118
6.
HIGH VACUUM, LOW-VOLTAGE “TUBES
'
charge was discovered by Lee de Forest 2 in 1907. He introduced
into the tube a third electrode Which consisted of a grid of wires
-*
located between the lament and the plate, as at G in gure 6—4.
.
This three-electrode vacuum tube is referred to as a triade.
A posi'
tive potential on the grid, With -
respect to the lament, serves to
reduce the space charge and increase
the plate current. On the
other hand, a negative grid adds
'
_
‘
+
its effect to that of the space charge
’
_
-Eand reduces the plate current.
_
___—
'
Eg ___—Ep
That the grid is more effective
_
in controlling the plate current than
+
is the plate, appears reasonable since
_
.Ip
the grid is placed directly in the
_
F1G 6—4
A three—electrode vacuum
region Of the Space
It has
'
'
tube.
been found that in a given tube, for
example, the plate voltage had to
'
_
be changed by ten volts in order to produce the same change in
'
plate current as could be accomplished by a change of only one
'
volt of the potential on the grid.
_
.
The variation of plate current l,, With grid voltage B,; is shown
"
-
I ' Ë
Ëz oo=E; :
*
'
_
,
190 = Ep
,‘
_
'
A
-
,
'
‘
']
‘
"
*
'-4 -3
0
EG
.
;. Î
curves of a triode.
Plate current v‘s. grid
_
-
for
voltage—s. E,, in the family of characteristic curve!
Î*
:r Asmd1cated
horizontal“ line between Âand
"
B ,4favchange
"
volts on the plate may- be counteractedbva
*
serra—ge'ëo‘f only? “one Volt . onf= 'tähé£ grid“; The ratio of these voltage