The Democrat-herald., 1922-06-23
POSITION AND SIZE
OF GRID IMPORTANT
Determine vaIse Of Negative Po-
tent'al Necessary to Reduce
. the Plate Current to Zero.

In using a three. .aIeclro,le vacuum
tube In Il radio, set ills preferable t"
maintain the grid negative with re-
spect 10 the fllnment In order to re-
qulre the minimum amount of energy
in the control of the plate circuit.
The relative position of the grid
with respect to the humcnt and the
plate and the size of the mesh Of the
griG, determine the value OF l.. Il 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 puten-
tinl that Will caus.. the. maximum or
saturntion current to flow In the plate
circuit, is also determined by the rela-
tive position of the grid with respect
to the filamcnt and plate. If the grid
is of very fine mesl, the value Of L
ls small because the electrons in pass
Ing through the small mesh of the
grid on their Journey from the fla
ment to the plate viii negatively
charge the grid and will be repcllcd
Similarly small positive charge up
piled to fine mesh will tend to ac-
celerate the velocity of the elcctrons.
In case of II very coarse mesh grid,
the p.ectrons can pass through the ap.
ertures In the grid without coming in
SO close to the charge on II and IL
relatively high potential Will be re-
quired on the grid to control the elcc-
trOn stream, or In other words, the
current flowing In the plate circuit.
Referring to Fig. VIII, O-G, is the
lnf x.,ilI In, al
cult when there Is no potential ap-
piled to the grid. Suppose II positive
potential .ns OIL is applied TO the grid.
The corresponding plate circuit current
will be I'D or BID, more than II was
When the grid hud no potential ap-
piled to ii.
A negative potential Of OH IS now
applied to the grid where OH is squal
in value to OIL, but opposite in sign.
The application of the negative po-
tcntinl when applied to the grid will
cause the plate current to be reduced
to n value H-C or A-C, less than it
was when there was no potential up
piled to the grid. So 1t is seen that
negative potential when applied to the
grid does not reduce the plate circuit
current ns 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 a detcctor or rcctiiicr of
radio signals
The incoming radio signal Is a high
frequency alternating current. Let us
apply an alternating difference of po-
tentinl whose maximum positive val
| ue Is equal to OIL and whose maximum
! negative value is equal to OH, to the
| grid OF the three-clectrode tube
| whose characteristic curve is the same
as that shown in Fig. VII. In Fig
IN Is shown the alternating difference
of potential applied to the grid.
Through the first quarter OF cycle,
from zero at J to maximum mega
tive value mt X, equal to OH, the
! plate circuit current will vary from
| O-G, Its value at = when no grid po-
| tentinl is applied to Il value at Q equal
I 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 by the
: portion Of the plate current curve
On.
During the next or third quarter
| Of n cycle the applied grid potential
! increases from zero Mt l. to maxi
mum positive value at NJ, equal to
+ OIL, and causes the plate circuit to in.
crease from I-R, it's value when the
Plate potential is zero to Id. an In
, crease in plate current equal to BID.
| During the remaining fourth quar-
ter OF a cycle us the applied grid po.
tential varies from n maximum posi-.
tive value ut n V zaro at n. the plate
circuit current ynries from u ynine S
to y.
Assuming that the characteristic
curve rs shown in Fig. VIII and Fig.
IN was with n potential of 40 volts
on the plate, then, If the pinte current
Is TO be reduced to zero by varia-
tiOn OF plate voltage-with no potcn-
| tinl applied to the grid-thc plate
voltage must be reduced to zero or Il
reduction of 40 volts must be made.