Electricity is a study that is frequently explained in terms of
opposites. The term that is the opposite of resistance is CONDUCTANCE. Conductance is the
ability of a material to pass electrons. The factors that affect the magnitude of
resistance are exactly the same for conductance, but they affect conductance in the
opposite manner. Therefore, conductance is directly proportional to area, and inversely
proportional to the length of the material. The temperature of the material is definitely
a factor, but assuming a constant temperature, the conductance of a material can be
Resistance is a property of every electrical component. At times, its effects will be undesirable. However, resistance is used in many varied ways. RESISTORS are components manufactured to possess specific values of resistance. They are manufactured in many types and sizes. When drawn using its schematic representation, a resistor is shown as a series of jagged lines, as illustrated in figure 1-29.
Composition of Resistors
One of the most common types of resistors is the molded composition, usually referred to as the carbon resistor. These resistors are manufactured in a variety of sizes and shapes. The chemical composition of the resistor determines its ohmic value and is accurately controlled by the manufacturer in the development process. They are made in ohmic values that range from one ohm to millions of ohms. The physical size of the resistor is related to its wattage rating, which is the ability of resistor to dissipate heat caused by the resistance.
Carbon resistors, as you might suspect, have as their principal
ingredient the element carbon. In the manufacturer of carbon resistors, fillers or binders
are added to the carbon to obtain various resistor values. Examples of these fillers are
clay, bakelite, rubber, and talc. These fillers are doping agents and cause the overall
conduction characteristics to change.
Fixed and Variable Resistors
There are two kinds of resistors, FIXED and VARIABLE. The fixed resistor will have one value and will never change (other than through temperature, age, etc.). The resistors shown in A and B of figure 1-29are classed as fixed resistors. The tapped resistor illustrated in B has several fixed taps and makes more than one resistance value available. The sliding contact resistor shown in C has an adjustable collar that can be moved to tap off any resistance within the ohmic value range of the resistor.
There are two types of variable resistors, one called a POTENTIOMETER and the other a RHEOSTAT (see views D and E of fig. 1-29.)An example of the potentiometer is the volume control on your radio, and an example of the rheostat is the dimmer control for the dash lights in an automobile. There is a slight difference between them. Rheostats usually have two connections, one fixed and the other moveable. Any variable resistor can properly be called a rheostat. The potentiometer always has three connections, two fixed and one moveable. Generally, the rheostat has a limited range of values and a high current-handling capability. The potentiometer has a wide range of values, but it usually has a limited current-handling capability. Potentiometers are always connected as voltage dividers. (Voltage dividers are discussed in Chapter 3.)