Experiment E2
Electromechanical Indicating Instruments
E2. 1 - Objectives:

This comprises 3 sections, where the requirements are stated in items A and
B, below:
A- Section (1): To determine the Internal resistance, (Rm), of the well-known
device referred to it as D'Arsonval (Galvanometer).
B- This involves section 2 ad 3, as given below, where the requirements are
to use this device of D'Arsonval movement as a Fundamental unit for the
design of each of the following instruments.
Section (2): Ammeter.
Section (3): Voltmeter.
E. 2 – Procedure:
See Appendix E2 for the theoretical approaches applied in this respect for the
following 3- sections.
Section (1): to determine the internal resistance (Rm):
Part A: Variable resistor's method:
1. Assemble the circuit shown in Fig E2.1. Set the potentiometer to maximum
and the voltage source to minimum.
2. Slowly increase the voltage to 1 V.
3. Slowly decrease the resistance of the potentiometer until a full scale reading
is obtained of the meter.
4. Turn the power off, carefully disconnect the potentiometer and measure the
value of its resistance. Record this value as R1.
5. Reconnect the circuit and slowly vary the potentiometer until the meter
reads exactly half-scale. Turn off the power again and carefully disconnect
the potentiometer and measure the value of it resistance. Record this value
as R2. The internal resistance of the meter movement can then be calculated
as : Rm1 = R2 – 2R1
Fig. E2.1
Part B: Potentiometer method:
1. Assemble the circuit shown in Fig E2.2 below. Set the potentiometer to
maximum and the voltage source to minimum.
2. Slowly increase the voltage to 1 V.
3. Slowly decrease the resistance of the potentiometer until a full scale reading
is obtained on the meter.
4. Use the DMM to measure the voltage across the D'Arsonval meter Vm, and
compute the internal resistance of the meter by: Rm2 = Vm / I full-scale
Fig. E2.2
Part C: Shunt resistor method:
1. Assemble the circuit shown in Fig E2.3 below. Vary the 50 k
potentiometer until a full scale reading is obtained on the meter under test.
2. Close switch S and vary the 10 k potentiometer until the meter reads
exactly half-scale.
3. Turn the power off, carefully disconnect the 10 k potentiometer and
measure the value of its resistance. Record this value as Rm3.
Fig. E2.3
** Calculate the average internal resistance of the meter under test as:
Rm = ( R m1 + R m2 + R m3 ) / 3.
Section (2): Design and construction of Ammeter, as
follows:
1. Calculate the value of the ohm-per-volt sensitivity of the meter as the
inverse of Im.
2. Calculate the value of the shunt resistors necessary for current range of
Ifsd = 10 mA and Ifsd = 100 mA.
3. Construct the two range milliammeter. Check the accuracy of the meter by
connecting the circuit of Fig. E2.4 (Rvr = 10 k). Use the DMM as the
standard.
4. For each range, use the variable resistor to take 5 different current readings
of both the DMM and the meter under the test. (e.g.: 1/10 Fsd, 3/10 Fsd,
5/10 Fsd, 8/10 Fsd and 10/10 Fsd).
5. Construct a table of DMM reading, meter under the test reading, error and
percentage error.
6. Plot Error vs. meter under test readings.
Fig. E2.4
Section (3): Design and construction of Voltmeter:
This involves 3 parts, (A,B &C) as given below. It requires the use of the
results obtained in section (1) for the calculations to be conducted here.
Part A: Building a voltmeter:
1. Using the meter movement, design a voltmeter having the following four
ranges:
a) 0 to 1V
b) 0 to 2V
c) 0 to 5V
d) 0 to 10V
2. Construct the experimental circuit shown in Fig. E2.5. Use fixed carbon
composition resistors for the multipliers (series resistance). Record the
measured values of Rs1, Rs2 and Rs3.
3. For the 4 ranges apply a voltage for full scale deflection in the movement.
Measure the applied voltage on each range with the DMM.
4. Calculated and record the percentage error of each range. Tabulate your
readings of V analog, VDMM, and percentage error.
Fig. E2.5
Part B: Voltmeter calibration:
This procedure is for range 1 and 5V only.
1. Choose suitable potentiometers to replace Rs1 and Rs3. Use the DMM to adjust
the pot's to the calculated values in step (A -1).