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1、直流双臂电桥测低值电阻电阻按照阻值大小区分，大致可分三类：在1以下的为低电阻；在1100k之间的为中值电阻；在100k以上的为高电阻。不同阻值的电阻，测量方法是不尽相同的。用惠斯通电桥测量中值电阻时，可以忽略导线本身的电阻和接点处的接触电阻的影响（总称为附加电阻，一般附加电阻约为10-3左右）。若待测Rx是低电阻，则这些附加电阻就不能忽略了。对惠斯通电桥加以改进而成的双臂电桥（又名开尔文电桥）消除了附加电阻的影响，适用于10-6102电阻的测量。常用来测量金属材料的电阻率、电机、变电器绕组的电阻、低阻值线圈电阻、电缆电阻、开关接触电阻以及直流分流器电阻等。【实验目的】1 学习用双臂电桥测低电阻
2、的原理和方法。2了解测低值电阻时接线电阻和接触电阻的影响及其避免的方法。3. 测定金属棒的电阻率。【实验仪器】QJ44型携带式直流双臂电桥，待测电阻若干，直尺，螺旋测微器等。【实验原理】1、 电阻的四端接法图1中C1、C2是电流端，通常接电源回路，从而将这两端的引线电阻和接触电阻折合到电源回路的其它串联电阻中；P1、P2是电压端，通常接测量电压用的高电阻回路或电流为零的补偿回路，从而使这两端的引线电阻和接触电阻对测量的影响大为减少。采用这种接法的电阻称为四端电阻。2、本实验使用的是QJ44型双臂电桥，其工作原理如图2所示。图2中待测电阻Rx和比较用的标准低电阻RN均采用四端接法。从电桥中看出，
3、当桥臂电阻R1、R2、R3、R4取值较大时（10），与它们对应的导线电阻与接触电阻都可以忽略不计。待测电阻电阻Rx和标准低电阻RN的电压端P1、P1、P2、 P2的附加电阻由于和高阻值桥臂串联，其影响就大大减少；两个靠外侧的电流端C1、C1的附加电阻串联在电源回路中，对电桥没有影响；两个内侧的电流端C2、C2的附加电阻和连线电路总和为r，只要适当调节R1、R2、R3、R4的阻值，就可以消除r对测量结果的影响。调节电阻R1、R2、R3、R4，使流过检流计G的电流为零，电桥达到平衡，根据基尔霍夫定理得到以下三个回路方程： 将上面三式联解，并消去I1、I2和I3可得 （1）式（1）就是双臂电桥的平衡
4、条件，可见r对测量结果是有影响的。为了使待测电阻RX的值便于计算及消除r对测量结果的影响，可以设法使第二项为零。通常把双臂电桥做成同轴联动式，使得在调整平衡时R1、R2、R3和R4同时改变，而始终保持成比例。即 （2）在此情况下，不管r多大，第二项总为零。于是平衡条件简化为 （3）或 （4）从上面的推导看出，双臂电桥的平衡条件和单臂电桥的平衡条件形式上一致，而电阻r可以消除在平衡条件中，因此r的大小并不影响测量结果，这是双臂电桥的特点。【仪器简介】1、QJ44型双臂电桥面板布置图（1）检流计；（2）电桥外接工作电源接线柱；（3）检流计电源开关；（4）检流计灵敏度调节旋钮；（5）滑线读数盘；（6
5、）步进读数开关；（7）检流计按钮开关；（8）电桥工作电源按钮开关；（9）倍率读数开关；（10、13）被测电阻电流端接线柱；（11）检流计电气调零旋钮；（12）被测电阻电位端接线柱。如图3所示：图32、仪器结构（1）QJ44型双臂电桥比例臂由100、10、1、0.1和0.01所组成。读数盘由一个十进盘和一个滑线盘组成的。（2）检流计包括一个调制型放大器、一个调零电位器和一个调节灵敏度电位器以及一个检流计表头。表头上备有机械调零装置，在测量前，可预先调整零位。当放大器接通电源后，若表针不在中间零位，可用调零电位器，调整表针至中央零位。（3）在检流计和电源回路中设有可锁住的按钮开关。（4）QJ44型
6、双臂电阻电桥的原理线路如图4所示。图43、电桥各量程，有效量程，等级指数和基准值如下表所示：量程倍率有效量程（）等级指数（C）基准值RN（）1001110.210100.11.10.2110.010.110.2待添加的隐藏文字内容30.10.10.0010.0110.50.010.010.00010.001110.0014、使用方法（1）将仪器连好专用电源线，插入220V插座,打开后面的电源开关，电桥就能工作。（2）将被测电阻，按四端连接法，接在电桥相应的C1、P1、C2、P2的接线柱上，如图5所示，AB之间为被测电阻。图5（3）“B1”开关打到“通”位置，检流计工作电源接通，等待5min后，
7、调节检流计指针指在零位上。（4）估计待测电阻值大小，选择适当量程倍率位置，先按下“G”按钮，再按下“B”按钮，调节步进和滑线读数盘，使检流计指针在零位上，电桥平衡，被测电阻按下式计算：待测电阻值（RX）= 量程倍率读数 （步盘读数滑线盘读数）（5）在测量未知电阻时，为保护检流计指针不被打坏，检流计的灵敏度调节旋钮应放在最低位置，使电桥初步平衡后再提高检流计灵敏度。在改变检流计灵敏度或环境等因素的影响，有时会引起检流计指针偏离零位，在测量之前，随时都可以调节检流计零位。【实验内容与步骤】1、 用开尔文直流双臂电桥与惠斯顿直流单臂电桥，分别测出三个待测电阻的电值，对结果进行比较，并作分析说明。2、
8、用直尺和螺旋测微计测出金属棒的直径d，有效长度l及对应的阻值R，用作图法计算金属棒的电阻率。【注意事项】1、在测量电感电路和直流电阻时，应先按下“B”按钮，再按下“G”按钮，断开时，应先断开“G”按钮，后断开“B”按钮。2、测量0.1时以下阻值时，“B”按钮应间歇使用。3、在测量0.1以下阻值时，C1、P1、C2、P2接线柱到被测量电阻之间的连接导线电阻为0.0050.01，测量其它阻值时，连接导线电阻应不大于0.05。4、电桥使用完毕后，“B”和“G”按钮应松开，检流计工作电源开关应处于关闭位置，避免消耗电能，同时也能防止内部元件发热影响测量精度。【思考题】1、 开尔文直流双臂电桥与惠斯顿直
9、流单臂电桥有哪些异同？2、 直流双臂电桥的基本原理是什么? 它是如何消除附加电阻的影响的?3、 被测低电阻为何具有四个端连接？如果电位端与电流端连接错误会有什么现象？Winger Tuivasa-Sheck, who scored two tries in the Kiwis 20-18 semi-final win over England, has been passed fit after a lower-leg injury, while Slater has been named at full-back but is still recovering from a knee inj
10、ury aggravated against USA.Both sides boast 100% records heading into the encounter but Australia have not conceded a try since Josh Charnleys effort in their first pool match against England on the opening day.Aussie winger Jarryd Hayne is the competitions top try scorer with nine, closely followed
11、 by Tuivasa-Sheck with eight.But it is recently named Rugby League International Federation player of the year Sonny Bill Williams who has attracted the most interest in the tournament so far.The Kiwi - with a tournament high 17 offloads - has the chance of becoming the first player to win the World
