Gandhinagar institute of technology

• Sub –Electrical Measurements and measuring Instruments

• Topic – Measurement of low resistances • Guided by – Prof. Rahil Parikh• Sem – 3rd• Branch – Electrical (b3)• Academic year – 2015&2016

Prepared By:-

• Sodha Manthansinh (140120109057)

Introduction

• The measurement of resistances is as important as the measurement of any other electrical parameter.

• The basic knowledge of resistance measurement is necessary to understand the working of the other instruments used for the measurement of other electrical quantities.

Classification of resistances

• (1) Low Resistances • (2) Medium Resistances • (3) High Resistances

Different methods used for measurement of Low Resistances

• (1) Voltagemeter-Ammeter Method• (2) Kelvin Double Method • (3) Potentiometer Method

(1) Voltmeter-Ammeter Method

• Rx = Resistance value to be measured• V = Voltmeter reading • I = Ammeter reading

• The two types of connections are used in this method. The connections are shown in the fig (a) and (b).

Kelvin Bridge

Ry = represents resistance of connecting lead from R3 to Rx.

Connection of G• To point ‘m’• To point ‘n’

Value of Rx

• Higher than the actual

• Lower than the actual

Figure (2)

Calculation Of Unknown Resistance by Kelvin Bridge

• From Fig.(2)

• The balance condition of bridge is+ = + Rmp)

• Substituting from eq.(1) Rx + = +

Rx =

The problems with the above method are :-

• the method is not practical

• difficult to find correct galvanometer null point

Kelvin’s Double Bridge Method

To Overcome the problem Of Kelvin Bridge , The New Bridge is Introduced ,which is used for precise measurement of low resistance called Kelvin’s Double Bridge.

Construction• Consist of 2 ratio arms• Connected resistances are P, Q,

p,q,r,S,R.• r is the resistance of slide wire• R is the unknown resistance• Rg is regulating resistance • Galvanometer (G) is connected

between point ‘F’ and ‘H’.

Working• By adjusting the balanced

condition, we can find the unknown resistance

• At Balance condition Ig = 0. • Hence voltage across ‘P’ = voltage across ‘R’ + voltage across ‘p’ VP = VR + Vp

I1P = IR + I2p ……(3)• Similarly voltage across ‘Q’ = Voltage across ‘S’ + voltage across ‘q’ VQ = VS + Vq

I1Q = IS + I2q …….(4)• Voltage across ‘r’ = voltage across (p+q)Vr = V(p+q)

So,

I2 = So from eq.(1) I1P = IR + ……….(5) I1Q = IS + ………..(6)

Calculation Of Unknown Resistance by Kelvin’s Double Bridge

• Divide eq. (5) & (6)

• Finally we get the below equation• R = …….(7)• Usually, ratio is adjusted is equal to . So eq.(7) can be expressed as

• R = • For accurate measurement of ‘R’ two readings are taken by reversing the direction of current,

the average value of these two values is taken as magnitude of unknown resistance

Calculation Of Unknown Resistance by Kelvin’s Double Bridge

Thank You