The resistance measured by Wheatstone electric bridge

School of Electronics & Applied Physics, Hefei University of Technology

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.

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.

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

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

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.

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

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

(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

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