Free trader-journal and Ottawa fair dealer. (Ottawa, Ill.) 1920-1927, 1922-07-10

Erid, determine the value of L.. T. il
the negative potential which must be-
applied to The grid in order to reduce
the plate current 10 zero. The value
of +. which Is the positive gr1 pOten-
tial that Will cause the maximum or
sDtiIrati0n current Ia How In the plate
circuit, is also determined by the rela.
tise position OF the grid with respect
10 the itumcnt and plate. If the grid
Is Of very fine mesh. the value of l.
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charge the grid and viii be repealed
similarly small positive charge al,-
piled To q fine mesh will tend to ac-
ceierate the velocity of the electrons
In case Of very coarse mesh grid
the eleetrons can pass through the ap.
ertures In the grid without coming ia
so close to the charge on it and n
relatively nigh potential will he 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. VII, O-G, is the
current that will flow In the plate cir
cult when there is no potential ap-
piled to the grid. Suppose Il positive
potential rs I'll 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 10 it.
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 POI
tential when applied to the grid will
cause the plate current to be reduced
10 . value He or A-t, less than it
was when there WAS no potential APP
piled to The grid So It is seen that
negative potential when applied to the
ard does not reduce the plate circuit
etrrcn as much Is the same positive
potential increased the plate circuit
current. This Irregular c0nductiyty of
the tube is made use of when the tube
is used rs a detector or rectifier of
radio signals.
The incoming radio signal is high
frequency alternating current. Let us
apply an alternating difference of pc.
tential whose maximum positive val
ne is equal to Oh and whose maximum
negative value Il equal to OH, to the
Grid Of the tlreeelectrode tube
whose characteristic curye'is the same
as that shown IN Fig, VIII In 9ig.
TX Is shown the alternating difference
If potential applied 10 the grid.
Through the first quarter Of cycle
from zero At J to n maximum mega.
tive value st X, equal to OH, the
plate circuit current will vary from
&C, its value al 7 when no grid po
tential js applied to value at Q equal
10 He.
During the next quarter of a cyclt
the gri7l potential changes from
maximum negative value at X to zero
at IT, The corresponding values Of
plate circuit current are shown by the
portion az 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 NJ. equal to
OF. and causes the plate circuit TO in.
crease from I-B. its value when the
plate potential is zero, to ID, an III
crease in plate current equal to BID
During the remaining fourth quar-.
ter of Il cycle as the applied grid DO.
tential varies from a maximum posi-.
tive value al NJ to zero at n, the plate
circuit current varies from value S
TO I.
Assuming that the characteristic
curve as shown in Fig. VIII and Fig.
IN was with a potential Of 10 volts
on the plate, then, if the plate current
is to be reduced tn zero by yarla-
ton of Plate voliage-with no poten.
tial applied to the grld-the plate
voltage must be reduced to zero or
reduction Of 40 volts must be made.
Us fftkA
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