The main scale has 300 divisions covering a total length of 15 cm.
The value of one main scale division (MSD) is calculated as:
\(\text{MSD} = \frac{\text{Total Length}}{\text{Number of Divisions}} = \frac{15 \text{ cm}}{300}\)
\(\text{MSD} = 0.05 \text{ cm}\)
The vernier scale has 25 divisions that are equal to 24 main scale divisions.
This means:
\(25 \text{ VSD} = 24 \text{ MSD}\)
\(1 \text{ VSD} = \frac{24}{25} \text{ MSD}\)
\(1 \text{ VSD} = \frac{24}{25} \times 0.05 \text{ cm} = 0.96 \times 0.05 \text{ cm} = 0.048 \text{ cm}\)
The least count (LC) of the travelling microscope is the difference between one main scale division and one vernier scale division:
\(\text{LC} = \text{MSD} - \text{VSD}\)
\(\text{LC} = 0.05 \text{ cm} - 0.048 \text{ cm}\)
\(\text{LC} = 0.002 \text{ cm}\)
Alternatively, the least count can be calculated directly using the formula:
\(\text{LC} = \frac{\text{Value of smallest main scale division}}{\text{Number of divisions on the vernier scale}}\)
\(\text{LC} = \frac{0.05 \text{ cm}}{25}\)
\(\text{LC} = 0.002 \text{ cm}\)
Therefore, the least count of the travelling microscope is 0.002 cm.
In a metre bridge experiment, the positions of the cell, E, and galvanometer, G, are interchanged. We shall observe in the galvanometer :