The effect of temperature on a thermistor
This experiment, for advanced level students, shows that the current through a thermistor increases with temperature, as more charge carriers become available.
Apparatus and materials
timer or clock
Leads, 4 mm
Crocodile clip holder
Thermometer -10°C to 110°C
Thermistor - negative temperature, coefficient, e.g. 100 ohm at 25°C (available from Rapid Electronics).
Power supply, 5 V, DC or four 1.5 V cells
Beaker, 250 ml
Kettle to provide hot water
Digital multimeter, used as a milliammeter
Power supply, low voltage, DC, continuously variable or stepped supply with rheostat (>1 A)
Health & Safety and Technical notes
A thermistor may be described as:
- ntc 'negative temperature coefficient': its resistance decreases as the temperature increases
- ptc 'positive temperature coefficient': its resistance increases as the temperature increases
If you have both types available, students may be interested in comparing them.
a Set up the circuit as shown below.
b Pour boiling water into the beaker and take readings of the current through the thermistor as the temperature falls. Record the results.
c Plot a graph of current/ mA (y-axis) against temperature/ °C (x-axis).
d Assuming that the voltage is constant, describe how the conductance or resistance varies with temperature.
1 The thermistor is made from a mixture of metal oxides such as copper, manganese and nickel; it is a semiconductor. As the temperature of the thermistor rises, so does the conductance.
2 The increase in conductance is governed by the Boltzmann factor. Whether or not your students need to understand Boltzmann, they should be able to grasp that
- as the temperature goes up, the resistance goes down
- in this case, it happens because more charge carriers are released to engage in conduction.
This experiment comes from AS/A2 Advancing Physics. It has been re-written for this website by Lawrence Herklots, King Edward VI School, Southampton.