3.9.4 Experimental procedures
1)Measuring resistance by self-assembly Wheatstone bridge
(1)Realize the circuit according to the circuit diagram shown in Figure 3.9.1,where R1,R2 and R0 are substituted by three resistance boxes.The voltage of the power supply is set to 3 V.
(2)Adjust the values of R1 and R2 in order to achieve two different ratios of R1/R2 values:R1∶R2=500∶500 and 50∶500,which will be used for measuring different unknown resistance.
(3)Exchange the positions of Rx and R0 and then readjust the standard resistance R0,and a new balance can be reached again.
(4)Design the record form by yourself and calculate the uncertain resistance by Equation(3.9.6).
2)Measuring resistance by plate-type Wheatstone bridge
(1)Measure the rough value of the unknown resistance with a multimeter.
(2)Realize the circuit according to the circuit diagram shown in Figure 3.9.2.Adjust R0 so that the resistance values of R0 and Rx are roughly the same,and calibrate the zero point of the galvanometer carefully.
(3)Switch on Kg and move the slide button to the middle of the resistance wire.Switch on the power supply.(https://www.daowen.com)
(4)Check the deflection direction and the degree of the galvanometer pointer by jumper method(connect and then immediately disconnect the d button)in order to protect the galvanometer.The equilibrium state of the bridge can be realized by varying the ratio of R1/R2,which is achieved by sliding d button and adjusting the ratio of L1/L2,while keeping the R0 unchanged.Record the values of L1,L2 and R0.Exchange the positions of R0 and Rx while maintaining the ratio R1/R2 under the same condition,readjust the standard resistance R0,and a new balance can be reached again.Record the value of R′0.Finally,calculate Rx according to Equation(3.9.6).
(5)When the bridge is in balance,change the value of comparison arm R0 so that the galvanometer pointer deflects 1～2 grids(Δn)from the equilibrium position.Record the changing value ΔR0 and the deflection scaleΔn and calculate its sensitivity according to Equation(3.9.8).
(6)Change the initial value of R0 and repeat the measurements for 4～6 times.
(7)Select the data with the highest sensitivity as the final result.
Figure 3.9.2 Schematic of the plate-type Wheatstone
3)Tips and attentions
(1)Make sure to connect the circuit correctly so that the power supply is not in the short circuit condition.
(2)Make sure that the circuit current never exceeds the rated current of each resistance.
