Hello friends, in this blog article we will learn different methods for the measurement of resistance. We will first see, what are the different types of methods in short and will explain the same in details in separate blog articles.
Measurement of Resistance
Before learning different types of methods for measurement of resistance, we will first see the classification of resistance. From the point of view of measurement, we have three types of resistances as follows:
- Low resistance: < 1Ω
- Medium resistance: 1Ω to 100,000Ω
- High resistance: >100,000Ω
A. Measurement of Low resistance
1. Ammeter-Voltmeter method: This is a very popular method for measurement of medium resistances since instruments required for this method are usually available in the laboratory.
2. Kelvin’s double bridge method: The Kelvin double bridge is the modification of the Wheatstone bridge and provides greatly increased accuracy in measurement of low-value resistance.
3. Potentiometer method: The potentiometer is extensively used for calibration of voltmeters and ammeters and has, in fact, become the standard for the calibration of these instruments.
B. Measurement of Medium resistance
2. Substitution method: This is a more accurate method than ammeter-voltmeter method. The accuracy of this method is greatly affected if the emf of the battery changes during the time of readings.
3. Wheatstone bridge method: Another method of measuring the value of a resistor is the Wheatstone bridge. This device sets up a parallel resistor system that measures the differences in voltage between two legs of a circuit. If there is a difference of voltage between the branches it will be detected by the galvanometer.
4. Ohmmeter method
C. Measurement of High resistance
1. Direct deflection method: Direct deflection method for measurement of high resistance
2. Loss of Charge method: In the loss of charge method unknown resistance is connected in parallel with the capacitor and electrostatic voltmeter. The capacitor is initially charged to some suitable voltage by means of a battery of voltage V and then allowed to discharge through the resistance.
3. Megohm bridge: Megohm bridge is another important method for measurement of high resistances. It has one three terminal high resistance located in one arm of the bridge.
4. Megger: We know that the ratiometer ohmmeters may be designed to cover a wide range of resistances. The principle of ratiometer ohmmeters is particularly adapted to application in portable instruments measuring insulation resistance. This principle forms the basis of insulation testing instrument known as Meggar.
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