Plus 3.4 Mil-amperes
Plus 8.0 Volts and Plus 4.3 Mil-amperes

And so on. Fig. 21 shows the new road.

[Illustration: Fig 21]

One member of the Council jumped up and said "But what if the grid is
made negative?" The Commissioner had forgotten to see what happened so
he went home to take more readings.

He shifted the battery clip along, starting at _c_ of Fig. 22. At
the next meeting of the Council he brought in the following list of
readings and hence of points on his proposed road.

Minus 1.0 Volts and Plus 0.6 Mil-ampere
" 2.0 " " " 0.4 " "
" 3.0 " " " 0.2 " "
" 4.0 " " " 0.1 " "
" 5.0 " " " 0.0 " "

Then he showed the other members of the Council on the map of Fig. 23
how the Audion Characteristic would look.

[Illustration: Fig 22]

There was considerable discussion after that and it appeared that
different designs and makes of audions would have different
characteristic curves. They all had the same general form of curve but
they would pass through different sets of points depending upon the
design and upon the B-battery voltage. It was several meetings later,
however, before they found out what effects were due to the form of the
curve. Right after this they found that they could get much better
results with their radio sets.

Now look at the audion characteristic. Making the grid positive, that is
going on the positive side of the zero volts in our map, makes the plate
current larger. You remember that I told you in Letter 6 how the grid,
when positive, helped call electrons away from the filament and so made
a larger stream of electrons in the plate circuit. The grid calls
electrons away from the filament. It can't call them out of it; they
have to come out themselves as I explained to you in the fifth letter.

[Illustration: Fig 23]

You can see that as we make the grid more and more positive, that is,
make it call louder and louder, a condition will be reached where it
won't do it any good to call any louder, for it will already be getting
all the electrons away from the filament just as fast as they are
emitted. Making the grid more positive after that will not increase the
plate current any. That's why the characteristic flattens off as you see
at high values of grid voltage.

The arrangement which we pictured in Fig. 22 for making changes in the
grid voltage is simple but it doesn't let us chang