  My Dear Young Student: Although there is much in Letters 7 and 8 which it is well to learn and to think about, there are only three of the ideas which you must have firmly grasped to get the most out of this letter which I am now going to write you about the audion. First: Electric currents are streams of electrons. We measure currents in amperes. To measure a current we may connect into the circuit an ammeter. Second: Electrons move in a circuit when there is an electron-moving-force, that is an electromotive force or e.m.f. We measure e.m.f.’s in volts. To measure an e.m.f. we connect a voltmeter to the two points between which the e.m.f. is active. Third: What current any particular e.m.f. will cause depends upon the circuit in which it is active. Circuits differ in the resistance which they offer to e.m.f.’s. For any particular e.m.f. (that is for any given e. m. f.) the resulting current will be smaller the greater the resistance of the circuit. We measure resistance in ohms. To measure it we find the quotient of the number of volts applied to the circuit by the number of amperes which flow. In my sixth letter I told you something of how the audion works. It would be worth while to read again that letter. You remember that the current in the Connect an ammeter in the plate- or B-circuit, of the tube so as to measure the plate-circuit current. You will find that almost all books use the letter “I” to stand for current. The reason is that scientists used to speak of the “intensity of an electric current” so that “I” really stands for intensity. We use I to stand for something more than the word “current.” It is our symbol for whatever an ammeter would read, that is for the amount of current. Another convenience in symbols is this: We shall frequently want to speak of the currents in several different circuits. It saves time to use another letter along with the letter I to show the circuit to which we refer. For example, we are going to talk about the current in the B-circuit of the audion, so we call that current IB. We write the letter B below the line on which I stands. That is why we say the B is subscript, meaning “written below.” When you are reading to yourself be sure to read IB as “eye-bee” or else as “eye-subscript-bee.” IB therefore will stand for the number of amperes in the We are going to talk about e.m.f.’s also. The letter “E” stands for the number of volts of e.m.f. in a circuit. In the filament circuit the battery has EA volts. In the plate circuit the e.m.f. is EB volts. If we put a battery in the grid circuit we can let EC represent the number of volts applied to the grid-filament or C-circuit. The characteristic relation which we are after is one between grid voltage, that is EC, and plate current, that is IB. So we call it the EC–IB characteristic. The dash between the letters is not a subtraction sign but merely a dash to separate the letters. Now we’ll find the “ee-see-eye-bee” characteristic. Connect some small dry cells in series for use in the grid circuit. Then connect the filament to the middle cell as in Fig. 19. Take the wire which comes from the grid and put a battery clip on it, then you can connect the grid anywhere you want along this series of batteries. See Fig. 18. In the figure this movable clip is represented by an arrow head. You can see that if it is at a the battery will make the grid positive. If it is moved to b the grid will be more positive. On the other hand if the clip is at o there will be no e.m.f. applied to the grid. If it is at c the grid will be made negative. Between grid and filament there is placed a voltmeter which will tell how much e.m.f. is applied to the grid, that is, tell the value of EC, for any position whatever of the clip. Now we set the battery clip so that there is no voltage applied to the grid; that is, we start with EC equal to zero. Then we read the ammeter in the plate circuit to find the value of IB which corresponds to this condition of the grid. Next we move the clip so as to make the grid as positive as one battery will make it, that is we move the clip to a in Fig. 19. We now have a different value of EC and will find a different value of IB when we read the ammeter. Next move the clip to apply two batteries to the grid. We get a new pair of values for EC and IB, getting EC from the voltmeter and IB from the ammeter. As we continue in this way, increasing EC, we find that the current IB increases Before I tell you why this happens I want to show you how to make a picture of the pairs of values of EC and IB which we have been reading on the voltmeter and ammeter. Imagine a city where all the streets are at right angles and the north and south streets are called streets and numbered while the east and west thorofares are called avenues. I’ll draw the map as in Fig. 20. Right through the center of the city goes Main Street. But the people who laid out the roads were mathematicians and instead of calling it Main Street they called it “Zero Street.” The first street east of Zero St. we should have called “East First Street” but they called it “Positive 1 St.” and the When they came to name the avenues they were just as precise and mathematical. They called the main avenue “Zero Ave.” and those north of it “Positive 1 Ave.,” “Positive 2 Ave.” and so on. Of course, the avenues south of Zero Ave. they called Negative. The Town Council went almost crazy on the subject of numbering; they numbered everything. The silent policeman which stood at the corner of “Positive 2 St.” and “Positive 1 Ave.” was marked that way. Half way between Positive 2 St. and Positive 3 St. there was a garage which set back about two-tenths of a block from Positive 1 Ave. The Council numbered it and called it “Positive 2.5 St. and Positive 1.2 Ave.” Most of the people spoke of it as “Plus 2.5 St. and Plus 1.2 Ave.” Sometime later there was an election in the city and a new Council was elected. The members were mostly young electricians and the new Highway Commissioner was a radio enthusiast. At the first meeting the Council changed the names of all the avenues to “Mil-amperes” Then the Highway Commissioner who had just been taking a set of voltmeter and ammeter readings on an audion moved that there should be a new Zero Volt and Plus 1.0 Mil-ampere And so on. Fig. 21 shows the new road. One member of the Council jumped up and said “But what if the grid is made negative?” The Commissioner had forgotten to see what happened so he went home to take more readings. He shifted the battery clip along, starting at c of
Then he showed the other members of the Council on the map of Fig. 23 how the Audion Characteristic would look. 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 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 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 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. You might spend a little time thinking over this, seeing what happens when an alternating e.m.f. is applied to the grid of an audion, for that is going to be fundamental to our study of radio. |
|
