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Carbon resistor consists of a high grade ceramic rod or cone (called the substrate) on which a thin resistive film of carbon is deposited. They are commonly used in electrical equipments.

## Colour Code

The numerical value of carbon resistor is represented by colour code which consists of bands, starting from left to right. The numerical value of carbon resistor is represented by colour code which consists of bands, starting from left to right. The codes of different colours are given in the following table.

## Interpretation of colour bands

1. The first band indicates the first digit of the value of resistance.
2. The second band gives the second digit.
3. The third band is decimal multiplier. It gives the number of zeros after first two digits. The third band is decimal multiplier. It gives the number of zeros after the first two digits.
4. The fourth band gives the tolerance value.

## Tolerance

The possible variation in the resistance of carbon resistor from marked value is called tolerance.

• Silver band indicates the tolerance of +_10%.
• Golden band indicates the tolerance of +_5%.
• If there is no fourth band then the tolerance value is +_20%.

## Rheostat

It is a wire wounded variable resistance.

## Construction

1. The ends of wire are connected to two fixed terminals A and B.
2. It consists of a bare manganin wire (Cu, Fe, Mn, Ni alloy) wound on an insulating cylinder.
3. A third terminal C is attached to a sliding contact which can be move over the wire. A third terminal C is attached to a sliding contact which can be move over the wire.

## Working

It can be use in two ways :

1. As a potential divider
2. As a variable resistor

## 1. Rheostat as a potential divider

If the potential difference V is applied across two fixed terminals A and B of rheostat then the current flowing through it is

I = V/R

The potential difference points B and C of the wire is VBC. So,

VBc = I × r

Thus, the potential can be varied from 0 to V volts which depends upon the position of sliding contact. If the sliding contact C is moved towards the end A, the length and hence the resistor r of the portion of wire increases which increases VBc and vice versa. The potential can be varied from 0 to V volts which depends upon the position of sliding contact. If the sliding contact C is moved towards the end A, the length and hence the resistor r of the portion of wire increases which increases VBc and vice versa.

## 2. Rheostat as a variable resistor

If one of the fixed terminal say A and the sliding terminal C are connected in the circuit, the resistance of wire between A and sliding contact C is used. When the sliding contact is shifted away from A, then the resistance increases and when the sliding contact is moved towards A, the resistance decreases. If one of the fixed terminal say A and the sliding terminal C are connected in the circuit, the resistance of wire between A and sliding contact C is used. When the sliding contact is shifted away from A, then the resistance increases and when the sliding contact is moved towards A, the resistance decreases.