## Introduction

In the electrical field, resistance is an important property. And we always discuss about resistance. because many of the phenomena occurred due to the resistance of the material.

## What is resistance?

‘The property of a material by virtue of which it opposes the flow of current is called resistance.’

If we talk about every type of material that is available on earth, every material has a resistance property. But some materials have low resistance, and others have high resistance. It depends on the nature of the material.

Metal has lower resistance than nonmetals. Most of the nonmetals called as insulators because of highest resistance. Due to very high resistance, current can’t flow through an insulator.

Metal, which have low resistance and can flow current through it. But different types of metal have different resistance. Like silver, it has the lowest resistance. And can flow current easily. But copper and aluminium have higher resistance than silver. Although we use aluminium and copper metal for wire and cable, because silver metal is very costly. And this metal has low mechanical strength.

Resistance denoted by R. And unit of resistance is Greek letter Ω (ohm).

### Resistivity

To not flow current through metals. It is called the resistivity of that metal. Resistivity presents the strength and quality of the material in terms of resistance. It depends on the metal’s quality. The unit of the resistivity is ohm – meter (Ω- m). Resistivity depends on the nature of the material.

### Conductivity

Conductivity is the reciprocal of resistivity. Unit of conductivity is ℧ (mho) (reciprocal of ohm Ω).

## Classification of resistance

Initially, we can divide in three parts of resistance.

- Low resistance
- Medium resistance
- High resistance

## Low resistance

when the resistance of the material is less than 1 Ω. It is called low resistance. For example, the resistance of a DC motor’s armature is 0.2 ohm. which has low resistance. Kelvin double bridge method used for measuring low resistance.

## Medium resistance

If the resistance range is 1Ω – 5000 Ω, it is called the medium resistance of the metal.

*What is Buchholz Relay? And why it is so important?*

## High resistance

If the resistance of a material is above 5000Ω. This resistance range is considered high for the material.

## Formula

Suppose that you have a material. And you want to know the resistance of that material. So, there is a formula. which can help find out the resistance of that material.

**R = ρ (l/A)**

Where ‘R’ is resistance of the material. ‘ρ’ is resistivity of the material. ‘A’ is the cross-sectional area of the material. And “l” is the length of the material.

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