The component which is having ‘opposing force’ similar in many respects to mechanical friction is called a resistors.
The symbol of resistors is, the opposing force of resistors is called a ‘resistance’.
The resistance is measured in ohms.
In resistors they are of two groups 1) fixed, 2) variable.
1) Fixed resistors:-
The most commonly used in the fixed type resistors is ‘moulded carbon composition resistors.’
The resistive material used in this resistor is of carbon clay composition. The leads are made of tinned copper. Resistors of this type are readily available in values ranging from few ohms to about 22m having range of 5 to 20%. The relative sizes of all fixed resistors changes with the wattage (power rating).The sizes increased wattage rating in order to with stand the higher currents and dissipation losses.
A wire wound resistors:-
These resistors use a length of resistance wire, such as nichrome. This wire wound onto a round, hollow porcelain core. The ends of the winding are attached to metal pieces inserted in the core. Tinned copper wire leads are attached to these metal pieces. This assembly is coated with enamel containing powdered glass. It is then heated to develop a coating known as vitreous enamel. This coating is very smooth and gives mechanical protection to the winding it also helps in conducting heat away from the unit quickly.
Color coding and resistors values.
Some resistors are large enough in size to have their resistance in ohm printed on the body. And some are too small in size. Therefore a system of color coding is used to indicate their values. For the fixed moulded composition resistors four color bands are printed on one end of the outer casing as shown in following
The numerical value associated with each color is indicated in given table. The color bands are always read left to right from the end that has the bands closest to it. The first and second bands represent the 1st and2nd significant digits respectively of the resistance values. The third band is for the number of zeros that follow the second digit. In case it represents a multiplying factor 0.1 or 0.01. The fourth band represents the manufacturers tolerance.
2) Variable resistors:-
In electronic circuits sometimes it becomes necessary to adjust the values of current and voltages. For e.g. It is often desired to change the volume of sound, brightness of a television, picture, etc. Such adjustments can be done using variable resistors.