Experiments based on using Vernier callipers and screw gauge

The experiments based on using vernier calliper and screw gauge deal with measuring the linear dimensions of an object. In simple words, it gives the distance between two points in a straight line. Vernier callipers and screw gauge provide accurate dimensional values of the object, with high sensitivity to any change in the dimensions. 

These precision instruments are predominantly used in making medical instruments and mechanical spares. They are easily employed to measure the diameter of spherical surfaces as they hold the objects right in place.

Vernier Calliper

Vernier calliper, an instrument used for measuring the linear dimensions of an object, is accurate provided there is no error in the procedure. Pierre Vernier, a mathematician in France, was the first to introduce a calliper in the year 1631. It can measure things having a thickness as small as 0.1 mm. Currently, there is even a digital version available. It can be used to measure 

• The distance between the outer surfaces
• The inner distance between two surfaces of an object
• The depth of a hollow object

Experiment based on using a vernier calliper

Aim: To experiment using a vernier calliper to measure the diameter, a linear dimension, of a cylindrical object(e.g., blackboard chalk).

Materials required: The given sample object (blackboard chalk), vernier calliper, stationery.

Parts and Working mechanism:

There are two scales in a vernier calliper: 

• One is the Main scale, which has graduation where 1 division equals 1 mm. The scale has regular numerical markings above the longest lines extended upwards, vertically. The distance between two consecutive long lines measures 10 mm or 1 cm. It is fixed in its position.
• The other is the Vernier scale. It has a scale of 9 mm in total. Thus, each division is 0.1 mm or 0.01 cm long. It is movable along the horizontal axis.

The vernier calliper hosts two jaws:

• The upper jaws are used for measuring the inner dimensions of an object.
• The lower jaws are for measuring the other dimensions of an object.

Formulae and Principle:

• The least Count of vernier calliper is the minimum measurement that the vernier calliper can measure. It is calculated using the formula,

Least count = magnitude of the smallest division of the main scale /

Total number of divisions in the vernier scale

 = 1 mm / 10

Least count = 0.1 mm or 0.01 cm

• Zero error is a condition where the ‘0’ of the major scale does not align with the ‘0’ of the vernier scale. In such cases, the division that coincides exactly in both scales is the zero error value.

If the zero of vernier is left to the zero of the major scale, it is called negative zero error. The value will be negative. If it is to the right, it is called positive zero error. The value will be positive.

• Reading = main scale reading + vernier scale reading – (∓ zero error × least count)

Procedure:

• Place the lower jaws closed and check if the zeros of both scales coincide.
• Now, loosen the screw of the vernier scale and move it towards the right side. 
• Place the given object (blackboard chalk) and tighten the vernier scale.
• Hold the instrument exactly below your eyes to avoid parallax error, which may result in false results.
• Now, note down the division of the main scale that coincides with the zero of the vernier scale. If the zero does not coincide exactly, then take the closest value to the left of the zero. That is the primary scale reading. 
• Then, note which vernier scale division coincides exactly with any main scale division. That is the vernier scale reading.
• Substitute the values in the final reading formula and calculate the answer.
• Similarly, take three to four sets of values by rotating the chalk. The average of all the values gives the diameter of the object.

Screw Gauge

A screw gauge is more efficient than a vernier calliper to measure linear distances between two points. It can measure objects having dimensions as small as 0.01 mm. A micrometre screw gauge is used, even now, because of its easy adaptation and high reliability. It is combined with other meteorological devices such as telescopes to measure the stellar and planetary objects in the sky. Its original features pave the way for its usage for around 150 years.

Experiment based on using a screw gauge (micrometre):

Aim: To experiment using a screw gauge to measure the diameter of a cylindrical object (e.g., a wire).

Materials required: The given sample object, screw gauge, stationery. 

Parts and Working mechanism:

The parts of a screw gauge include,

• U-frame – A U-shaped metal attached to a small nut called anvil with a plain parallel face and a long screw on the other side.

• Screw – The screw, called the spindle, is on the right-hand side of the screw gauge. It is movable and has consecutively arranged threads (the spirals) equally distanced. One whole rotation that leads from one thread to the following thread is called the pitch of the screw. 

• Scales – A micrometre screw gauge houses two scales. One scale is the primary linear, and the other is the secondary circular scale. The secondary scale that is circular gives the measure of the pitch of the screw. This scale rotates over the linear scale to provide the linear scale value. It has a total of 100 divisions. The major scale has a graduation that measures 1 mm between two consecutive lines.

Formulae and Principle:

• Least count of screw gauge – The minor count of a screw gauge is calculated with the formula,

Least count = pitch of the screw / total divisions in the circular scale

         = 1 mm / 100

Least count = 0.01 mm or 0.001 cm

• Zero error – If the zeros of the scales do not coincide exactly, it is called zero error. In such cases, the division that coincides precisely in both the primary and circular scales is taken as the zero error value.

If the zero of the circular scale is below the zero of the linear scale, it is called negative zero error. On the other hand, if it is above the zero of the linear scale, it is a positive zero error. 

• Reading = linear scale reading + circular scale reading × least count

Procedure

• Check for zero error by aligning the scales.
• Loosen the screw to place the wire between the nut and the spindle.
• Tighten the screw to hold the object in place and measure the scale readings.
• The linear scale division that lies near the left end of the circular scale is the primary scale reading.
• The circular scale division that coincides with the linear scale line is the circular scale reading.
• Substitute the values in the formula and obtain the final reading.
• Repeat the steps to get 3 – 4 values and calculate the average. It gives the diameter of the wire.

Conclusion

The experiments based on using vernier callipers and screw gauge (micrometre) are inevitable when the dimension of the apparatus influences the result of the study. They are efficient in measuring objects that are almost micrometre in size. Due to their resolved precision, they find an ever dying place in any mechanical and instrumental manufacturing.