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.] LETTER 6 THE AUDION DEAR SON: