DCbalancedbridgeforresistancetestIntroduction.DC balanced bridge method is a commonly used resistance measurement method,mainly including DC single arm bridge and DC double arm bridge. The single-armbridge is mainly used for median resistance test (10-10° ohms).The DC dual-arm bridgemainly measures low-value resistance that below 10 ohms.Experimental objectives(1) Understand the classification of the bridge, and let the students understand andmaster the basic principles and measurement methods ofthe Wheatstone bridge andthe Kelvin bridge resistance through teaching,(2)Master the characteristicsand conditions of thebalanced bridge;(3) Improve students' ability to analyze and solve problems through experiments, andinprove students'professional ability to use instruments and meters proficiently.Experimental InstrumentsWiringboard,galvanometer,wire,standard six-position resistancebox,slidingvaristor,toggle switch, reset switch, resistance to be tested, fixed value resistor, DC powersupply,QJ44 typeDC double-arm bridge, wire to be tested (copper Silk,aluminumwire), spiral micrometerExperimental principle and procedure1. DC single-arm bridge (Wheatstone bridge) measured median resistanceThe Wheatstone bridge principle is shown in the figure on the right. The four resistorsRi, R2, Rs and the resistor Rx to be tested form the four arms of the bridge. Thegalvanometer branch between B and D is called “bridge".Adjust Rs to make thepotentials between B and D equal. At this time, the galvanometer indicator is zero, thatis, the bridge reaches equilibrium. When the bridge is balanced, the resistance of thefour resistors is proportional (equilibrium condition), and the resistance of the resistortobetested isobtained:
DC balanced bridge for resistance test Introduction： DC balanced bridge method is a commonly used resistance measurement method, mainly including DC single arm bridge and DC double arm bridge. The single-arm bridge is mainly used for median resistance test (10-106 ohms). The DC dual-arm bridge mainly measures low-value resistance that below 10 ohms. Experimental objectives (1) Understand the classification of the bridge, and let the students understand and master the basic principles and measurement methods of the Wheatstone bridge and the Kelvin bridge resistance through teaching; (2) Master the characteristics and conditions of the balanced bridge; (3) Improve students' ability to analyze and solve problems through experiments, and inprove students' professional ability to use instruments and meters proficiently. Experimental Instruments Wiring board, galvanometer, wire, standard six-position resistance box, sliding varistor, toggle switch, reset switch, resistance to be tested, fixed value resistor, DC power supply, QJ44 type DC double-arm bridge, wire to be tested (copper Silk, aluminum wire), spiral micrometer Experimental principle and procedure 1. DC single-arm bridge (Wheatstone bridge) measured median resistance The Wheatstone bridge principle is shown in the figure on the right. The four resistors R1, R2, Rs and the resistor Rx to be tested form the four arms of the bridge. The galvanometer branch between B and D is called “bridge”. Adjust Rs to make the potentials between B and D equal. At this time, the galvanometer indicator is zero, that is, the bridge reaches equilibrium. When the bridge is balanced, the resistance of the four resistors is proportional (equilibrium condition), and the resistance of the resistor to be tested is obtained:
(3-5)Rx=R1/R2RsThe task objectives of the experiment are proposed:(1). correctly connect the circuit according ti the principle;(2).Adjust the bridge balance;(3). Measure the resistance of the four resistances to be tested (472, 4702, 3k2,47k2).2. DC dual-arm bridge (Kelvin bridge) measuring low-value resistanceThe Kelvin bridge principle is shown on the right. Compared with the single-armbridge, the double-arm bridge adds two high-value resistors R3 and R4 to form a six-arm bridge.Because there are two proportional arms, it is called a double-armed bridgeThe bridge always meets:(3-5)R//R2=R3/R4By deriving, the balance conditions of the double-arm bridge and the one-arm bridgecan be obtained in the same form, eliminating the influence of the additional resistancer, and thereforesuitableformeasuringlow-valueresistors.The switch remains off during the connection of the circuit. The key to emphasizethe experiment isthe choiceof bridgemagnification.Accordingto theresistance oftheresistanceto be tested,the appropriatemagnification is selected.The selection principleis that the highest-order position of the standard six-position resistance box is not zero.The galvanometer should be zeroed before use, and the pointer should not be fullybiased for a long time during the experiment. Set the power supply voltage to 4V, andthe resistance box has the largest resistance. Turn on the circuit switch, touch the resetswitch, the galvanometer isfully biased, and the bridge is unbalanced.To adjust thevariable resistor Rs, the principle of “from large to small" should be followed. Firstadjust the ×10000 knob, then adjust the ×1000, ×100, ×10, ×1, ×0.1 knob to improvethe experimental efficiency. When the bridge reaches the approximate balance range,the toggle switch on the bridge can be turned to the first gear and the bridge can beconnected.Continue to adjust the resistance of Rsto makethe bridge completelybalanced.Write down the resistance of the resistor box and the value of the override.Introduce the working panel of the QJ44 double-arm bridge. Warm up for five minutes.Connect the wire to be tested to the corresponding terminal of the bridge box accordingto the four-terminal twisting method. Select the appropriate override value and adjustthe galvanometer sensitivity knob to a lower sensitivity (coarse adjustment)Simultaneously press the circuit switch "B" and the galvanometer button switch "G" toadjust the comparison arm progress knob and the slide wire disc (according to the
