Letters of a Radio-Engineer to His Son

by John Mills

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.

70We 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.

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 71 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.

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 72next 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.

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.”