The document discusses various resistance measurement methods, including voltmeter-ammeter, substitution, ohmmeter, and bridge methods, highlighting their advantages and limitations. It details the Wheatstone bridge's operation, measurement errors due to thermal EMF and contact resistance, and provides schematics and calculations related to galvanometer deflection. The document also includes assignments and answers for further understanding of resistance measurements.

1. Resistance Measurement Methods
(1) Voltmeter-Ammeter (2) Substitution
(3) Ohmmeter (4) Bridge

2. Voltmeter-Ammeter Method
(1) Simple and requires only two meters
(2) Subject to error

3. Ohmmeter Method
(1) Only one meter
(2) Non-linear scale

4. Using Bridge Circuit
4
• Bridge Circuit is a null method, operates on the
principle of comparison
• A known (standard) value is adjusted until it is equal to
the unknown value

5. 5
Wheatstone Bridge
Ratio arms (R1 , R2) & Standard arm (R3 )

6. 6
Wheatstone Bridge Apparatus
The Ratio Control switches the
ratio in decade steps
The remaining four step switches
set the resistance of the standard
arm
R1 , R2 10 Ω, 100 Ω, 1 kΩ 10 kΩ
R3 Min. step 1 Ω, Max value 11110 Ω
Example

7. 7
Measurement Errors

8. Thermal EMF
8
Creates error in measurement of low resistance

9. 9
Contact Resistance
On the micro scale, surface roughness
limits surface to surface contact
Measurement using four terminals
to avoid contact resistance
Standard resistance model with contact
resistance

10. 10

11. 11

12. 12

13. 13
Figure below show the schematic diagram of a Wheatstone
bridge with the bridge elements. The battery is 5 V and its
internal resistance negligible. The galvanometer has a
current sensitivity of 10 mm/ µA and an internal resistance
of 100 Ω. Calculate the deflection of the galvanometer
caused by the 5 Ω unbalance in arm BC.
Assignment
Ans: 33.2 mm

14. 14
The galvanometer in the previous question is replaced by
one with an internal resistance of 500 Ω and a current
sensitivity of 1 mm/µA. Assuming that the minimum
deflection that can be observed on this galvanometer is 1
mm, determine what is the minimum unbalance in the
resistance that can be detected in arm BC.
Assignment Continued

15. Answers to Assignment-2
1) 2.48 Ω
2) 1.06 %
3) (a) 99.9 Ω, 0.195 Ω, 0.195 %
(b) (i) 80 (ii) 98
(c) (i) 68 (ii) 95
4) (a) 38.3 % (b) 68.3 % (c) 95.5 % (d) 15.7 %
5) For series: For parallel:
6) (a) 8.7 Ω (b) 7.1 Ω (c) 5.6 Ω
Rnσ R
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