Lab 1 D’ARSONVAL GALVANOMETER

                                       

                   

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LAB 1 D’ARSONVAL GALVANOMETER 

Objective :

- To find the internal resistance and the current sensitivity of the galvanometer. 

- To Calculate the sensitivity of a galvanometer using formular and find error range.

- To Understand how to connect a galvanometer into a circuit.

Equipment and components :

• d’Arsonval Galvanometer (35-0-35 mA)

• DC power supply

• digital multimeter

• 1 potentiometer, 1kΩ

• Composite resistor 510Ω, 220Ω, 10Ω, 5Ω, 2Ω 

Introduction : 

    The galvanometer contains a coil of wire in a magnetic field, which will experience a torque  when a current passes through the wire of the coil. The coil is attached to a pointer and a spring  so that the amount of deflection of the pointer is proportional to the current in the wire of the  coil. 

    The value of the load resistor (R1) will be set to a specified value and the potential difference  provided by the power supply will be varied to obtain a full-scale deflection of the pointer of  the galvanometer. The voltage (VFS) required to obtain full-scale deflection will be recorded,  without changing the applied voltage (VFS), Add a shunt resistor (RS) in parallel with the  galvanometer. Vary the load resistance to get the full-scale deflection in the galvanometer.  The new load resistance, R2 will be recorded. In both circuits, the potential difference supplied  by the power supply is the same as is the current passing through the galvanometer (full-scale  deflection in both circuits).

    Application of Kirchhoff’s rules to the two circuits results in the following expression for the  value of the internal resistance of the galvanometer (Rg). Assume Rg is 1.2Ω for the given  galvanometer. 

            The current sensitivity (K) can be obtained form the measurement by using this:

where, N = number of major divisions of the galvanometer scale for a full-scale deflection of the pointer 

     R1 = load resistor in circuit 1 

     R2 = load resistor in circuit 2 

     RS = value of shunt resistor parallel to galvanometer 

     VFS = voltage at full-scale deflection of the pointer in galvanometer

Exercise:

1. Set the potentiometer to 510 Ω. Connect the circuit as in the Figure 1.1 and keep the  voltage source in minimum position such that voltage output from the voltage terminals  are 0V.  

2. Increase the voltage supply and the galvanometer pointer will deflect towards right  hand side or left hand side. You can replace the galvanometer with the DMM/Ammeter  in the Multisim. Assume Rg is 1.2 Ω for the given galvanometer. Vary the voltage  supply until the galvanometer shows its maximum deflection, which the pointer should  comes to 30 positions (divisions). 

3. Connect the multimeter across the voltage terminal to measure the voltage and record  this value as VFS.  

4. Turn OFF the supply and do not disturb the voltage source. Now connect the resistance,  RS and construct the circuit as in Figure 1.2. Select the 10 Ω for RS. Switch ON the  instrument, then the pointer of the galvanometer will return back by a few divisions.  

5. Without disturbing the voltage source, adjust the potentiometer, R2 until the pointer  scale comes to full scale deflection which is 30 positions (divisions). 

6. Turn OFF the supply and disconnect R2. Measure the absolute values of R2 using  multimeter and record the value in the Table 2.1. 

7. The values of VFS, R1, R2, RS, are known, determine the galvanometer resistance, Rg by  calculation. Then calculate its current sensitivity, K. Repeat from step 5 for RS = 5 Ω  and 2 Ω and R1 = 220 Ω. 

8. Finally disconnect the galvanometer from the circuit and connect multimeter across the  terminal of galvanometer to measure its internal resistance. Record the value as Rg (measured). 

Result:

Screenshots:

Calculation:

Discussion:

1. Connect the circuit in Figure 1.3 and run the simulation for both load resistance values for 10kΩ and 100kΩ. Observe the measured voltage using a voltmeter/DMM and  conclude the answer with the prove of the diagram. 

Solution:

2. Connect the circuit in Figure 1.4 and run the simulation for. The actual voltage across  R2 is 7.5 V. Configure the circuit with the voltmeter has an internal resistance of 1 MΩ,  10MΩ and 100MΩ. Observe the measured voltage using a voltmeter/DMM and  conclude the answer with the prove of the diagram. 

Solution:

Conclusion:

    From this experiment, I learned how to connect a galvanometer to a circuit. In order to measure current in point or node of a circuit, a galvanometer must connected in parallel to measure the current. In practical, a galvanometer will have its internal resistance which cannot be avoid when connected to the circuit. In order to overcome this problem, a initial voltage source is set slightly higher until it get the same current as the voltage in full scale, VFS. In this experiment, I learned how to use software “multisim” to create a circuit and use a multimeter as a galvanometer to measure the current. On the other hands, I also learn how is the error percentage is like when compared to calculated value and practical value. The practical value mostly is slightly different from the value calculated since it have encounter issues like temperature of surrounding or slightly vary input power. The lower the error percentage, the better the circuit performance since it has higher accuracy. Finally, I also learn how to calculate sensitivity, K for a galvanometer. The sensitivity is the value measured for every one scale based on the galvanometer.

- END OF LAB 1 -