Letters of a Radio-Engineer to His Son

by John Mills

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 68plate circuit can be controlled by the e. m. f. which is applied to the grid circuit. There is a relationship between the plate current and the grid voltage which is peculiar or characteristic to the tube. So we call such a relationship “a characteristic.” Let us see how it may be found and what it will be.

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 I_B. 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 I_B as “eye-bee” or else as “eye-subscript-bee.” I_B therefore will stand for the number of amperes in the 69 plate circuit of the audion. In the same way I_A would stand for the current in the filament circuit.

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 E_A volts. In the plate circuit the e. m. f. is E_B volts. If we put a battery in the grid circuit we can let E_C 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 E_C, and plate current, that is I_B. So we call it the E_C–I_B 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.