Rx=R1/R2 Rs (3-5) The task objectives of the experiment are proposed: (1). correctly connect the circuit according ti the principle; (2). Adjust the bridge balance; (3). Measure the resistance of the four resistances to be tested (47Ω, 470Ω, 3kΩ, 47kΩ). 2. DC dual-arm bridge (Kelvin bridge) measuring low-value resistance The Kelvin bridge principle is shown on the right. Compared with the single-arm bridge, the double-arm bridge adds two high-value resistors R3 and R4 to form a sixarm bridge. Because there are two proportional arms, it is called a double-armed bridge. The bridge always meets: R1/R2=R3/R4 (3-5) By deriving, the balance conditions of the double-arm bridge and the one-arm bridge can be obtained in the same form, eliminating the influence of the additional resistance r, and therefore suitable for measuring low-value resistors. The switch remains off during the connection of the circuit. The key to emphasize the experiment is the choice of bridge magnification. According to the resistance of the resistance to be tested, the appropriate magnification is selected. The selection principle is that the highest-order position of the standard six-position resistance box is not zero. The galvanometer should be zeroed before use, and the pointer should not be fully biased for a long time during the experiment. Set the power supply voltage to 4V, and the resistance box has the largest resistance. Turn on the circuit switch, touch the reset switch, the galvanometer is fully biased, and the bridge is unbalanced. To adjust the variable resistor Rs, the principle of “from large to small” should be followed. First adjust the ×10000 knob, then adjust the ×1000, ×100, ×10, ×1, ×0.1 knob to improve the experimental efficiency. When the bridge reaches the approximate balance range, the toggle switch on the bridge can be turned to the first gear and the bridge can be connected. Continue to adjust the resistance of Rs to make the bridge completely balanced. Write down the resistance of the resistor box and the value of the override. Introduce the working panel of the QJ44 double-arm bridge. Warm up for five minutes. Connect the wire to be tested to the corresponding terminal of the bridge box according to the four-terminal twisting method. Select the appropriate override value and adjust the galvanometer sensitivity knob to a lower sensitivity (coarse adjustment). Simultaneously press the circuit switch "B" and the galvanometer button switch "G" to adjust the comparison arm progress knob and the slide wire disc (according to the
principle of "first big and then small"), so that the galvanometer pointer points to thezero scale, the bridge balance. Adjust the galvanometer sensitivity to the highest (finetuning),adjust the comparison arm to zero thegalvanometer, and balancethebridgeagain.Follow the instructions in time to correct problems in the student's experiment. Thereare 4 common problems that you should check:1.Check whether there is a condition that thegalvanometer is fully biased for a longtime,2. Check whether the student circuit connection is correct, and whether themagnification selection is correct,3.Check the data record and the experimental report:the effective number of the wirediameter measurement result, the sliding line, the effective number of the slide diskreading, the unit of resistivity, etc.,4. Check whether the instrument power is off
principle of "first big and then small"), so that the galvanometer pointer points to the zero scale, the bridge balance. Adjust the galvanometer sensitivity to the highest (fine tuning), adjust the comparison arm to zero the galvanometer, and balance the bridge again. Follow the instructions in time to correct problems in the student's experiment. There are 4 common problems that you should check: 1. Check whether there is a condition that the galvanometer is fully biased for a long time; 2. Check whether the student circuit connection is correct, and whether the magnification selection is correct; 3. Check the data record and the experimental report: the effective number of the wire diameter measurement result, the sliding line, the effective number of the slide disk reading, the unit of resistivity, etc.; 4. Check whether the instrument power is off
Experimental data recording and processingExperimentaldatarecordingandprocessing1.DC single arm bridge measurement median resistanceFirstmeasurement(incm)MagnificationR.(2)CalculatedR to be measured (2)ComparisonR,(2)Ri/R2value474703K47K2.DC dual-arm bridge for low-value resistance test2.1Themeasuredwirelengthand resistanceMagnificationThe measuredWireScribe diskwire lengthMagnificationstep readingreading()resistance()(2)(cm)CopperwireWire2.2The measured wire diameterPlease measure the wire diameter for 6 different positions using a micrometerDi(mm)D2(mm)D3(mm)D(mm)Ds(mm)D(mm)CopperwireWire2.3Calculatethe resistivityof the wireSRx=DRxAccording to the formula : p= S4LLGetting.Pcu=PFe=
Experimental data recording and processing Experimental data recording and processing 1. DC single arm bridge measurement median resistance First measurement (in cm): R to be measured (Ω) Magnification R1/R2 Comparison Rs (Ω) Rx(Ω) Calculated value 47 470 3K 47K 2. DC dual-arm bridge for low-value resistance test 2.1 The measured wire length and resistance Magnification Magnification step reading (Ω) Scribe disk reading(Ω) Wire resistance(Ω) The measured wire length （cm） Copper wire Wire 2.2 The measured wire diameter Please measure the wire diameter for 6 different positions using a micrometer. D1(mm) D2(mm) D3(mm) D4(mm) D5(mm) D6(mm) Copper wire Wire 2.3 Calculate the resistivity of the wire According to the formula ：ρ = 𝑆𝑅𝑥 𝐿 = 𝜋𝐷 2 𝑅𝑥 4𝐿 Getting: 𝜌𝐶𝑢= 𝜌𝐹𝑒=
Questionsforstudents:(1) Briefly describe the source of error in the process of measuring the medianresistance ofa single-arm bridge.(2) Briefly describe how the double-arm bridge eliminates the influence of theresistance of the button end and the wire on the measurement results(3)Summarizethemethodsyouknowaboutmeasuringresistanceandtalk abouttheadvantages and disadvantages of each
Questions for students： (1) Briefly describe the source of error in the process of measuring the median resistance of a single-arm bridge. (2) Briefly describe how the double-arm bridge eliminates the influence of the resistance of the button end and the wire on the measurement results. (3) Summarize the methods you know about measuring resistance and talk about the advantages and disadvantages of each