The effect of temperature on a thermistor
Practical Activity for 14-16
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
- Heat-resistant mat
- 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:
negative temperature coefficient: its resistance decreases as the temperature increases
positive temperature coefficient: its resistance increases as the temperature increases
If you have both types available, students may be interested in comparing them.
- Set up the circuit as shown below.
- Pour boiling water into the beaker and take readings of the current through the thermistor as the temperature falls. Record the results. Analysis
- Plot a graph of current/ mA (y-axis) against temperature/ °C (x-axis).
- Assuming that the voltage is constant, describe how the conductance or resistance varies with temperature.
- 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.
- 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.