resistance measurement methods

P and Q are known as the ratio arms. 2 wires carry the current, known as the source …

An EMF source is connected between points a and b while a galvanometer is connected between points c and d.The figure below shows the circuit diagram for resistance measurement of an unknown resistance R. S is a standard variable resistance and r is a regulating resistance.Following are few methods used for measurement of high resistance values-We normally utilize very small amount of current for such measurement, but still owing to high resistance chances of production of high voltages is not surprising.

Before proceeding further we will refresh about basics of resistance.The property of the material which oppose the flow of electrons is called as Resistance of the material. Figure below shows two connections on voltmeter and micro ammeter to measure R, one without guard circuit and one with guard circuit.In this method we utilize the equation of voltage across a discharging In this method we use the famous Wheatstone bridge philosophy but in a slightly modified way. Some of the methods areDirect deflection method with the use of high sensitivity galvanometerIn this post we will discuss about the Ohmmeter in detail.Ohmmeter is an electrical apparatus used to measure electrical resistance. It uses one ammeter to measure current, I and one voltmeter to measure voltage, V and we get the value of resistance asThis is the simplest and the most basic bridge circuit used in measurement studies. Example - Cable porcelain insulators, Insulation resistance of machines etc,Several methods are available to measure the resistance in each class.

The two types of connections employed for ammeter voltmeter method are shown in below figure. Thus to eliminate this issue small valued resistance are constructed with four terminals. The full-scale deflection on the milliammeter is made zero on the resistance scale. If current I is at a maximum value and voltage E is constant, then resistance R = E/I is at a minimum value.

Some basic methods are suitable for low as well as medium resistances.

Some basic methods are suitable for low as well as medium resistances. Example - Multiplier resistance, wire and cable resistance etc. In electrical systems, the measurements of insulation resistance are very important among all high-resistance measurements.

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The four wire (Kelvin) method of measurement is preferred for resistance values below 100Ω, and all Seaward milliohmmeters and microhmmeters use this method. Figure be…

There is no single method of measuring the entire range in one apparatus. It comprises of a permanent magnet similar to that of a Following are the methods employed for measuring a resistance whose value is in the range 1Ω – 100kΩ –This is the most crude and simplest method of measuring resistance. The methods are listed below.

Ammeter- voltmeter method; It mainly consists of four arms of resistance P, Q; R and S. R is the unknown resistance under experiment, while S is a standard resistance. The concept of guard circuit is to bypass the leakage current from the ammeter so as to measure the true resistive current.

This resistance measurement method is very popular since the instruments required for this test are usually available in the laboratory. The major problem in measurement of low resistance values is the contact resistance or lead resistance of the measuring instruments, though being small in value is comparable to the resistance being measured and hence causes serious error. The methods are listed below. (1) Measurement of Low Resistance. The symbol is the capital Greek letter omega (Ω) .The opposition to a steady current in a circuit is called as resistance. This opposition is due to the collisions between electrons and between electrons and other atomsin the material.

For the convenience of measurement, the resistances are classified as Example The resistance of series filed windings ammeter shunts, cable resistance, machine armatures. Two terminals are current terminals and other two are potential terminals.

Due to this we encounter several other problems such as-Hence to solve the problem of leakage currents or capacitive currents we use a guard circuit. The foundation of resistance measurement, Ohm’s Law states that the ratio of potential difference (V) across the ends of a conductor of resistance (R) to the current (I) flowing in that conductor will be constant, provided that the temperature also remains constant. In both the cases, is readings of ammeter and voltmeter are taken, then the measured value of resistance is given by : This method is derived from the 4-point method, which is used for soil resistivity measurements.

The methods employed for measurement of low resistances are:-It is an electromechanical instrument used for measurement of low resistances. So the milliammeter can be calibrated directly in ohms and thus a non-linear scale is used.

This method of measurement which avoids errors caused by wire resistance is called the Kelvin, or 4-wire method.