or filament, of metal. The ends of this filament extend out through the glass so that we may connect a battery to them and pass a current of electricity through the wire. If we do so the wire gets hot.