b.
Adjustable
Resistors.
Adjustable resistors are used where it
is necessary to change or adjust the
value of the resistance in a circuit
from time to time. In its usual form,
the adjustable resistor is wire wound,
and has one or more sliding collars
which
may
be
moved
along
the
resistance
element
to
select
any
desired resistance value (A of fig.
10).
The collar is then clamped in
place.
c. Variable Resistors.
Variable
resistors are used in a circuit when a
resistance
value
must
be
changed
frequently.
Depending on the power
requirements, variable resistors are
either
of
carbon
or
wire-wound
construction.
The actual resistance
element of the variable resistor is
usually circular in shape, and the
sliding tap or "arm" which makes
contact with it is provided with a
knob and a shaft by means of which the
resistance can be changed smoothly.
(1) If
both
ends
of
the
resistance
element
are
provided
with
terminals
(in
addition
to
the
sliding
arm).
the
variable
resistor
is
called a potentiometer (B
of fig. 10).
(2) If only one end of the
resistance element and the
sliding arm are brought
out
to
connection
terminals,
the
variable
resistor
is
called
a
rheostat.
A rheostat is
shown in C of figure 10.
Current flows only through
the
section
of
the
resistance
element
included
between
the
sliding
arm
and
the
connecting
and
the
connecting terminal.
Figure 10. Adjustable and
variable resistors.
308
14
Previous Page
