Figure of Merit of a galvanometer

# Materials Required

• A weston type galvanometer
• A battery or battery eliminator
• Two resistance boxes
• Two one-way keys
• Connecting wires

# Real Lab Procedure

## Resistance of galvanometer by half deflection method :

• Arrange the components on a table and connect them as per the circuit diagram.
• Make sure that plugs of the resistance boxes are tight.
• Take out a high resistance from the resistance box 1 and insert the key k1.
• Adjust the resistance from this resistance box to get maximum galvanometer deflection.
• Note the deflection and record it as θ in the tubular column.
• Insert the key k2 also, without changing the value on the resistance box.
• Now, adjust the resistance from the low resistance box such that galvanometer shows deflection which is exactly half of the previous reading.
• Record the value of low resistance box.
• We can repeat the experiment by changing the value of high resistance R and adjusting low resistance S.
• The resistance of the given galvanometer can be calculated each time by using the relation G= RS/(R-S).

## Figure of merit of the galvanometer :

• Release key k2 from the connection. Insert key k1.
• Adjust the value of R such that the galvanometer shows a certain deflection.
• Record the observations in a tabular column.
• Repeat the experiment by changing the value of R and note the galvanometer deflection each time.
• We can find the figure of merit of the galvanometer by using the equation, k= E/(R+G)θ.

# Simulator Procedure (as performed through the Online Labs)

• Select the ‘Galvanometer’ form the drop down list.
• To see the circuit diagram, click on the ‘Show circuit diagram’ check box seen inside the simulator window.
• Connections can be made as seen in the circuit diagram by clicking and dragging the mouse from one connecting terminal to the other connecting terminal of the devices to be connected.
• Once all connections are made, click and drag key 1 to insert it into the switch.
• You can select the desired large resistance from the resistance box.
• The galvanometer shows a deflection.
• Now click and drag key 2 to insert it into the switch without changing the value of the resistance.
• Select the resistance from the small resistance box such that the galvanometer shows deflection which is exactly half of the previous reading.
• To see the result, click on the ‘Show result’ check box.
• To redo the experiment, click on the ‘Reset’ button.

# Observations

## Resistance of the galvanometer by half deflection method.

 No. Resistance, R ($\Omega$) Deflection in the galvanometer,  $\theta$ Shunt Resistance, S  ($\Omega$) Half deflection,  $\frac{\theta }{2}$ Galvanometer resistance $G=\frac{RS}{R+S}$ ($\Omega$)

## Figure of merit of the galvanometer

 No. Resistance, R  ($\Omega$) Deflection, $\theta$ (div.) Figure of Merit,  $\small K=\frac{E}{(R+G)\theta }$ ( Amp/div )

# Calculations

Calculate the value of G in each case and record it in the tabular column. The mean of these calculated values will give the resistance of the given galvanometer.

Calculate the value of k in each case and record it in the tabular column. The mean of these calculated values will give the figure of merit of the given galvanometer.

# Results

The resistance of the given galvanometer, G     =--------- Ω
The figure of merit of the given galvanometer, k =---------- Amp / div.

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