Measurement of Resistance

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         9.1. Voltmeter-ammeter method

Figure 31 (a) as well as (b) demonstrates two ways of relations for measurement of the resistance

Figure 31 Ways for measuring the resistance

Here Ra is called resistance of ammeter ( it is extremely small)

Rv is called resistance of voltmeter (it is the big value)

R indicates the resistance measured

V is called reading of voltmeter, then

I is called reading of ammeter

Figure 31 (a) demonstrates the links for measurement of the low resistances.

Figure 31 (b) demonstrates the links for the measurement of the high and the low resistances

9.2. Substitution Method.

 The links for the measurement of the resistance by a substitution is shown in Figure 32

Figure 32 Measurement of the resistance by a substitution

Here R can be a flexible resistance and it can be transformed in minor steps, say like 0.1 Ω.

  • Reference Figure 32 (a) Leading resistance X is placed in a circuit and the rate of current is noted.

At that time resistance X is detached then it is exchanged by a recognized flexible resistance ‘R’ which is diverse so that a value of a current is equivalent in both of the matters. This importance of R is alike to the unidentified resistance.

(ii)                        If a R is of secure importance note the analyses of an ammeter for the succeeding things:

For resistances of the X and the R in series

At what time resistance X is indifferent

Let the analyses of an ammeter for these matters be I1 then I2

9.3. Measurement of Resistance by the Wheatstone bridge

 In the Wheatstone bridge way of evaluating resistances, a device of unidentified resistance is composed against devices of recognized resistances. Though constructed on link of resistances, it is some of the best accurate ways of evaluating resistances by way of it is free of the standardization of the representative instrument then bank on the null point way.

The four subdivisions of the system ABCDA (see Figure33) has two recognized resistances P plus Q, a recognized flexible resistance R as well as the unidentified resistance X. A and E are connected over a switch S1 to connections A then C; as well as a galvanometer G, a flexible resistor Z as well as a switch S2 are in sequence diagonally Band D. The purpose of Z is only to look after G beside a moderate current have to be the system very out of balance at what time S2 is sealed.

Next closing S1 as well as S2, here R is accustomed till there is not any swerve on G by the resistance of Z condensed to nil. Links Band D are at that time at the equivalent potential,

Hence that p.d. in the middle of A and B is the equivalent as that in the middle of A as well as D, and the p.d. in the middle of B as well as C is the same as that in the middle of C as well as D.

Figure 33 Wheatstone bridge

I1 in addition to I2 are the flows from side to side P and R in turn when the link is stable. After Kirchhoff’s first rule it trails that from the time when here is no current in G, the currents in Q as well as X are too I1 as well as I2 respectively.

which is the relative of Wheatstone bridge. P and Q are generally called the proportion arms and X as adjust or rheostat arm. The battery and the galvanometer can be exchanged without influencing the connection.

Note. The resistances P and Q may appear as the resistance of a slide-wire, in which case R might be a settled esteem and adjust is acquired by moving a sliding contact along the wire. On the off chance that the wire is homogeneous and of uniform segment, ‘the proportion of P to Q is the same as the proportion of the lengths of wire in the individual arms.


For the precise estimation of potential contrast, current and resistance the potentiometer is a standout amongst the most helpful instruments.

Its guideline of activity is that an obscure e.m.f. or.p.d. is measured by adjusting it, entirely or to some degree, against a known distinction of potential.

Development, The potentiometer, in its easiest frame, comprises of wire LM of uniform cross-segment, extended close by a scale and associated over a gatherer B of adequate limit. A standard cell of known e.m.f. E1 is associated amongst L and terminal 1 of a two-way switch S, care being taken that the relating terminals of Band E1 are associated with L.


– Slider N is squeezed quickly against wire LM and its position balanced until the galvanometer avoidance is zero when N is reaching LM. Give l1 a chance to be the comparing separation between Land N. The fall of potential over length l1 of the wire is then the same as the e.m.f. E1·

– Then move the change S to 2, consequently supplanting the standard cell by another cell, the e.m.f E2 of which is to be measured. Change the slider N again to give zero diversion on G. On the off chance that I2 be the new separation between Land N, then

Uses of potentiometer.

 The accompanying are the applications/employments of potentiometers:

Estimation of little e.m.fs (upto 2 volts).

Correlation of e.m.fs of two cells

Estimation of high e.m.fs (say 250 volts)

Estimation of resistance

Estimation of current

Alignment of ammeter

Alignment of voltmeter

Illustration 11

Utilizing a Weston cadium cell of 1.0183 V and a standard resistance of 0.1 Ω a potentiometer was balanced so that 1.0183 m was identical to the e.m.f. of the cell; when a specific direct current was moving through the standard resistance, the voltage crosswise over it compare to 150 cm. What was the estimation of current?


