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10
TEMPERATURE COEFFICIENT OF RESISTANCE
OBJECT
To measure the temperature coefficient of both thermistors and copper wire.
APPARATUS
Thermistor assembly, copper wire assembly, electrically heated water bath, temperature sensor, slidewire
Wheatstone bridge, galvanometer, Power supply, resistance box, eight wires
THEORY
Most metals used as electrical conductors exhibit a linear relation between resistance and temperature.
The
temperature coefficient of resistance
of such metals is defined by the expression
=
o
o
t
tR
R
R
(1)
where
t
R
:
value of resistance of the metallic conductor at temperature t
C
o
R
:
value of resistance of the metallic conductor at temperature 0
C
Of course, equation (1) may be rewritten as
R
t
=
R
o
t
+
R
o
(2)
when we recognize immediately that equation (2) is simply the slopeintercept form of a straight line,
namely, y = mx+b.
Thus, if we were to plot a series of values of resistance versus temperature for a given
specimen, the resulting straight line would intercept the resistance axis at the value R
o
while the attendant
slope divided by R
o
is the value of
Most metals have a small positive temperature coefficient of
resistance.
In contrast with metals, thermistors have large negative temperature coefficients.
Moreover, with
thermistors the change of resistance with a change in temperature is nonlinear.
The resistance of a
thermistor is given by the equation
R = R
a
)
/
1
/
1
(
a
t
t
where
R:
resistance in ohms at temperature t (K)
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 Fall '09
 JohnJames
 Resistance, Heat

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