122
6.
HIGH VACUUM, LOW—VOLTAGE TUBES
which is seen to be the reciprocal of the slope of the curve of gure
6—9 at the point P, the increments AEp and
being taken as
_
small as possible. It is to be noted that this differs from the dc
denition of resistance Which would be the reciprocal of the slope _,
of the line OP, i.e., Ep/lp.
Since thecurve of gure 6—9 is never exactly a straight line,
73,, variés as the voltage of the plate battery is changed but, W1th
P
=“?
'
/’
//
AE,;
’
;}:ê‘iä*
'
'
’
…
FIG. 6—9: Plate current-plate voltage curve of a three-electrode tube.
'
-
voltage, may be taken as a constant in applying equation"
6—10.
Values of 73, at normal operating voltages range in d1'erent;
commercial triodes from 800 to 150,000 ohms.
_
The plate resistance may be measured by means of a bridge
circuit3 such as that in gure 6—10, where the normal operatmg
,
.
|
.
,
.
.
%
E
'
°
°
°
-
ltl‘ll !
‘
“+
_FIG' 6_10the-plate résistance rp.
2
_
s
.
gr1d,lament
_—plàte” voltages are applied to the tube. The
are Varied until there is no sound in the
Phones—_ …Th‘en
_Wheat‘stone
equation is appll—