E.m.f. of a standard cell, E1 is 1.0183 V

Potentiometer modification is I1 = 1.0183 m

What’s more, I2 = 150 cm = 1.5 m

Voltage over the standard resistance of 0.1 Ω comparing to I2,



Meggers (or the megohmmeters) are tools which measure the protection resistance of electric circuits in respect to earth and each other.

A megger comprises of an e.m.f. source and a voltmeter. The size of the voltmeter is adjusted in ohms (kilo-ohms or megohms, all things considered). In estimations the e.m.f. of the independent source must be equivalent to ·that of the source utilized as a part of alignment.

Fig. 35 indicates diagrammatically a megger whose readings are autonomous of the speed of theself-contained generator. The moving framework consolidates two curls 1 (current loop) and 2 (weight loop) mounted on a similar shaft and set in the field of a lasting magnet (not appeared) 90° separated, The generator empowers the two curls over particular wires. Associated in arrangement with one loop is a settled resistance R1 (or a few distinct resistances keeping in mind the end goal to expand the scope of the instrument). The obscure resistance Rx is associated in arrangement with the other curl. The streams in the curls connect with the attractive field and create contradicting torques

Figure 35 Circuit outline of megger

The diversion of the moving framework relies on upon the proportion of the streams in the curls and is autonomous of the connected voltage. The obscure resistance is perused straightforwardly from the size of the instrument. (The precision of estimation is unaffected by varieties in the speed of the produce in the vicinity of 60 and 180 r.p.m.).


This one is not feasible to conned instruments and meters specifically to the lines in high voltage circuits. Rather instrument transformers are utilized. The accompanying are the two essential preferred standpoint inborn in this strategy:

(i) Standard evaluated instruments might be utilized.

(ii) Operating staff interacting with the instruments are not subjected to high voltage and current of the lines, thus there is less peril to them. Indeed, even with a low-voltage framework, instrument transformers are utilized for measuring substantial streams, so overwhelming prompts to the instrument board and to the ammeter and other current terminals are maintained a strategic distance from.

The guideline of the instrument transformer is on a very basic level the same as that of the power transformer. The instrument transformers are delegated takes after:

1.Potential transformers

2.Current transformers

12.1. Potential Transformers (P.T.)

A potential transformer is a stage down transformer utilized alongside a low range voltmeter for measuring a high voltage. The essential is associated over the high voltage supply and the optional to the voltmeter or potential loop of the wattmeter. Since the voltmeter (or potential curl) impedance is high, the auxiliary current is little and the potential transformer carries on as a conventional two winding transformer working on no-heap. Fig. 36 demonstrates a potential transformer used to gauge the voltage of a circuit. It might be noticed that the optional is grounded. This is done as such that if the protection separates, the high voltage does not jeopardize work force who might read the meters

Figure 36 Potential transformer associations.

These transformers are made with superb iron center working at low flux densities so that the polarizing current might be little. Watchful plan guarantees least variety of voltage proportion with load and least stage move amongst info .and yield voltages. Potential transformers auxiliary are normally intended for a yield of 110 V.

12.2. Current Transformers or (C.T.)

Similarly as a shunt expands the scope of a D.C. ammeter, so does the present transformer play out a similar capacity in A.C. circuits. In this way a high size exchanging current can be effortlessly measured by a blend of a present transformer and a low range ammeter.

The essential of a present transformer (C.T.) comprises of a couple turns of thick cross-area associated in arrangement with the high current line. All the time the essential is only one turn, shaped by taking the line conductor through the optional winding (Fig. 37). The auxiliary twisting comprises of an extensive number of turns of fine wire intended for either 5A or 1 A rating. Hence a present transformer is venture up transformer. The present transformer

has the optional successfully short circuited through the low impedance of the ammeter. Fig. 38 demonstrates the present transformer association.

Figure 38 Current transformer associations

Figure 37 Line conductor acting as primary.

The present transformer proportion is not equivalent to the proportion of optional to essential turns, for the most part due to the impact of the charging current. The essential current can be considered as the whole of two streams, the first to adjust optional current so that essential and auxiliary m.m.fs. may adjust and the second being the no-heap current I0, The parts other than being in charge of a slight blunder in the present proportion, is likewise in charge of a stage point mistake. The transformer must be painstakingly intended to limit the proportion and stage point blunder.

It might be noticed that present transformer should never be worked on open-circuit for the accompanying two reasons:

(i) There will be no optional m.m.f. also, since the essential current (and m.m.f.) is settled, the center flux will increment massively. This will bring about huge whirlpool current and hysteresis misfortunes and the subsequent high temperature may harm the protection or even the center.

(ii) A high voltage will be initiated in the multi-turn auxiliary and this high voltage might be risky both to life and to the protection.

Figure 39 demonstrates the wiring graphs for potential and current instrument transformers.


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