Resistance is one of the important parameter in electrical parameters. Thus it is essential to known the resistance of any circuit to understand the behavior of any element in the circuit. Measurement of resistance is also used for the measurement of other electrical quantities. Depending upon the value of resistance they are classified into three categories,
- Low Resistance - Resistance of the order of 1Ω and below are classified as low resistance.
- Medium Resistance - Resistance ranging from 1Ω to 100Ω are classified as medium resistances
- High Resistance - Resistance of the order of 100kΩ and above are classified as high resistances
Even though there are multimeters for the measurement of resistance. In order to obtain accurate value, suppose if the resistance is very low and very high various methods are implemented. Hence for the measurement of resistance, the above classification is done and different techniques are applied for low, medium, and high values of resistances.
Measurement of Low Resistance (<1Ω) :
The various methods that can be employed for the measurement of low resistance are,- Ammeter-voltmeter Method
- Kelvin's Double Bridge Method
- Potentiometer Method
Ammeter-voltmeter Method :
This method is the simplest of all. It only requires an ammeter and a voltmeter which is easily available in the laboratory. However, it is not so accurate because of the voltage drop across the ammeter and the shunting effect of the voltmeter.
Kelvin's Double Bridge Method :
This method is most accurate compared to other methods. It eliminates the error, due to the contact and lead resistances.
Potentiometer Method :
It is also one of the accurate method of obtaining the value of resistance, but the accuracy of this method depends on the availability of a stable dc supply.
Measurement of Medium Resistance (1Ω to 100kΩ) :
The various methods that can be employed for the measurement of medium value resistance are,- Ammeter-voltmeter Method
- Substitution Method
- Wheatstone Bridge Method
- Ohmmeter Method
Ammeter-voltmeter Method :
The ammeter-voltmeter method being simple can also be used for the measurement of medium resistance, but the accuracy is again less owing to the same reason as that of low resistance measurement.
Substitution Method :
Compared to the ammeter-voltmeter method, the substitution method is quite accurate. The error due to ammeter voltage drop and voltmeter shunts are not present in this method. Even though the above errors are eliminated, there are many other causes of errors in this method, i.e., error due to variations in battery emf, error due to sensitivity of the instrument, etc. Hence this method is not used frequently.
Wheatstone Bridge Method :
This is one of the most common and popular method used for the measurement of medium resistance. It is the most accurate and reliable method of measuring the resistance. However, errors due to thermo emf and heating effect are present in it. Also, the cost of the Wheatstone bridge is quite high.
Ohmmeter Method :
The value of resistance can be directly measured by using an ohmmeter. The accuracy of an ohmmeter is moderate. But it is very simple and easy to use.
Measurement of High Resistance (>100kΩ) :
The various methods that can be employed for the measurement of high value resistance are,- Direct Deflection Method
- Loss of Charge Method
- Megger
Direct Deflection Method :
In this method, the resistance to be measured is obtained according to the deflection of the galvanometer. The galvanometer used here must be very sensitive and should have high resistance. It is quite costly, as it requires a high degree of insulation to avoid leakage currents.
Loss of Charge Method :
This method is the simplest of all, as it requires a voltmeter and a capacitor to determine the unknown resistance R. Also it is the most effective method for the measurement of high insulation resistance. However, this method is time consuming and the results are not so accurate due to the internal resistance of the voltmeter and leakage resistance of the capacitor.
Megger :
It is a portable instrument for measuring high resistances in which the voltage range can be controlled by using a voltage selector switch. It is not only used for the measurement of high resistances (insulation resistance) but also can be used for testing the insulation resistance.
The added advantage of this method is that the deflection of the instrument is independent of the field strength of the magnet. Megger is also used for testing the continuity between two points in a circuit.