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 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.

Vbc≠0, V BC =0 ，  If the potential difference of BC is Vbc≠0, current flow through the galvanometer I g ≠0. If we appropriately adjust Ra, Rb, R resistance and make V BC =0 ， no current flow through the galvanometer I g = 0, we call the state as bridge balance.  According to the bridge balance:  V AB = V AC,V BD = V CD, that is  I 1 * R x = I 2 * R a, I 1 *R = I 2 *R b (1)  we find: R x = (R a /R b ) R (1) Experimental principle

 If we make K = R a / R b, 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 principle

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

 R x = （ R a /R b ） R= （ a/b ） R  (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:  R x = （ a/b ） R r （ 1 ） R x = （ b/a ） R l （ 2 ）  swap the location of the Rx and R: R x = （ b/a ） R l （ 2 ）  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