4.2.4 Experimental procedures
1)Median method or substitution method for measuring Rg
(1)The connection of the median method is shown in Figure 4.2.1.
①Adjust E to 0 V,and connect E(0～2 V),RW,refitted meter and standard ammeter.
②Turn off the switch S(the resistance box R2 is not connected),adjust E and RW to make the refitted gauge head full deflective and then write down the reading of the standard meter.This current is the full-scale current of the refitted gauge head Ig.
③Turn on the switch S(the resistance box R2 is connected),and change the value of R2,E or RW,so that the pointer of the measured gauge head is deflected from full scale Ig to
Ig,yet the total current intensity remains Ig unchanged.The value of the resistance box R2 is the internal resistance of the measured gauge head.
(2)The connection of the substitution method is shown in Figure 4.2.2.
①Adjust E to 0 V,and connect E(0～2 V),RW,refitted meter and standard ammeter.
②Adjust E and RW to make the refitted gauge head full deflective,and write down the reading of the standard meter.This current is the full-scale current of the refitted gauge head Ig.
③Disconnect the refitted gauge head,replace the gauge head with the resistance box R2,and adjust R2(be careful not to adjust E and RW),so that the current of the standard ammeter is the only recorded value Ig.Then the value of the resistance box R2 is the internal resistance of the measured gauge head Rg.
2)Refitting the meter into a 5 mA ammeter
(1)According to the range of the ammeter and Rg value,calculate the shunt resistance value by Equation(4.2.1).
(2)Adjust E to 0 V,connect the circuit as shown in Figure 4.2.3,set the shunt resistance as the calculated value in step(1),adjust E(0～2 V)and the sliding rheostat RW to make the pointer of the refitted meter points to the full scale and record the standard meter reading(be aware of the fact that as a current-limiting resistor,the resistance value should not be adjusted to the minimum value).Next,gradually decrease the reading of the refitted meter every 0.5 mA until zero and then gradually increase to the full scale at the original interval.And record the corresponding reading of the standard meter each time.(https://www.daowen.com)
(3)Take the reading of the refitted meter as the abscissa,and the average value of the two readings of the standard meter.When the standard meter is adjusted from large to small or small to large as the ordinate.Make the calibration curve of the ammeter on the graph paper,and determine the accuracy rating of the refitted meter according to the maximum error value of the two meters(see 4.2.8 Appendix 2 for details).
3)Refitting the gauge head into a 1.0 V voltmeter
(1)According to the range of the voltmeter and the parameters of the gauge head,calculate the extending resistance value by Equation(4.2.2).
(2)Adjust E to 0 V,connect the circuit as shown in Figure 4.2.4,set the divider resistance RM as the calculated value in step(1),and adjust E(0～10 V)as well as the sliding rheostat RW.When the pointer of the refitted meter points to the full scale,the 1.0 V voltmeter is obtained.Write down the standard meter reading at this time.Next,gradually decrease the reading of the refitted meter every 0.1 V until zero,and then gradually increase to the full scale at the original interval.Then record the corresponding reading of the standard meter each time.
(3)Take the reading of the refitted meter as the abscissa,and the average value of the two readings of the standard meter.When the standard meter is adjusted from large to small or from small to large as the ordinate.Make the calibration curve of the voltmeter on the graph paper,and determine the accuracy rating of the refitted meter according to the maximum error value of the two meters.
4)Refitting the gauge head into an ohmmeter and calibrating the surface scale
(1)According to the parameters Ig and Rg of the gauge head along with the power supply voltage E,set RW as 4.7 kΩ(turn to maximum),and set R3 as 10 kΩ.
(2)Zero adjustment of the ohmmeter.Connect the circuit as shown in Figure 4.2.5(a),adjust E=1.5 V,short-circuit the two terminals a and b.And adjust RW to make the gauge head indicating zero.
(3)Measure the median resistance of the refitted ohmmeter.Connect the resistance box(R1+R2)(equivalent to RX)to terminals a and b of the ohmmeter.And adjust(R1+R2)so that the pointer of the gauge head points to the center.At this time,the value of the resistance box(R1+R2)is the median resistance.
(4)Set the resistance of the resistance box as a set of specific values RXi and read the corresponding deflection grid number div.Then draw the dial of the refitted ohmmeter by the obtained readings RXi and div.
(5)Determine the power supply range of the refitted ohmmeter.Short-circuit the terminals a and b,set the working power supply in the 0～2 V gear and adjust E about 1 V.Turn RW counterclockwise to the end,adjust E until the pointer of the gauge head is fully deflective,and record E1;then turn RW clockwise to the end,adjust E until the pointer of the gauge head is fully deflective,and record E2.E1～E2 is the power supply range of the ohmmeter.
