Internal micrometer is a measuring intrument used to measure internal diameter (ID) of a large cylinder bore with high accuracy. Construction is shown in figure. There is one fixed rod B (to the right in figure) and one moved rod A (to the left in figure). It is based on the particle of advancement of a screw when it is rotated in a nut with internal threads. Main scale reading can be directly seen on the hub which is fixed with respect to rod B. When the cap is rotated rod A moves in or cut depending on direction of rotation. The circular scale reading is seen by checking which division of circular scale coincide with the references line

This is to be multiplied by LC to get circular scale reading.
Least count = value of 1 circular scale division `= ("pitch")/("number of division on circular scale")`
Length of rod A is chosen to match the `ID(PQ)` to be measured. Zero error is checked by taking reading between standard blocks fixed at normal value of `ID` to be measured. Zero error is positive if cap end is one the right of the main scale and negative it is on the left side.

In the above instrument, while measuring an internal diameter. `ID` is set of 321 mm with no zero error. It cap end is after `n^(th)` division adn `17^(th)` division of main scale coincides with the reference line, the `ID` is -

Options

A. 321.717 mm
B. 321.87 mm
C. 328.17 mm
D. 324.67 mm

Answer

D

Explanation

`IR=321+7(0.5)+17(0.01)=324.67mm`