Classification of Resistance

Measurements

• Low resistances (0-1) Ω

• Medium resistances (1-0.1M)Ω

• High resistances (≥ 0.1MΩ)

1

Medium Resistance Measurement

Methods

(1) Voltmeter-Ammeter (2) Substitution

(3) Ohmmeter (4) Bridge

3

Voltmeter-Ammeter Method

As we know that the internal resistances of the voltmeter and ammeter are large and small, respectively.

4

5

Voltmeter-Ammeter Method

(1) Simple and requires only two meters

(2) Subject to error

Substitution Method

• Keep at position 1 and

vary the Rh up to finite

value of current flow

through the circuit.

• Shift to position 2 with the

same value of Rh in

previous case, change

the S value until the

same/previous current

flow through the circuit.

7

Ohmmeter Method

(1) Only one meter

(2) Non-linear scale

Using Bridge Circuit

9

• 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

10

Wheatstone Bridge

Ratio arms (R1 , R2) & Standard arm (R3 )

11

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

12

13

14

Measurement Errors

Thermal EMF

15

Creates error in measurement of low resistance

16

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

17

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19

20

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

21

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

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

Rn R

n n