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The least value or the smallest value that any instrument can measure is called the least count. For example, the least count of a metre scale is 1mm. Metre scale is the most common instrument to measure length. The other measuring instruments are vernier caliper and screw gauge.
Vernier Caliper
Vernier Calipers are used to measure smaller dimensions. It has a main scale and a sliding vernier scale. The main scale of the vernier caliper is graduated just like a metre scale. The graduation is done in mm and cm on one side and the other side is marked in inches. The vernier scale is the sliding scale. It slides on the main scale. The vernier scale is 9 mm long and consists of 10 divisions. Therefore, the length of each division is 0.9 mm. This instrument is used to measure the outer dimension, inner dimension and depth of the object. Vernier calipers have two lower jaws and two smaller upper jaws. The object is placed between the lower jaws to measure the outer dimension. The upper jaws are used to measure the inner dimensions.
Least count of Vernier Caliper
Least count of Vernier Caliper = Smallest division on the main scale/ Number of divisions on the Vernier Scale
Therefore, the least count of vernier calliper = 1mm/10 = 0.1 mm
Since the least count of the vernier caliper is 0.1mm, it can be said that we can get an accuracy of 0.1 mm in the measurement when the vernier caliper is used.
Zero Error in Vernier Caliper
When the zero of the main scale and the vernier scale coincides then the error is zero
Positive Zero Error
When the jaws are in contact and the zero on the vernier scale lies to the right of the zero on the main scale then the error is positive and the correction is negative.
Negative Zero Error
If the zero on the vernier scale lies on the left of the zero on the main scale, then the error is negative and the zero correction is positive.
Screw Gauge
The screw gauge is used to measure even smaller dimensions than vernier calipers. It has a U shaped frame fitted with the jaws. The screw gauge has a pitch scale and a circular head scale. The pitch scale is graduated in millimetres and the head scale has 100 equal divisions.
Least Count of Screw Gauge
Pitch of the screw gauge is the distance moved by the screw on the pitch scale in n rotations
Pitch = distance travelled in n rotations/ n rotations
The total number of divisions on the circular scale is equal to 100. So, what is the least count of screw gauge?
Least count of the screw gauge = Pitch/Total number of divisions on the circular scale
Least count of the screw gauge = 1mm/100 = 0.01 mm
Zero Error of Screw Gauge
Positive Zero Error
If the zero mark on the head scale is below the baseline of the main scale, the error is positive. The division on the circular scale coinciding with the baseline is multiplied with the least count to get the positive zero error
Negative Zero Error
When the zero marks on the head scale are above the baseline of the pitch scale, then the error is negative. The number on the circular scale that coincides with the baseline is subtracted from the total divisions on the circular scale and then multiplied with the least count of the screw gauge. This gives the negative zero error.
