Experiment to determine galvanometer resistance using the half deflection method in the apparatus theory and the viva half deflection method.
Apparatus
A low-resistance galvanometer, a cell, two resistance boxes, two one-way plug keys, connecting wires, and sandpaper are required. These items are used to perform the Resistance of a galvanometer by half half-deflection method.
Theory
Set up the circuit as shown in the diagram. When only key K₁ is closed, the galvanometer current is:
I = E / (G + R)
Where:
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G = resistance of the galvanometer
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E = e.m.f. of the cell
Since the deflection is proportional to the current, you can determine G using the half deflection method. Compare the deflection with and without a shunt to calculate resistance accurately.
Procedure
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Draw a neat and labeled circuit diagram. Ensure all connections are tight. Keep both keys K₁ and K₂ open.
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Remove a plug from the resistance box R to set a high resistance. Then, close K₁. Adjust the resistance to obtain a large deflection. Always note even divisions. Record the deflection and R.
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Close key K₂. The deflection decreases. Adjust shunt resistance S until the deflection becomes exactly half of the previous one. This step follows the half-deflection method principle. Record S.
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Repeat the experiment for at least four deflection values. Doing so ensures consistent and reliable readings.
Observations and Calculations
Mean value of G ……..Ohms
Precautions
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Ensure all connections are tight and neat.
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Introduce a large resistance R before closing key K₁.
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Note only even divisions on the galvanometer scale.
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Avoid parallax errors while reading.
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Do not press keys for too long to prevent heating.
Viva Questions
Q1: Why does a galvanometer have resistance?
Ans: The galvanometer has resistance due to the wire in its coil.
Q2: Why does the deflection become half after closing the shunt key?
Ans: Half of the current flows through the galvanometer, and the remaining half passes through the shunt, following the half deflection method.
Q3: What is meant by Ig?
Ans: Ig is the current required for full-scale deflection, causing the needle to reach its maximum position.