Circuit 42 shows how you can
investigate the resistance of a thermistor.
The voltmeter is wired in parallel with
the thermistor, the p.d. V is measured in volts (V).
The variable resistor allows you to vary
the p.d. and current flow.
The ammeter, wired in series, gives you
the current I reading in amps (A).
You must decide on the initial p.d. and
see how the current varies.
You calculate the resistance of the
thermistor from Ohm's Law equation: V = IR, so R = V/I
Somehow you need to vary the temperature
of the thermistor resistor e.g. dipping it into a beaker of water of varying
temperature, making sure the circuit is insulated from the water.
You can make measurements from 0 to
60oC by using ice and then warm-hot water and try to get
measurements for every 5 or 10oC incremental rise in
should find that the resistance falls with increase in temperature because
a thermistor is a temperature dependent resistor.
The higher its temperature, the lower
a thermistor's resistance (e.g. tens of ohms) and much higher at low
temperatures (e.g. thousands of ohms).
High resistance in a cool environment and
low resistance in a warm environment.
You can see this trend clearly in the resistance -
temperature graph for a thermistor.
Thermistors can therefore respond to changes in temperature.
Uses of thermistors
Thermistors can act as temperature
detectors and are used in thermostats, temperature sensors - cooling systems
in car engines etc.
32 shows in principle how to control a cooling fan in a room.
Or any other heating system in
(real thermistor circuits are more
The fixed resistor and cooling fan are
wired in parallel. This means they always have the same potential difference
However, the thermistor is a variable
The p.d. of the power supply is shared
out between the thermistor and the 'loop' consisting of the fixed resistor
and fan wired in parallel.
The output component (fan) and the
thermistor are wired in series.
(I've indicated this with blue arcs -
not meant to be wires!)
The greater the component's resistance,
the greater proportion of the p.d. it takes.
If the room gets hotter, the resistance
of the thermistor decreases, so it takes a smaller shared of the p.d.
Therefore the p.d. across the fixed
resistor and fan rises (V1 increases, V2 decreases).
The fixed resistor and cooling fan
motor are wired in parallel, so have the same p.d. V1 across
The greater the p.d. across the fan, the
faster it goes as the power output can increase (P = IV).
If the room cools, the thermistor's
resistance increases and the process reverses and the fan slows down or
Thermistors are used as
temperature detectors e.g. electronic thermostats in heating and cooling
systems in the home or electric kettles (relatively low temperatures), or in
high temperature situations like a car engine.
on the I-V graph for a thermistor
on the right)
The graph of current versus voltage for a
thermistor is similar to that of a filament bulb.
Its a non-linear graph and the
phrase non-linear component may be used.
When the current (A) is NOT
proportional to the p.d (V) so the thermistor is described as a
(doesn't obey Ohm's Law, a non-ohmic resistor!).
passage of current heats up the filament and the rise in
temperature causes the resistance to increase.
As the current increases, more heat energy is released and the
filament gets hotter and hotter, so further increase in temperature
further increases the resistance.
This decreases the rate at which the current increases with
increase in potential difference.