EXPERIMENT 5—1
113
.
lament Will not be obtained but the test of the form of Richardson’s equation Will not be invalidated.
_
If an optical pyrometer is not available, the temperature of a
pure tungsten lament may be determined in terms of its resistance.
It can be shown13 that, in this case
R/Ro =
(5—10)
Where R is the resistance of the lament at the temperature T and
Ro is the resistance at zero degrees centigrade (To = 273).
It has
been found suñiciently accurate to measure Ro at room tempera—
ture (To = 273 +
°
C.) by means of
bridge. The
,
current used to operate the bridge Will heat the
appreci—
ably unless a high resistance is used in series With a single dry cell
_
and the current is only ashed through the lament for a brief
instant while 'obtaining the balance.
It is possible, if greater
accuracy is desired, to measure Ro With various observed bridge
currents and extrapolate back 'Eo zero current. The high temperature resistance R maybe measured by means of an ammeter
in series and a voltmeter in parallel With the lament. Ohm’s law
is then used to give R. ‘
”
—
.
Ÿ
Adjust the lament to its maximum brightness, i.e., to its
normal.operati-ng value. ;Now increase the plate voltage V until
the platecurrent i no longer increases, Or at least rises very slowly.
This saturation or
iri’ equation 5—3_for this
particular temperature T of the lament. The
required to
produce saturation mayamoü‘nt to 200 or{more vOlts.
_
'
4. Keep the
voltage at’the max1mum
found in (3),
‘
orhigher, and reCord values
fér_ a,seriesof temperatures
below
.
the normal operating
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