Instruments Used in measuring.

Instruments Used in measuring length.

(i) Tape-measure: (Accurately measures length greater than 1metre:𝑙 > 1𝑚). E g length of a foot ball field, length of a plot of land etc.

(ii) Metre-rule 🙁 Accurately measures length greater than 12 centimetres but less than 1metre:12𝑐𝑚 < 𝑙 < 1𝑚). Eg length of a desk, breadth of a window, etc. A metre rule gives readings in cm to 1dp.

(iii) Vernier calipers or Slide calipers : Accurately measures length greater than 1cm but less than 12 cm:2.5cm < 𝑙 < 12 cm. E.g Internal and External diameters of test tubes and beakers, breadth of a metre rule.etc. A vernier caliper gives readings in cm to 2dp.

image 98

Engineer calipers

image 99

The distance between the jaws is after wards measured on an ordinary scale like a metre-rule.

How to read a vernier Caliper

image 100
image 102

Reading of vernier calipers,

  1. Record the reading on the main scale to two places in cm.
  2. Look along the Vernier scale carefully until you see
    division on it which coincides with the main scale, this gives
    the second decimal place.


image 103
Main scale = 2.40cm
Vernier scale = 0.04cm
Final reading = 2.44cm

What readings are represented in the diagram?

image 104

(iv) Micrometer screw gauge: (Accurately measures length less than 1centimetre: 1𝑚𝑚 < 𝑙 < 25𝑚𝑚). Eg Diameter of wires, Diameter of ball beairings and pendulum bob, etc.

A micrometer screw gauge gives readings in cm to 2dp.

image 105
For each turn the spindle moves through 0.5mm.The fraction of each turn is indicated on the thimble. This has a
image 106

Example I:

image 107
Sleeve scale reading = 14.50mm
Thimble scale reading = 0.44mm
= 14.94mm

Example II:

image 108
Sleeve scale reading = 9.5 00mm 00mm
Thimble scale reading = 0.29mm
= 9.79mm

Precautions taken when using a micrometer screw gauge

  • The faces of the anvil and the spindle must be cleaned to remove dust so as to get accurate readings.
  • The reading must be checked.