惠斯顿电桥实验教案(英文)Wheatstone bridge

EXPERIMENT 2 WHEATSTONE BRIDGE1.AimTo determine the resistance of an unknown electrical resistance using a Wheatstone bridge.2.TheoryFor measuring accurately any electrical resistance, single bridge, also known as Wheatstone bridge is widely used. Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. In many cases, the significance of measuring the unknown resistance is related to measuring the impact of some physical phenomenon (such as force, temperature, pressure, etc.) which thereby allows the use of Wheatstone bridge in measuring those elements indirectly.The apparatus used in this experiment is a simple form of the Wheatstone bridge. The basic circuit is shown in Fig. 1. It consists of four bridge arms, R x represents an unknown resistance and R0, R1, R2 are decade variable resistances. In addition, the circuit contains a "bridge" balance indicator (usually a galvanometer), working power E and slide-wire rheostat.Fig.1 Principle of Wheatstone bridge circuitAfter R1, R2were suitably selected, by adjusting the variable resistor, then the voltage between the two midpoints (B and D) will be zero and no current will flow through the galvanometer. Thus the bridge is in the status of balance. Detecting zero current with a galvanometer can be done to extremely high accuracy. Therefore, if R1, R2 and R0are known to high precision, then R x can be measured tohigh precision. Very small changes in R x disrupt the balance and are readily detected.When the bridge is balanced, that is V BD =0, thus1122I R I R =, 102x I R I R = (1)So we can get that1122102xI R I R I R I R = (2) That is120xR R R R = (3) So 201x R R R R =(4) In fact, it is difficult to exactly decide the value of 21/R R , so we use "exchange measure method" to obtain R x . Exchange the positions of R 1 and R 2, or R 0 and R x , and adjust R 0 to a new value 0′R , the bridge can get a new balanced state, thus102x R R R R '=(5) Combine equation (4) and (5), we can getx R = (6)This equation has nothing to do with R 1 and R 2. In this way, we can make the error only relate to the instrument error of R 0, which is the instrument error of the resistor box. 3. Procedure(1) Connect the circuit as shown in Fig.2, and ensure that all connections are correct. Zero the galvanometer. Meanwhile, adjust the slide-wire rheostat to a maximum value in the circuit. (2) Adjust the variable resistance box R 0 (it is R 3 in Fig.2) and then press the electric button in the galvanometer, judging whether the galvanometer reads zero. If not, adjust R 0 until the galvanometer reads zero (see gradual approach ). Note that the R 1 and R 2 should be given the same value.Gradual approach: ① Adjust R 0 to a small value R 31 (eg. 600Ω ) and the galvanometerpointer is tilted to one side (see Fig.3a). ② Adjust R 0 to some large value R 32 (eg.900Ω), the pointer is tilted to the other side (see Fig.3b). When the bridge is balanced, R 0 must between R 31 and R 32. ③ Select a new resistance value R 33 between R 31 and R 32, and observe the pointer deflection again. ④ Determine the new resistance range based on the result of pointer deflection after selecting R 33. ⑤Repeat this procedure until the current through the galvanometer is made zero.(3) Adjust the two slide-wire rheostats to zero value in the circuit, then adjust R 0 until the current through the galvanometer is made zero. Record R 0 (R 3).(4) Exchange the positions of R 0 and R x , repeat procedures (1)-(3), adjust R 0 to a new value 0′R and make the bridge to get a new balanced state, then record 0R '(R 3).(5) Adjust R 0 (R 3) to make the galvanometer pointer deflect 1, 2 and 3 div, respectively, and record the corresponding R 3 value, then R 0∆can be obtained.Fig.2 Set-up of Wheatstone bridge apparatus.Fig.3 Deflection of the galvanometer pointer with different value of R 0.R 2R 0 (R 3)R 1GalvanometerR x4. DataTable 1 Test of the unknown resistorTimes123R 1()Ω R 2()ΩR R 03()()ΩR R '03()()Ωx R ()ΩxR ()ΩTable 2 Sensitivity of the bridgeTimes1 2 3 R 0()ΩR 0∆()ΩNumber of the needle deflectionn 0∆1 2 3n S R R 000(/)∆=∆S5. Result & error analysisLevel of resistance box: α = 0.1%α⋅=∆x Rx R =xx x R R R =±∆=S =5. ConclusionsIn no more than 100 words write a succinct summary of what you have done. State the goal and method of the experiment. State your main results, i.e.: What value did you get for the unknown resistor? Is your result in agreement with the accepted value? If your result is not consistent with the accepted value can you suggest why not ?。