The Bemidji daily pioneer. [volume] (Bemidji, Minn.) 1904-1971, 1922-06-26

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 F, which Is the positive grid poten-
tial 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charge the grid and will be repealed.
Similarly a small positive charge ap-
piled to a 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 APP
ertures in the grid without coming in
so close to the. charge on it and
relatively high potential will be re-
quired on the grid to control the cleo
iron 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 OIL 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 &C. less than it
was when there was no potential ap-
piled to. the grid. So. it is seen that a
negative potential when applied to the
grid does not reduce the plate. circuit
current us 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 q. detector or rectifler OF
radio signals.
The incoming radio signal Is high
frequency alternating current. Let us
apply ain alternating difference of po-
tentlal whose maximum positive val
lie Is equal to Oh and:whOse maximum
negative value is equal to OH, to the
grid Of the, three-electrOde tube
whose characteristic curve is the same
CHI, In Y,~
IN is shown the alternating difference
Of potential applied to the grid.
Through the first quarter Of cycle,
from zero at j to a. maximum mega
tive, value at X, equal to OH, the
plate circuit current will vary from
O-G, its value at 2 when no grid po-
tential Is applied to a value at Q equal
to H.Cs
During the next quarter of a cycle
the grid potential changes from
maximum negative value at X 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 u cycle the applied grid potential
increases from zero at L to a maxi
mum positive. value at NJ; equal to
OIL, and causes the plate circuit to in.
crease from lib, its value when the
plate potential 10, zero, to, I'D, an in.
crease in plate current equal to BID.
. During the remaining fourth quar-
ter Of cycle as the applied grid po-
tential varies from maximum posi-
tive value at NJ to zero at N, the plate
circuit current varies from value g.
to Z.
Assuming that the characteristic
curve as shown in Fig. VIII and Fig.
IN was, with potential of 10 volts
on the plate, then, if the plate current
is. to be reduced to zero by a varia-
ton Of plate voltage-with no poten-
tial applied, to the grid-the plate
voltage must be reduced to zero or
reduction of 40 volts must be made.
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