Potentiometers and rheostats are types of variable resistors. Potentiometers have three terminals and are used to control voltage in a circuit by varying the position of a wiper arm along a resistive element. Rheostats have two terminals and are used to control current by adjusting the amount of current flowing through a high resistance wire element. Thermistors are temperature sensitive resistors with a negative temperature coefficient of resistance - their resistance decreases with increasing temperature. They are made of semiconductor ceramics and find applications in temperature measurement and control.

1. Dr.R.Hepzi Pramila Devamani,
Assistant Professor of Physics,
V.V.Vanniaperumal College for Women,
Virudhunagar

2.  These are variable resistors either of carbon or wire
– wound type often used for controlling voltage and
current in a circuit.
 They generally have carbon composition resistance
element and are connected across a voltage source.
 They have three terminals, the centre one being
connected to the variable arm which is used for
varying voltage division in the circuit as shown in
the figure.

3.  By moving the variable arm B over the
fixed resistance R between the points A
and C, any part of the input voltage can
be tapped off.
 Since in the figure 5.4, B happens to be
at the middle value of R, output voltage
is half the input voltage.
 Most variable resistors used in radios
are potentiometers meant for controlling
volume ofr tone. When used as a
volume control, it picks off a voltage
between zero and the full available
voltage as shown in figure 5.5.
 By moving up and down , any desired
amount of voltage (signal) can be
picked up between maximum signal
value at point A and zero signal value at
C.

4.  The tone control circuit shown if figure 5.6 uses only two
terminals on the potentiometer. The resistor allows the capacitor
to by-pass to ground either more or less of high frequencies in an
audio circuit.
 When B is at point A, the capacitor becomes a direct by-pass to
ground for higher frequencies in the audio signal.
 Consequently, radio sound becomes ‘bassy’.
 When B is at terminal C, it increases the amount of resistance in
series with the capacitor.
 Hence less amount of high frequencies is by-passed to ground and
consequently, there is more trebble in the sound.
 Potentiometers are commonly used as control devices in
amplifiers, TV sets and various types of meters. Typical
applications include volume and tone controls, balace controls,
linearity and brightness controls in TV receivers.

6.  The resistance element of rheostats is made of high
resistance wire.
 It has two terminals and is connected in series with a
circuit for adjusting the amount of current flowing
through it.
 Rheostats are commonly used to control relatively high
currents such as those found in motor and lamp circuits
for controlling its current. Only resistance BC is
connected into the circuit.
 Though similar in construction to potentiometers, they
are usually larger in size because they possess much
high power rating.

7.  A given potentiometer
can be used as rheostat.
One method is just to
use two ends only
leaving the third end
unconnected as shown
in figure 5.8 (a).
 The other method is to
wire the third unused
terminal to the central
terminal as shown in
figure. 5.8 (b).

8.  Since fixed carbon composition
resistors having axial leads are
physically small, they are
colour-coded to indicate their
resistance in ohms.
 The system is based on the use
of colours as numerical values.
 The dark colours like black and
brown correspond to the lowest
numbers of zero and one
respectively, the light colours to
next higher numerals and lastly
white colour to nine.
 The colours used with code and
the numbers they represent are
given in the table 5.1.

10.  It is a thermal resistor i.e., a temperature-sensitive
sensor.
 Most thermistors have negative temperature
coefficient of resistance though devices having
positive temperature coefficient are also available.

11.  Thermistors are made by heating under pressure
(sintering) semiconductor ceramic materials made
from mixture of metallic oxides.
 Various mixtures for this purpose are those of
manganese, nickel, cobalt, copper and iron etc.
 These are pressed into desired shapes and then
looked at high and then baked at high temperature
for making different types of thermistors.
 Electrical connections are made either by including
wires during shaping process or by silvering the
surfaces after sintering.

12.  As shown in figure 15.33,
thermistors are made in the
shape of beads (with diameters
ranging from 0.15mm to 2.5
mm), probes (beads sealed in
glass rods, discs or washers)
upto 25 mm in diameter and
rods from 6mm to 50 mm long.
 Beads may be glass coated or
enclosed in evacuated envelopes
for protection against corrosion.
 Washer shaped thermistors can
be bolted together for series or
parallel connection. The usual
symbol for the device is shown
in figure 15.33

13.  A typical curve is shown in
figure 15.34., the thermistor has
a resistance of about 3000Ω at -
200C and 40 Ω at 800C.
 Since a change of 100 0C has
resulted in a 75:1 change in
resistance which makes a
thermistor such a valuable
temperature measurement and
control device.
 There are two ways of changing
the resistance of the device
internally and externally.
 In other words, device
resistance depends on ambient
temperature and self heating.

14.  Thermistors find many applications such as
 For measurement and control of temperature as in
ovens.
 For temperature compensation i.e., cancelling the
effect of temperature on other electronic devices.
 As flow rate measuring devices.
 For regulating ac voltage (unlike zener diode which
being polarity sensitive, can regulate only dc
voltages).
 For liquid level detection.

15.  It is another basic component commonly used in
electronic circuits.
 It is nothing but a coil wound on a core or former of
some suitable material.

16.  It consists of number of
turns of wire wound on
a former made of
ordinary card board
(fig.5.13a).
 Since there is air inside
the coil, an air-core
inductor has the least
inductance for a given
number of turns and
core length.

17.  It is that inductor in which a coil of wire is wound
over a solid or laminated iron core (fig.5.13b).
 Putting iron inside an inductor has the effect of
increasing its inductance as many times as the
relative permeability of iron.
 In order to avoid eddy current loss, iron core is
laminated i.e.,it is made up of thin iron laminations
pressed together but insulated from each other. Such
an inductor is also called choke.

18.  The iron core has been
found to work more
efficiently at low
frequencies if it is in the
form of a closed core.
i.e.,
 If the core not only goes
through the centre of the
coil but also surrounds it
on its two sides as
shown in figure 5.14.

19.  A coil of wire is wound on
a solid core made of
highly ferromagnetic
substance called ferrite.
 Ferrite is a solid material
consisting of fine particles
of iron powder embedded
in an insulating binder. A
ferrite core has minimum
current loss.
 The symbols for different
types of inductors are
shown in figure 5.15.