finally makes a large change in the plate current and hence in
the current through the telephone.

The method of shifting a slider along, every time the incoming signal
makes a complete cycle, is impossible to accomplish by hand if the
frequency of the signal is high. It can be done automatically, however,
no matter how high the frequency if we use a condenser in the grid
circuit as shown in Fig. 88.

[Illustration: Fig 88]

When the incoming signal starts a stream of electrons through the coil
_L_ of Fig. 88 and draws them away from plate 1 of the condenser
_C_ it is also drawing electrons away from the 1 plate of the
condenser _C_{g}_ which is in series with the grid. As electrons
leave plate 1 of this condenser others rush away from the grid and enter
plate 2. This means that the grid doesn't have its ordinary number of
electrons and so is positive.

If the grid is positive it will be pleased to get electrons; and it can
do so at once, for there are lots of electrons streaming past it on
their way to the plate. While the grid is positive, therefore, there is
a stream of electrons to it from the filament. Fig. 89 shows this
current.

All this takes place during the first half-cycle of the incoming signal.
During the next half-cycle electrons are sent into plate 1 of the
condenser _C_ and also into plate 1 of the grid condenser
_C_{g}_. As electrons are forced into plate 1 of the grid condenser
those in plate 2 of that condenser have to leave and go back to the grid
where they came from. That is all right, but while they were away the
grid got some electrons from the filament to take their places. The
result is that the grid has now too many electrons, that is, it is
negatively charged.

[Illustration: Fig 89]

An instant later the signal e. m. f. reverses and calls electrons away
from plate 1 of the grid condenser. Again electrons from the grid rush
into plate 2 and again the grid is left without its proper number and so
is positive. Again it receives electrons from the filament. The result
is still more electrons in the part of the grid circuit which is formed
by the grid, the plate 2 of the grid condenser and the connecting wire.
These electrons can't get across the gap of the condenser _C_{g}_
and they can't go back to the filament any other way. So there they are,
trapped. Finally there are so many of these trapped electrons that the
grid is so negative all the time as almost entirely to oppose the
effor