(i) Explain the use of screw gauge and vernier caliper in measuring smaller distances.
(ii) Write a note on triangultion method and radar method to measure larger distances.

Explanation

To measure small distance (diameter of a wire) with a screw gauge :
The least count and zero error of the screw gauge are noted down.
Turn the ratchet anti - clockwise so as to obtain the gap between A and B.
The wire is to be placed in the gap and then turn the ratchet in the clockwise so as to the wire gently.
Note the main scale reading. Note that division P of the circular scale which coincides with the base line of the main scale.
To get circular scale reading. Circular scale division P is multiplied with least count.
Add the circular scale reading to the main scale reading to obtain the total reading.
From the observed mean diameter subtract the zero error. [division of the circular scale multiplied with Least Count] if any with its sign to get the true diameter.

A model reading
`PSR = 6mm, HSC=40" division, Reading "=[6mm+(40xx0.01mm)]=6.40mm`

The least count and zero error of the Vernier calipers have to be noted.
The Jaw `''J_(2)''` is moved away from `''J_(1)''` and object is placed to be measured between the jaws. Now move the jaw `J_(2)` towards `J_(1)` till it touches the object. Now tighten the screw.
The main scale reading is noted.
A model reading
MSR = 2.2 cm, VSC = 4 divisions,
Reading `=[2.2 cm+(4xx0.01cm)]=22.4cm`
Note that division ''P'' on Vernier scale which coincides with the main scale. Multiply this Vernier division P with the Least Count to get Vernier scale reading.
The Vernier scale reading is added to the main scale reading. This gives the observed length.
From the observed length subtract zero error (division of Vernier coincides with any division of the main scale multiplied with least count) if any with its sign to get the true distance or length.