There was considerable discussion after that and it appeared that different designs and makes of audions would have different characteristic curves. They all had the same general form of curve but they would pass through different sets of points depending upon the design and upon the B-battery voltage. It was several meetings later, however, before they found out what effects were due to the form of the curve. Right after this they found that they could get much better results with their radio sets.
Now look at the audion characteristic. Making the grid positive, that is going on the positive side of the zero volts in our map, makes the plate current 75larger. You remember that I told you in Letter 6 how the grid, when positive, helped call electrons away from the filament and so made a larger stream of electrons in the plate circuit. The grid calls electrons away from the filament. It can’t call them out of it; they have to come out themselves as I explained to you in the fifth letter.
You can see that as we make the grid more and more positive, that is, make it call louder and louder, a condition will be reached where it won’t do it any good to call any louder, for it will already be getting all the electrons away from the filament just as fast as they are emitted. Making the grid more positive after that will not increase the plate current any. That’s why the characteristic flattens off as you see at high values of grid voltage.
The arrangement which we pictured in Fig. 22 for 76making changes in the grid voltage is simple but it doesn’t let us change the voltage by less than that of a single battery cell. I want to show you a way which will. You’ll find it very useful to know and it is easily understood for it is something like the arrangement of Fig. 14 in the preceding letter.
Connect the cells as in Fig. 24 to a fine wire. About the middle of this wire connect the filament. As before use a clip on the end of the wire from the grid. If the grid is connected to a in the figure there is applied to the grid circuit that part of the e. m. f. of the battery which is active in the length of wire between o and a. The point a is nearer the positive plate of the battery than is the point o. So the grid will be positive and the filament negative.
On the other hand, if the clip is connected at b the grid will be negative with respect to the filament. We can, therefore, make the grid positive or negative depending on which side of o we connect the clip. How large the e. m. f. is which will be applied to the grid depends, of course, upon how far away from o the clip is connected.
Suppose you took the clip in your hand and slid it along in contact with the wire, first from o to a 77 and then back again through o to b and so on back and forth. You would be making the grid alternately positive and negative, wouldn’t you? That is, you would be applying to the grid an e. m. f. which increases to some positive value and then, decreasing to zero, reverses, and increases just as much, only to decrease to zero, where it started. If you do this over and over again, taking always the same time for one round trip of the clip you will be impressing on the grid circuit an “alternating e. m. f.”
What’s going to happen in the plate circuit? When there is no e. m. f. applied to the grid circuit, that is when the grid potential (possibilities) is zero, there is a definite current in the plate circuit. That current we can find from our characteristic of Fig. 23 for it is where the curve crosses Zero Volts. As the grid becomes positive the current rises above this value. When the grid is made negative the current falls below this value. The current, IB, then is made alternately greater and less than the current when EC is zero.