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. [Illustration: Fig 18] 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 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. 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 _E_{C}_, for any position whatever of the clip.