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Vernier Callipers

The Vernier calliper is used to measure the length or thickness of an object with great accuracy. It was invented by Pierre Vernier in 1630. It can measure length less than 1 mm.

A Vernier calliper consists of a fixed main scale or ordinary scale, a sliding , movable jaws, and a tail. Parts of a Vernier Main Scale

The main scale consists of a steel metallic strip graduated in centimeters at one edge and in inches at the other edge . It consists of a fixed external jaw and a fixed internal jaw. Vernier Scale A vernier scale slides on the strip. It can be fixed in any position by the retainer. On the Vernier scale, 0.9 cm is divided into ten equal parts. It consists of a fixed external jaw and a fixed internal jaw. External Measuring Jaws

The outer measuring jaws (one of the main scale and the other of the vernier scale) helps to take the outer dimension of an object Internal Measuring Jaws The inner measuring jaws (one of the main scale and the other of the vernier scale) helps to take the inner dimension of an object. Retainer

The retainer helps to retain the object within the jaws of the Vernier calipers to fix firmly. Depth Measuring Strip

The depth measuring strip (prong) helps to measure the depth of an object. Least Count

The smallest length that can be measured accurately with a device is called the least count of that device. The least count of a Vernier calliper is known as the Vernier constant. The least count or the smallest reading which you can get with the instrument can be calculated as: Least Count = One main scale division - one Vernier scale division ------(i) or Least Count = [(One main scale division) / (Number of divisions in Vernier scale) ] Calculate the least count and only then place the object between the two jaws. Record the position of zero of the Vernier scale on the main scale.

Parts and Functions of Vernier Callipers

Part Function

Main scale Main scale measures the length accurately up to 1mm.

Vernier scale Vernier scale helps to measure length correctly up to 0.1 mm.

Outside jaws or Outside jaws or External jaws to measure length of a rod, diameter of a External jaws sphere, external diameter of a hollow cylinder.

Inside jaws or Inside jaws or Internal jaws measure the internal diameter of a hollow Internal jaws cylinder of a hollow cylinder or pipe.

Strip Strip is used to measure depth of a beaker or a bottle.

Zero Errors If the zero on the main scale coincide with the zero on the Vernier scale, then the Vernier calliper is said to have no zero error.

If the zero on the main scale does not coincide with the zero on the Vernier scale, then the Vernier calliper is said to have zero error.

There are two types of zero errors i. Positive zero error ii. Negative zero error.

If the two jaws are in contact, the zero of the main scale and the zero of the Vernier scale should coincide. If both the zeros do not coincide, there will be a positive or negative zero error.

Calculation When a body is placed between the jaws of the Vernier Calipers; If the zero of the vernier scale lies ahead of the nth division of the main scale, then the main scale reading (MSR) is; MSR = n If nth division of Vernier scale coincides with any division of the main scale, then the Vernier scale reading (VSR) is; VSR = n × L.C, where n is the main scale reading and L.C is the least count of the vernier calllipers Total reading, Total Reading = MSR + VSR Total Reading = [n + ( n × L.C)]

Note The Vernier calliper can also be used to find the inner diameter and depth of a hollow cylinder.

Screw An ordinary scale can be used to measure lengths up to 1 mm accurately. The Vernier callipers can be used to measure up to 0.1 mm accurately, but not any dimension less than that.

A screw gauge is used to measure up to 0.01 mm with greater accuracy. It can be used to measure the thickness of thin objects like wires, glass sheets, and so on. The screw gauge works on the principle of a screw in a nut.

The screw gauge consists of a U-shaped metallic frame, a stud or , a sleeve cylinder, a spindle, a barrel, the main scale, the head scale and a ratchet. Pitch The pitch of the screw is equal to the distance travelled by the screw on the pitch scale divided by the number of rotations made by the head scale.

Least count ( L.C) = Pitch of the screw / Number of head scale divisions

Description of a Screw gauge

Screw gauge consists of a U shaped metallic frame(F). To one end of the frame, a stud S1 is fixed. On the opposite end of F, a hollow cylinder ‘C’ is fixed. This cylinder has fine threads inside it and acts as a nut.

On the outer surface of the cylinder, a line-parallel to its axis, called index line ‘I’ is drawn and is divided into some equal divisions, called Pitch Scale ‘P’.

A screw ’S’ with a flat tip S2 having threads exactly identical to the threads cut inside the cylinder, moves through the cylinder ‘C’.

To the other end of the screw, a milled head ‘M’ is connected. To this milled head, one end a barrel ‘B’ is connected and this barrel forms a jacket to the hollow cylinder ‘C’.

The other end of the barrel is tapered and has 100 or 50 equal divisions on it. This is called Head Scale ‘H’.

Determination of Diameter Aim: To determine the diameter of a wire by using screwgauge Procedure  Find the least count of the Screw gauge by this formula: Least count ( L.C) = Pitch of the screw / Number of head scale divisions  The Zero error of the Screw gauge is noted, if any.

 The wire is placed between the shafts S1 and S2 and the head Scale is rotated anti-clock wise, so that the shafts holds the wire tightly.  The value of the pitch scale division which just precedes edge of the head scale is noted as pitch scale reading (P.S.R)  The value of the head scale division which just coincides with the index line is the observed Head Scale Reading. (H.S.R)  Observed reading = Pitch scale reading + (Head scale reading × Least count).  i.e., The Diameter of the wire = P.S.R. + (H.S.R. x L.C) The wire is placed at different positions in between the studs and the observations are recorded. Zero Errors

With the tip of the screw in contact with the stud, if the zero of the head scale does not coincide with the index line, then the screw gauge is said to have zero error. Zero error can be positive or negative. Correction is always the negative of the error.

Positive Zero Error

When the two studs S1 and S2 are in contact with each other, if the Zeroth division of Head Scale is below the index line, then the error is said to be Positive Zero error and the correction is negative. Positive zero error is subtracted from the observed reading

Negative Zero Error

When the two studs S1 and S2 are in contact with each other, if the Zeroth division of Head Scale is above the index line, then the error is said to be Negative Zero error and the correction is Positive. Negative zero error is added to the observed reading.

Note: Correction is always the negative of the error.

Parts and Functions of Screw Gauge:

Part Function

Ratchet Ratchet is used to advance the screw by turning it till the object is gently held between the stud and spindle of screw.

Sleeve Sleeve is used to mark main scale and base line.

Thimble Thimble is used to mark circular scale.

MainScale Main scale is used to read length correct up to 1 mm.

Circular Scale Circular scale helps to read length correct up to 0.01 mm.