on in the receiving circuit of
Fig. 63 and that its characteristic under these conditions is given by
Fig. 56. I've redrawn the figures to save your turning back. The audion
will not act as a detector because an incoming signal will not change
the average value of the current in the plate circuit. If, however, we
connect a C-battery so as to make the grid negative, we can shift this
characteristic so that the incoming signal will be detected. We have
only to make the grid sufficiently negative to reduce the plate current
to the value shown by the line _oa_ in Fig. 85. Then the signal
will be detected because, while it makes the plate current alternately
larger and smaller than this value _oa_, it will result, on the
average, in a higher value of the plate current.

[Illustration: Fig 86]

You see that what we have done is to arrange the point on the audion
characteristic about which the tube is to work by properly choosing the
value of the grid voltage _E_{C}_.

There is an important method of using an audion for a detector where we
arrange to have the grid voltage change steadily, getting more and more
negative all the time the signal is coming in. Before I tell how it is
done I want to show you what will happen.

Suppose we start with an audion detector, for which the characteristic
is that of Fig. 56, but arranged as in Fig. 86 to give the grid any
potential which we wish. The batteries and slide wire resistance which
are connected in the grid circuit are already familiar to you.

When the slider is set as shown in Fig. 86 the grid is at zero potential
and we are at the point 1 of the characteristic shown in Fig. 87. Now
imagine an incoming signal, as shown in that same figure, but suppose
that as soon as the signal has stopped making the grid positive we shift
the slider a little so that the C-battery makes the grid slightly
negative. We have shifted the point on the characteristic about which
the tube is being worked by the incoming signal from point 1 to point 2.

[Illustration: Fig 87]

Every time the incoming signal makes one complete cycle of changes we
shift the slider a little further and make the grid permanently more
negative. You can see what happens. As the grid becomes more negative
the current in the plate circuit decreases on the average. Finally, of
course, the grid will become so negative that the current in the plate
circuit will be reduced to zero. Under these conditions an incoming
signal