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12. MEASUREMENT OF LOW RESISTANCES 12.1. Task of the measurement 12.1.1. Measurement using voltmeter-ammeter method (Ohm’s method). Connect the measuring circuit according to Fig. 12.1. Use suitable measurement procedure to exclude influence of thermo-electrical voltages. Calculate measured resistance RX from the measured voltage and current and estimate the expanded uncertainty of measured resistance (coverage factor k = 2). 12.1.2. Measurement using series comparison method. Connect the measuring circuit according to Fig. 12.2. Measure the voltage drop across the resistance standard RN and across the measured resistor RX. Use suitable measurement procedure to exclude influence of thermo-electrical voltages. Calculate measured resistance RX and estimate the expanded uncertainty of measured resistance (coverage factor k = 2).

12.2. Schematic diagram RP

C A

A C DC POWER SUPPLY

RX

DV

RP

DC POWER SUPPLY

Fig. 12.1 Measurement of low resistance by V-A method

RX

RS

DV

DV

Fig. 12.2 Measurement of low resistance by series comparison method

12.3. List of the equipment used DC power supply A RP RX RS DV C

- type ...; - moving coil ammeter, accuracy class ..., range: …; - current limiting resistor ... Ω; - measured resistor; - resistance standard ... Ω, tolerance ... %; - digital voltmeter, type ..., error ... + ..., measurement range ...; - commutator

Theoretical background See the textbook: M. Sedláček, J. Holub, D. Hejtmanová: Laboratory Excercises in Electrical Measurements, CTU Publishing House, Prague, 2005

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