Mexico weekly ledger., 1922-07-06

grid, determine the value of u. + Is
the negative potential which must be
applied to the grid in order to reduce
the plate current to zero, The value
of T, 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 grl1 with respect
to the tlanent and plate. If the grid
is of very fine mesh, the value of II
is small because the elecrrOns 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. 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 ap-
ertures In the grid without coming In
10 close to the charge on 1t and a
relatively. high potential will be re-
Quired on the grid to control the cleo
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 a positive
potential as Oh is applied to the grid.
The corresponding plate circuit current
will be I'D or BID, more than 1t was
when the grid had no potential ap-
piled to 1t.
A negative potential of OH RS now
applied to the grid where OH Is SquaI
in value to OIL, 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 a value H-C or A-C, less than 1t
was when there was no potential ap-
piled to the grid. Sc, 1t 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 detector or rectifier of
radio signals.
The Incoming radio signal is a high
frequency alternating current. Let US
apply an alternating difference of po-
tential whose maximum positive val
ue 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
as that shown in Fig, VIII. In Fig
TX Is shown the alternating difference
Of potential applied to the grid.
Through the first quarter or cycle,
from zero at J to maximum mega
tive value at X, equal to OH, the
plate circuit current will vary from
0-G, its value at P when no grid po-
tential is applied to a value at Q equal
to H-Ca
During the next quarter Of a cycle
the grid potential changes from a
maximum negative value at X to zero
at L. The corresponding values Of
plate circuit current are shown hy the
portion Of the plate current curve
OR.
During the next or third quarter
OF q cycle the applied grid potential
increases from zero at L to a maxi
mum positive value at M, equal to
OF, and causes the plate circuit to In
crease from lib, It's value when the
plate potential Ia zero, to I'D. an In.
crease In plate current equal to BID.
During the remaining fourth quar-
ter cf q cycle as the applied grid po-
tential varies from maximum pOsi-
tive value at N to zero at N, the plaes
circuit cuaTaat varies from value 3
to 4.
Assnsniag that ths characteristic
carve AZ Shown IN Fig, vITI and Ear.
TX was viii. i potential Of 40 yelia
on the plate, then, II the plate current
ID to be reduced to zero BY q yaria.
| tion of plate voltage with no poten.
tial applied to the grid---the plate
voltage must he rednced to zero or
Eeduccisn Of 40 volts mast be made.