Resistance is a measure of the opposition of a particular volume of
material at a temperature of 20 degrees centigrade.
NO! NO! NO! That is part of the definition of specific resistance. The full definition goes like this: The specific
resistance of a material is the resistance of a unit volume of that material at a temperature of 20 degrees
centigrade. Now go back and read the statements on Page 1-5A again.
Fixed, tapped, and variable is exactly right. DON'T FORGET THE
Variable resistors are of two kinds, RHEOSTATS or POTENTIOMETERS. Some variable resistors have
only two terminals and these can only be used as rheostats. A three-terminal variable resistor connected as a
rheostat has only two leads connected to the electrical circuit to adjust the current flow in a circuit or a portion of
If the three terminals of a variable resistor each connect to different parts of a circuit, it is connected as a
potentiometer. A potentiometer does not vary the total resistance between the terminal ends; instead, it varies the
amount of resistance between each end and the center contact. The potentiometer is usually smaller in physical
size than a rheostat and its purpose is to vary the amount of voltage applied to an electrical device. Essentially, it
is a voltage divider.
Complete the statements below by filling in the blanks.
a. The two kinds of variable resistors are the
b. The purpose of a rheostat is to adjust the
c. The purpose of a potentiometer is to vary the
(Continued on Page 1-25)