# Resistance Measurement Methods

### Measurement of Resistance Introduction:

In this post we will discuss about various methods of measuring the resistance of the material. Before proceeding further we will refresh about basics of resistance.

#### Resistance Introduction:

The property of the material which oppose the flow of electrons is called as Resistance of the material. This opposition is due to the collisions between electrons and between electrons and other atoms
in the material. This process will convert electrical energy into another form of energy such as heat.

The unit of measurement of resistance is the ohm. The symbol is the capital Greek letter omega (Ω) .

#### Types of Resistance:

• The opposition to a steady current in a circuit is called as resistance.
• In electrical and electronic circuits, wide range of resistance values ( fractional ohm to several mega ohms) are used.
• There is no single method of measuring the entire range in one apparatus.

For the convenience of measurement, the resistances are classified as

1. Low Resistance of value less than 1 Ohm
Example The resistance of series filed windings ammeter shunts, cable resistance, machine armatures.
2. Medium Resistances of values 1 ohm to 10 Kohm
Example - Multiplier resistance, wire and cable resistance etc.
3. High resistances above 10 KOhm
Example - Cable porcelain insulators, Insulation resistance of machines etc,

#### Resistance Measurement Methods

Several methods are available to measure the resistance in each class. Some basic methods are suitable for low as well as medium resistances. The methods are listed below.

(1) Measurement of Low Resistance

1. Ammeter- voltmeter method
2. Potentiometer method
3. Kelvin double bridge method
4. Ohm-meter method

(2) Measurement of Medium resistance

1. Ammeter- voltmeter method
2. Whetstone bridge method
3. Carey-Foster bridge method

(3) Measurement of high resistance
In electrical systems, the measurements of insulation resistance are very important among all high-resistance measurements. Some of the methods are

1. Megger Method
2. Direct deflection method with the use of high sensitivity galvanometer
3. Loss of charge method

In this post we will discuss about the Ohmmeter in detail.

#### Measurement of Low Resistance by Ohmmeter Method:

Ohmmeter is an electrical apparatus used to measure electrical resistance. The basic ohmmeter circuit is shown below.

• As shown it has an internal voltage source, ie. battery. Remember that in case of ammeter or voltmeter, they receive the energy necessary for their operation from the circuit under test.
• Initially , the terminals x-x are short circuited and R is adjusted to give the full-scale deflection on the milliammeter.
• If current I is at a maximum value and voltage E is constant, then resistance R = E/I is at a minimum value.
• The full-scale deflection on the milliammeter is made zero on the resistance scale.
• When terminals XX are open circuited no current flows and R(V/0) will be infinity.
• So the milliammeter can be calibrated directly in ohms and thus a non-linear scale is used.
• When an unknown resistor is placed between terminals x-x, the resistance value can be determined from the position of the pointer on the scale.

Continuity tester:
An ohmmeter designed for measuring low values of resistance is called as continuity tester.

Megger:
An ohmmeter designed for measuring high values of resistance is called as insulation resistance tester or megger.

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