This ppt is about different Methods for measuring low resistance. 1) Voltmeter- Ammeter Methods 2) Kelvin's Bridge 3) Kelvin's Double Bridge

1. Government Engineering
College, Modasa
Electrical Measurement And Measuring Instruments
Module Name : Measurement Of Resistance
Topic Name : Measurement Of Low Resistance
Presented By:
Kazim Marfatiya

2. Classification Of Electrical Resistance
High resistance: under this category resistance is greater than
0.1 M ohm.
Medium resistance: under this category resistance is ranging
from 1 ohm to 0.1M ohm.
Low resistance: under this category resistance value is lower
than 1 ohm.

3. Measurement of Low Resistance
R<1Ω
• The figure shows the construction
of low resistance
• For the protection of low
resistance, it is constructed with
four terminals
 CC’ = Current Terminals
 PP’ = Potential Terminals
• The value of low resistance is
measured between the potential
terminal P and P’.

4. Method of Measurement of Low Resistance
• Voltmeter and Ammeter Method
• Kelvin Bridge Method
• Kelvin’s Double Bridge Method
• Recall our classification done above, as we move from top to
bottom the value of resistance decreases hence, we require
more accurate and precise device to measure the low value of
resistance
• We need some modification in Wheatstone bridge itself, and
the modified bridge so obtained is Kelvin bridge, which is not
only suitable for measuring low value of resistance but has
wide range of applications in the industrial world

5. Voltmeter and Ammeter Method
• There are two methods of connecting voltmeter
and ammeter for measurement of resistance as
shown in figure.
• In both cases measured value of unknown
resistance is equal to the reading of voltmeter
divided by reading of ammeter.
 From fig.(1)
R = Rm ퟏ −
푹풂
푹풎
 Ideally R = Rm only when Ra = 0
 From fig.(2)
 R =
푹풎
ퟏ−
푹
풎
푹
풗  Ideally R = Rm when resistance of voltmeter
is ‘∞’

6. 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)

7. Calculation Of Unknown Resistance by Kelvin Bridge
• From Fig.(2)
Rnp
Rmp
= R1
R2
• The balance condition of bridge is
Rx +Rnp =
R1
R2
(R3 + Rmp)
• Substituting from eq.(1)
Rx +
R1
R2
Rmp =
R1
R2
R3 +
R1
R2
Rmp
Rx =
R1 R3
R2
 The problems with the above method are :-
• the method is not practical
• difficult to find correct galvanometer null point

8. 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

9. Calculation Of Unknown Resistance by Kelvin’s Double Bridge
• 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 = 풓
풑+풒+풓
푰
So from eq.(1) I1P = IR + 풓
풑+풒+풓
푰 풑 ……….(5)
I1Q = IS + 풓
푰 풒 ………..(6)
풑+풒+풓

10. Calculation Of Unknown Resistance by Kelvin’s Double Bridge
• Divide eq. (5) & (6)
• 퐼1푃
퐼1푄
=
퐼푅+
푟푝
푝+푞+푟
퐼
퐼푆+
푟푞
푝+푞+푟
퐼
• Finally we get the below equation
• R =
푃
푄
푆 +
푞푟
푝+푞+푟
푃
푄
−
푝
푞
…….(7)
• Usually, ratio
푃
푄
is adjusted is equal to
푝
푞
. So eq.(7) can be expressed as
• R =
푃
푄
푆
• 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