Resistance Measurement instruments

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  • Classification of Resistance MeasurementsLow resistances (0-1)

    Medium resistances (1-0.1M)

    High resistances ( 0.1M)

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  • 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.

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  • 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.

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  • 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 )

    ISEIM 2008*

  • *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

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  • *Measurement Errors

  • Thermal EMF*Creates error in measurement of low resistance

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    Contact ResistanceOn the micro scale, surface roughness limits surface to surface contactMeasurement using four terminals to avoid contact resistanceStandard resistance model with contact resistance

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  • *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

    ISEIM 2008

    ISEIM 2008*