We shall start with the filament heated to a deep red. The manufacturers of the audion tell the purchaser what current should flow through the filament so that there will be the proper emission of electrons. There are easy ways of finding out for one's self but we shall not stop to describe them. The makers also tell how many volts to apply to the plate, that is what value _E_{B}_ should have. We could find this out also for ourselves but we shall not stop to do so. [Illustration: Fig 19] Now we set the battery clip so that there is no voltage applied to the grid; that is, we start with _E_{C}_ equal to zero. Then we read the ammeter in the plate circuit to find the value of _I_{B}_ 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 _E_{C}_ and will find a different value of _I_{B}_ when we read the ammeter. Next move the clip to apply two batteries to the grid. We get a new pair of values for _E_{C}_ and _I_{B}_, getting _E_{C}_ from the voltmeter and _I_{B}_ from the ammeter. As we continue in this way, increasing _E_{C}_, we find that the current _I_{B}_ increases for a while and then after we have reached a certain value of _E_{C}_ the current _I_{B}_ stops increasing. Adding more batteries and making the grid more positive doesn't have any effect on the plate current. [Illustration: Fig 20] Before I tell you why this happens I want to show you how to make a picture of the pairs of values of _E_{C}_ and _I_{B}_ 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 next beyond "Positive 2 St.," and so on. West of the main street they called the first street "Negative 1 St." and so on. 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.