ignals.

There are just three possible answers to this question. The first answer
is: "No, there is a smaller number of electrons passing through the
plate circuit each second if the grid is being affected by an incoming
signal." The second is: "The signal doesn't make any difference in the
total number of electrons which move each second from filament to
plate." And the third answer is: "Yes, there is a greater total number
each second."

[Illustration: Fig 56]

Any one of the three answers may be right. It all depends on the
characteristic of the tube as we are operating it, and that depends not
only upon the type and design of tube but also upon what voltages we are
using in our batteries. Suppose the variations in the voltage of the
grid are as represented in Fig. 55, and that the characteristic of the
tube is as shown in the same figure. Then obviously the first answer is
correct. You can see for yourself that when the grid becomes positive
the current in the plate circuit can't increase much anyway. For the
other half of the cycle, that is, while the grid is negative, the
current in the plate is very much decreased. The decrease in one
half-cycle is larger than the increase during the other half-cycle, so
that on the average the current is less when the signal is coming in.
The dotted line shows the average current.

Suppose that we take the same tube and use a B-battery of lower voltage.
The characteristic will have the same shape but there will not be as
much current unless the grid helps, so that the characteristic will be
like that of Fig. 56. This characteristic crosses the axis of zero volts
at a smaller number of mil-amperes than does the other because the
B-batteries can't pull as hard as they did in the other case.

[Illustration: Fig 57]

You can see the result. When the grid becomes positive it helps and
increases the plate current. When it becomes negative it opposes and
decreases the plate current. But the increase just balances the
decrease, so that on the average the current is unchanged, as shown by
the dotted line.

On the other hand, if we use a still smaller voltage of B-battery we get
a characteristic which shows a still smaller current when the grid is at
zero potential. For this case, as shown in Fig. 57, the plate current is
larger on the average when there is an incoming signal.

If we want to know whether or not there is any incoming signal we will
not use the tube in the second