Little Falls herald., 1922-06-30

grid, determine the value of E. E is
the negative potential which must be
applied to the grid in order to reduce
the plate current to zero. The value
of +, which is the positive grid poten-
t1al that will cause the maximum or
saturation current to flow in the plate
circuit, is also determined by the rela-
tive position Of the grid with respect
to the filament and plate. If the grid
is of very fine mesh, the value of E
is small because the electrons in pass
ing through the small mesh of the
grid on their journey from the fila
ment to the plate will negatively
charge the grid and will be repelled.
Similarly small positive charge ap-
piled to fine mesh will tend to ac-
celerate the velocity of the electrons.
In case of very coarse mesh grid,
the electrons can pass through the ap-
ertures in the grid without coming in
SO close to the charge on it and a
relatively high potential will be re-
Quired on the grid to control the elec-
trOn stream, or in other words, the
current flowing in the plate circuit.
Referring to Fig. VIII, O-G, is the
current that will flow in the plate cir
cult when there is no potential ap-
piled to the grid. Suppose positive
potential as Oh is applied to the grid.
The corresponding plate circuit current
will be I'D or BID, more than it was
when the grid had no potential ap-
piled to it..
A negative potential of OH rs now
applied to the grid where OH is vqual
in value to Oh, but opposite in sign.
The application Of the negative po-
tential when applied to the grid will
cause the plate current to be reduced
to value H-C or A-C, less than it
was when there was no potential ap-
piled to the grid. So it is seen that
negative potential when applied to the
grid does not reduce the plate circuit
current as much as the same positive
potential increased the plate circuit
current. This irregular conductivity of
the tube is made use Of when the tube
is used as detectori or rectif1er of
radio signals.
The incoming radio signal is q high
frequency alternating current. Let us
apply an alternating difference of po-
tential whose maximum positive val
ue is equal to OF and whose maximum
negative value is equal to OH, to the
grid of the three-electrode tube
whose characteristic curve is the same
as that shown in Fig. VIII. In Fig.
TX Is shown the alternating difference
of potential applied to the grid.
Through the first quarter of a cycle.
from zero at to a maximum mega
tive value at X, equal to Oh, the
plate circuit current will vary from
O-G, Its value at p when no grid po-
tentfal Is applied to value at @ equal
to H-Oa
During the next quarter Of a cycle
the grid potential changes from a
maximum negative value at K to zero
at L. The corresponding values of
plate circuit current are shown by the
portion Of the plate current curve
OR.
During the next or third quarter
of cycle the applied grid potential
increases from zero at L to a maxi
mum positive value at M, equal to
Oh, and causes the plate circuit to in
crease from I-B, it's value when the
plate potential is zero, to I'D, an in
crease in plate >'lrrent equal to BID.
During the remaining fourth quar-
ter of a cycle as the applied grid po-
tential varies from a maximum posi-
tive value at M to zero at N, the plate
circuit current varies from a value S
to T.
Assuming that the characteristic
curve as shown in Fig. VIII and Fig.
TX was with a potential of 40 volts
on the plate, then, if the plate current
Is to be reduced to zero by varia-
tion Of plate voltage-with no poten-
tial applied to the grid-the plate
voltage must be reduced to zero or a
reduction of 40 volts must be made.