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The resistance measured by Wheatstone electric bridge
School of Electronics & Applied Physics, Hefei University of Technology
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The experiment teaching goal and requirements 1. Master the structure and the measurement
principle of Wheatstone bridge. 2. Master slide-wire Wheatstone bridge to
measure the value of resistance.3. master box-type Wheatstone bridge method of measuring the value of resistance.
4. measure resistance value.
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Experimental principleExperimental principle The basic circuit of
Wheatstone bridge is shown in figure 1. Resistance Rx to be measured and resistances Ra, Rb, R to be known constitute its four arms, points of A and D connect power E. The BC road is called as “bridge”, B and C connect a galvanometer G.
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If the potential difference of BC is Vbc≠0,Vbc≠0, current flow through the galvanometer Ig≠0. If we appropriately adjust Ra, Rb, R resistance and make VVBCBC=0 =0 ,, no current flow through the galvanometer Ig = 0, we call the state as bridge balance.
According to the bridge balance: VAB = VAC ,VBD= VCD, that is I1* Rx = I2* Ra, I1 *R = I2 *Rb
we find: Rx = (Ra/Rb) R (1(1))
Experimental principleExperimental principle
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If we make K = Ra / Rb , K is the ratio, R arm is comparison arm, the electric bridge balance is adjusted by two ways: (1) we maintain the same K, and regulate R; (2)we keep R constant, adjust K.
Many experimental results demonstrate that method to take the same rate of arm K and adjust comparative arm R is more accuracy.
Experimental principleExperimental principle
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Wheatstone bridge application in engineering technology
Wheatstone bridge application in engineering technology
Wheatstone bridge has a very wide range of applications in engineering. For example, in sensor technology (transducer / sensor technology) field, R or the Rx resistance will be replaced with thermal resistance, deformation resistance, inductance coil, piezoelectric ceramic, capacitors and so on. It can create a high sensitivity, good stability, low cost sensors and use for a variety of information, detection and control system, especially in the differential technique.
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1.Slide-wire bridge resistance measurement Experimental Process
Figure 2: we use a resistance wire AD to instead of Ra and Rb, we find:
slide-wire bridge
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RRxx== (( RRaa/R/Rbb )) R=R= (( a/ba/b )) RR
(1)Connect circuit according to Figure 2.(2) Regulate C point to make (a / b) = 1, adjust the variable resistor R to make the bridge to balance, record R.(3) Regulate C point to make (a / b) = 1 / 4 or 2 / 3 or 3 / 2 or 4 / 1, adjust the variable resistor R to make the bridge to balance, record R.(4) swap the location of the Rx and R , repeat the above process.
Experimental Process
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(1) Use box-type electric bridge to measure single resistance
(2) Use box-type electric bridge to measure series resistance
(3) Use box-type electric bridge to measure parallel resistance
Experimental Process
2.Resistance measured with box electric bridge
Box-type electric bridge
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When we use the slide-wire bridge to measure resistance, large errors should be caused due to the resistance wire AD uneven. Following method would be used to eliminate systematic errors:
RRxx== (( a/ba/b )) R R rr (( 11)) swap the location of the Rx and R:
RRxx== (( b/ab/a )) RRll (( 22)) Multiply the two equations, We find There is no a, b in the equation, it can eliminate
the system error.
Difficulty and Solution of the experiment