# Electric resistance

**Equation of resistance**

In the topic of Ohm’s law, a formula that states the relationship between the voltage (V), electric current (I), and electrical resistance (R) has been derived. Mathematically expressed through equations:

This equation shows that the electrical resistance (R) is directly proportional to the electric voltage (V) and inversely proportional to the electric current (I). If the mains voltage is greater then the electrical resistance is getting bigger, on the contrary, if the stronger the electric current gets bigger then the electrical resistance will be greater. This equation explains Ohm’s law only when the electrical resistance (R) is constant. If the electrical resistance is not constant then this equation does not explain Ohm’s law but explains the resistance of a conductor.

In the article about Ohm’s law, the relationship between the electrical conductor (R) resistance, type resistance or conductivity resistivity (ρ), conductor length (l), and conductor cross-section area (A) have been explained. Mathematically expressed in the equation:

Based on this equation it is concluded that electrical resistance is directly proportional to the resistance of the type and length of the conductor, but is inversely proportional to the cross-sectional area of the conductor. In other words, the electrical resistance increases when the resistance of the type and/or length of the conductor increases, whereas the electrical resistance decreases if the cross-sectional area of the conductor increases.

If the conductor length (l) and cross-sectional area (A) are constant, the electrical resistance (R) depends only on the resistance type (ρ). If the type of resistance increases, the electrical resistance increases, conversely if the type of resistance is reduced then the electrical resistance decreases. The resistance value of each type of conductor is directly proportional to the temperature, if the temperature gets higher then the type of resistance is getting bigger or vice versa if the temperature gets lower then the resistance of the type gets smaller.

The opposite of resistance type (resistivity) is conductivity. Conductivity is inversely proportional to resistivity, as stated in the equation below:

If expressed in conductivity, equation 2 changes to:

Based on this equation it can be concluded that the electrical resistance is inversely proportional to conductivity, if the conductivity of the conductor is greater then the electrical resistance becomes smaller or vice versa if the conductivity of the conductor gets smaller then the electrical resistance is greater.

The resistance value of each type of material is shown in the table below.

**Unit of resistance**

The electricity unit is Volt, the unit of electric current is Ampere so that the unit of electrical resistance is Volt / Ampere. Volt / Ampere is also called Ohm symbolized by Ω, appreciating Georg Simon Ohm (1789-1854), a German physicist. Besides Ohm, it is also used kilo-ohm and megaohm units. 1 kilo-ohm (kΩ) = 1000 Ω = 10^{3} Ω, while 1 mega-ohm (MΩ) = 1000,000 Ω = 10^{6} Ω.

**Measuring instrument of resistance**

The measuring instrument of electrical resistance is an ohmmeter. This measuring instrument has been integrated into a multimeter, a device for measuring electrical voltage, electric current, and electrical resistance.