Chapter VI: 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.

What do we mean when we say "the wire gets hot?" We mean that it feels hot. It heats the glass bulb and we can feel it. But what do we mean in words of electrons and atoms? To answer this we must start back a little way.

In every bit of matter in our world the atoms and molecules are in very rapid motion. In gases they can move anywhere; and do. That's why odors travel so fast. In liquids most of the molecules or atoms have to do their moving without getting out of the dish or above the surface. Not all of them stay in, however, for some are always getting away from the liquid and going out into the air above. That is why a dish of water will dry up so quickly. The faster the molecules are going the better chance they have of jumping clear away from the water like fish jumping in the lake at sundown. Heating the liquid makes its molecules move faster and so more of them are able to jump clear of the rest of the liquid. That's why when we come in wet we hang our clothes where they will get warm. The water in them evaporates more quickly when it is heated because all we mean by "heating" is speeding up the molecules.

In a solid body the molecules can't get very far away from where they start but they keep moving back and forth and around and around. The hotter the body is, the faster are its molecules moving. Generally they move a little farther when the body is hot than when it is cold. That means they must have a little more room and that is why a body is larger when hot than when cold. It expands with heating because its molecules are moving more rapidly and slightly farther.

When a wire is heated its molecules and atoms are hurried up and they dash back and forth faster than before. Now you know that a wire, like the filament of a lamp, gets hot when the "electricity is turned on," that is, when there is a stream of electrons passing through it. Why does it get hot? Because when the electrons stream through it they bump and jostle their way along like rude boys on a crowded sidewalk. The atoms have to step a bit more lively to keep out of the way. These more rapid motions of the atoms we recognize by the wire growing hotter.

That is why an electric current heats a wire through which it is flowing. Now what happens to the electrons, the rude boys who are dodging their way along the sidewalk? Some of them are going so fast and so carelessly that they will have to dodge out into the gutter and off the sidewalk entirely. The more boys that are rushing along and the faster they are going the more of them will be turned aside and plunge off the sidewalks.

The greater and faster the stream of electrons, that is the more current which is flowing through the wire, the more electrons will be "emitted," that is, thrown out of the wire. If you could watch them you would see them shooting out of the wire, here, there, and all along its length, and going in every direction. The number which shoot out each second isn't very large until they have stirred things up so that the wire is just about red hot.

What becomes of them? Sometimes they don't get very far away from the wire and so come back inside again. They scoot off the sidewalk and on again just as boys do in dodging their way along. Some of them start away as if they were going for good.

If the wire is in a vacuum tube, as it is in the case of the audion, they can't get very far away. Of course there is lots of room; but they are going so fast that they need more room just as older boys who run fast need a larger play ground than do the little tots. By and by there gets to be so many of them outside that they have to dodge each other and some of them are always dodging back into the wire while new electrons are shooting out from it.

When there are just as many electrons dodging back into the wire each second as are being emitted from it the vacuum in the tube has all the electrons it can hold. We might say it is "saturated" with electrons, which means, in slang, "full up." If any more electrons are to get out of the filament just as many others which are already outside have to go back inside. Or else they have got to be taken away somewhere else.

What I have just told you about electrons getting away from a heated wire is very much like what happens when a liquid is heated. The molecules of the liquid get away from the surface. If we cover a dish of liquid which is being heated the liquid molecules can't get far away and very soon the space between the surface of the liquid and the cover gets saturated with them. Then every time another molecule escapes from the surface of the liquid there must be some molecule which goes back into the liquid. There is then just as much condensation back into liquid as there is evaporation from it. That's why in cooking they put covers over the vessels when they don't want the liquid all to "boil away."

Sometimes we speak of the vacuum tube in the same words we would use in describing evaporation of a liquid. The molecules of the liquid which have escaped form what is called a "vapor" of the liquid. As you know there is usually considerable water vapor in the air. We say then that electrons are "boiled out" of the filament and that there is a "vapor of electrons" in the tube.

That is enough for this letter. Next time I shall tell you how use is made of these electrons which have been boiled out and are free in the space around the filament.

[Footnote 2: If the reader has omitted Letters 3 and 4 he should omit this paragraph and the next.]