A meter scale can measure accurately up to one-tenth section of one cm. Its least measurement 0.1 cm, is called least count of scale. There is limitation of scale that the meter scale cannot measure the value less than 0.1 cm. For bigger accuracy calculation we have devices such as,
(i) Vernier Callipers (ii) Screw Gauge
(i) Vernier Callipers: A vernier callipers provides with an auxiliary (or vernier) scale in addition to the main measurement scale. The vernier scale may be slide along the main scale. The vernier scale is so marked that the length of total number of divisions on it is smaller by length of one division on main scale.
The least count of vernier scale is computed by using the following formula
Least count of vernier scale (or vernier constant)= Value of main scale division/Total number of division in vernier callipers
or
Least count (vernier constant) = 1 M.S.D.(Main measurement division) - 1 V.S.D.(vernier scale division).
Zero error of vernier callipers: If the zero marking of main scale and vernier callipers does not coincide, necessary correction has to be created for this error which is known as zero error of the instrument. If the no error of the vernier measurement scale is to the right of the zero of the main scale the zero error is said to be positive & the correction will be negative otherwise vice versa.
(ii) Screw gauge: Screw gauge works on the rule of screw. It has a linear scale known as the main scale, and another scale known as the circular scale. The circular scale can be moved by a head screw. On moving the screw, the circular scale advances linearly on the main scale. The distance covered by the tip of screw when it is given one complete movement, is known as the pitch of the screw. Split the pitch of screw by the total number of division on the circular scale, we achieve the distance which the screw advances on rotating the screw by 1 division on its circular scale. This distance is known as the least count of the measurement device. Thus
Least count =Pitch x/ Total division n on the circuler scale
Generally, in a screw gauge the screw advances by 1 or ½ division on main scale when the screw is given one movement. If there are total 100 division on its circular scale and the value of 1 division on main scale is 0.1 cm, then least count = 0.001 cm
Zero error of screw gauge: In a perfect instrument the zeros of the main scale and circular scale coincides with each other, in that situation screw gauge has zero-error, otherwise the measurement device is said to have zero-error which is same to the cap reading with the gap closed. This error is positive when value is zero line or reference line of the cap lies above the line of graduation and corresponding corrections will be just opposite otherwise vice-versa.
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