12、 Cup in both rugby league and rugby union after triumphing with the All Blacks in 2011.Id give every award back in a heartbeat just to get across the line this weekend, said Williams.The (lack of) air up there Watch mCayman Islands-based Webb, the head of Fifas anti-racism taskforce, is in London fo
13、r the Football Associations 150th anniversary celebrations and will attend Citys Premier League match at Chelsea on Sunday.I am going to be at the match tomorrow and I have asked to meet Yaya Toure, he told BBC Sport.For me its about how he felt and I would like to speak to him first to find out wha
14、t his experience was.Uefa hasopened disciplinary proceedings against CSKAfor the racist behaviour of their fans duringCitys 2-1 win.Michel Platini, president of European footballs governing body, has also ordered an immediate investigation into the referees actions.CSKA said they were surprised and
15、disappointed by Toures complaint. In a statement the Russian side added: We found no racist insults from fans of CSKA. Baumgartner the disappointing news: Mission aborted.The supersonic descent could happen as early as Sunda.The weather plays an important role in this mission. Starting at the ground
16、, conditions have to be very calm - winds less than 2 mph, with no precipitation or humidity and limited cloud cover. The balloon, with capsule attached, will move through the lower level of the atmosphere (the troposphere) where our day-to-day weather lives. It will climb higher than the tip of Mou
17、nt Everest (5.5 miles/8.85 kilometers), drifting even higher than the cruising altitude of commercial airliners (5.6 miles/9.17 kilometers) and into the stratosphere. As he crosses the boundary layer (called the tropopause),e can expect a lot of turbulence.The balloon will slowly drift to the edge o
18、f space at 120,000 feet ( Then, I would assume, he will slowly step out onto something resembling an Olympic diving platform.They blew it in 2008 when they got caught cold in the final and they will not make the same mistake against the Kiwis in Manchester.Five years ago they cruised through to the
19、final and so far history has repeated itself here - the last try they conceded was scored by Englands Josh Charnley in the opening game of the tournament.That could be classed as a weakness, a team under-cooked - but I have been impressed by the Kangaroos focus in their games since then.They have be
20、en concentrating on the sort of stuff that wins you tough, even contests - strong defence, especially on their own goal-line, completing sets and a good kick-chase. Theyve been great at all the unglamorous stuff that often goes unnoticed in the stands but not by your team-mates.It is as though their
21、 entire tournament has been preparation for the final.In Johnathan Thurston, Cooper Cronk, Cameron Smith and either Billy Slater or Greg Inglis at full-back they have a spine that is unmatched in rugby league. They have played in so many high-pressure games - a priceless asset going into Saturday.The Kiwis are a lot less experienced but winning a dramatic match like their semi-final against England will do wonders for their confidence.They defeated Australia in the Four Nations final in 2010 and the last World Cup, and know they can rise to the big occasion.
