This document discusses various methods for measuring medium resistances between 1-0.1M ฮฉ. It describes four common methods: the voltmeter-ammeter method, substitution method, ohmmeter method, and bridge circuit method. The bridge circuit method operates on the principle of null comparison using a Wheatstone bridge configuration with ratio and standard arms. Sources of error in resistance measurements include thermal EMF and contact resistance. Worked examples are provided to calculate resistance values and measurement uncertainties.
7. Substitution Method
โข Keep at position 1 and
vary the Rh up to finite
value of current flow
through the circuit.
โข 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
9. Using Bridge Circuit
9
โข Bridge Circuit is a null method, operates on the
principle of comparison
โข A known (standard) value is adjusted until it is equal to
the unknown value
11. 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 โฆ, 100 โฆ, 1 kโฆ 10 kโฆ
R3 Min. step 1 โฆ, Max value 11110 โฆ
Example
16. 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
20. 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/ ยตA and an internal resistance
of 100 โฆ. Calculate the deflection of the galvanometer
caused by the 5 โฆ unbalance in arm BC.
Assignment
Ans: 33.2 mm
21. 21
The galvanometer in the previous question is replaced by
one with an internal resistance of 500 โฆ and a current
sensitivity of 1 mm/ยตA. 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
22. Answers to Assignment-2
1) 2.48 โฆ
2) 1.06 %
3) (a) 99.9 โฆ, 0.195 โฆ, 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 โฆ (b) 7.1 โฆ (c) 5.6 โฆ
Rnฯ R
n n
ฯ