Vernier calipers. Accurate measurement of the size of objects up to 20 centimetres in length can be obtained by using Vernier calipers. The object is placed between the fixed and moving arms (centre left). The reading on the vernier scale (the part that moves) is added to the reading on the fixed scale on the body of the caliper to obtain a reading. Vernier calipers can also be used to measure the diameter of the inside of objects by using the arms at top centre.