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76 ⏐⏐⏐ RESISTANCE
FIG. 3.22
Thick-film resistor networks. (Courtesy of
Dale Electronics, Inc.)
FIG. 3.20
Fixed resistors. [Parts (a) and (c) courtesy of Ohmite Manufacturing Co. Part
(b) courtesy of Philips Components Inc.]
FIG. 3.21
Miniature fixed resistors. [Part (a) courtesy of Ohmite Manufacturing Co. Parts
(b) and (c) courtesy of Dale Electronics, Inc.]
The symbol for a three-terminal potentiometer appears in Fig.
3.23(a). When used as a variable resistor (or rheostat), it can be hooked
up in one of two ways, as shown in Fig. 3.23(b) and (c). In Fig.
3.23(b), points a and b are hooked up to the circuit, and the remaining
terminal is left hanging. The resistance introduced is determined by
that portion of the resistive element between points a and b. In Fig.
3.23(c), the resistance is again between points a and b, but now the
remaining resistance is “shorted-out” (effect removed) by the connection
from b to c. The universally accepted symbol for a rheostat appears
in Fig. 3.23(d).
Most potentiometers have three terminals in the relative positions
shown in Fig. 3.24. The knob, dial, or screw in the center of the housing
controls the motion of a contact that can move along the resistive
element connected between the outer two terminals. The contact is connected
to the center terminal, establishing a resistance from movable
contact to each outer terminal.
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