than that of the C-battery
which is in the box no current can flow. Even if we do use a higher
voltage than the "negative C-battery" of the hidden grid-circuit there
will be a current only when the external battery is connected so as to
make the grid positive with respect to the filament.

Now suppose we take several cells of battery and try in the same way to
find what is hidden between the terminals _P_ and _F_. We
start with one battery and the ammeter as before and find that if this
battery is connected so as to make _P_ positive with respect to
_F_, there is a feeble current. We increase the battery and find
that the current is increased. Two cells, however, do not give exactly
twice the current that one cell does, nor do three give three times as
much. The current does not increase proportionately to the applied
voltage. Therefore we reason that whatever is between _P_ and
_F_ acts like a resistance but not like a wire resistance.

Then, we try another experiment with this hidden audion. We connect a
battery to _G_ and _F_, and note what effect it has on the
current which our other battery is sending through the box between
_P_ and _F_. There is a change of current in this circuit,
just as if our act of connecting a battery to _G-F_ had resulted in
connecting a battery in series with the _P-F_ circuit. The effect
is exactly as if there is inside the box a battery which is connected
into the hidden part of the circuit _P-F_. This concealed battery,
which now starts to act, appears to be several times stronger than the
battery which is connected to _G-F_.

Sometimes this hidden battery helps the B-battery which is on the
outside; and sometimes it seems to oppose, for the current in the
_P-F_ circuit either increases or decreases, depending upon how we
connect the battery to _G_ and _F_. The hidden battery is
always larger than our battery connected to _G_ and _F_. If we
arrange rapidly to reverse the battery connected to _G-F_ it
appears as if there is inside the box in the _P-F_ circuit an
alternator, that is, something which can produce an alternating e. m. f.

All this, of course, is merely a review statement of what we already
know. These experiments are interesting, however, because they follow
somewhat those which were performed in studying the audion and finding
out how to make it do all the wonderful things which it now can.

As far as we have carried our series of experiments the box might
contain two