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.

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"[3] and of all the
streets to "Volts."

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 road known as "Audion Characteristic." He said the road should pass
through the following points:

Zero Volt and Plus 1.0 Mil-ampere
Plus 2.0 Volts and Plus 1.7 Mil-amperes
Plus 4.0 Volts and Plus 2.6 Mil-amperes
Plus 6.0 Volts and