Resistance Measurement instruments
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Transcript of Resistance Measurement instruments
Classification of Resistance MeasurementsLow resistances (0-1)
Medium resistances (1-0.1M)
High resistances ( 0.1M)
Medium Resistance Measurement Methods(1) Voltmeter-Ammeter(2) Substitution(3) Ohmmeter(4) Bridge
*Voltmeter-Ammeter MethodAs we know that the internal resistances of the voltmeter and ammeter are large and small, respectively.
Voltmeter-Ammeter MethodSimple and requires only two metersSubject to error
Substitution MethodKeep 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.
Ohmmeter MethodOnly one meterNon-linear scale
Using Bridge Circuit*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
*Wheatstone Bridge Ratio arms (R1 , R2) & Standard arm (R3 )
*Wheatstone Bridge Apparatus The Ratio Control switches the ratio in decade steps
The remaining four step switches set the resistance of the standard armR1 , R2 10 , 100 , 1 k 10 kR3 Min. step 1 , Max value 11110 Example
Thermal EMF*Creates error in measurement of low resistance
Contact ResistanceOn the micro scale, surface roughness limits surface to surface contactMeasurement using four terminals to avoid contact resistanceStandard resistance model with contact resistance
*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/ A and an internal resistance of 100 . Calculate the deflection of the galvanometer caused by the 5 unbalance in arm BC.AssignmentAns: 33.2 mm
*The galvanometer in the previous question is replaced by one with an internal resistance of 500 and a current sensitivity of 1 mm/A. 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-21) 2.48 2) 1.06 %3) (a) 99.9 , 0.195 , 0.195 % (b) (i) 80 (ii) 98 (c) (i) 68 (ii) 954) (a) 38.3 % (b) 68.3 % (c) 95.5 % (d) 15.7 %
5) For series: For parallel:
6) (a) 8.7 (b) 7.1 (c) 5.6