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A. PRINCIPLE OF OPTICAL INSTRUMENTS
B. PRINCIPLE OF PNEUMATIC MEASURING INSTRUMENTS
C. PRINCIPLE OF ELECTRICAL MEASURING
INSTRUMENTS
D. PRINCIPLE OF ELECTRONIC MEASURING
INSTRUMENTS
E. PRINCIPLE OF MECHANICAL MEASURING
INSTRUMENTS

Measuring the dimension of a given component consist of a
comparing its dimension with a no standard.

This may be carried out by direct or indirect method.
i.
Reflections -: Reflections can be divided into two
types: Specular reflection and diffuse reflection .
Specular reflection describes the gloss of surfaces
such as mirrors, which reflect light in a simple,
predictable way.
ii.
When light is incident on a surface, it is partly
reflected and partly absorbed in the surface and the
remaining part may pass through it.

When a ray of light travel from one medium into
another, it bends while crossing the surface of
separation between two media.

The phenomenon of change in path of light as it goes
from one medium to another is called refraction.
There are two law of refraction -:
i.
The incident ray the normal to the refracting surface at
point and the refracted ray all lie in same plane.
ii.
The ratio of the sine of angle of incidence to the sine of
the angle of refraction is constant
A lens is a transparent medium bounded by two co-axial
curved refracting surface or one covered and one plane
surface.
Lens are divide two categories -:
i.
Converging lens
ii.
Diverging lens

A convex lens is thicker in the middle and converges a
parallel beam of light passing through it to a single point
known as the principal focus.

A real image is formed on the screen in this case.0
i.
DOUBLE CONVEX -: when both faces are bulging out.
ii.
PLANE – CONVEX -: when one face is plan and other
is convex.
iii.
CONCAVE – CONVEX -: when one of the surface is
concave and other is convex.

A concave lens is thinner in the middle and diverges the
parallel rays which appear to come from a focal point
behind the lens.

A virtual image formed on a screen in case of this type of
lens.
i.
PLANO – CONCAVE -: When one of the surface
is plane and other is concave.
ii.
DOUBLE CONCAVE -: When both faces are
bulging.
iii.
CONVEXO – CONCAVE -: When one of the
surface is convex and other is concave.

The one of the surface is plane and other is concave.

Interference of light means a phenomenon which
involves superposition of two light wave leading to
change in their intensity.

It may be define as “ the non-uniform distribution of
the energy in the medium due to the superposition of
two light waves.”

A prism is a transparent medium bounded by three
rectangular and two triangular plane surface.
The optical method of measurement are of two general
classes-:
i.
Which the object being measured is viewed by a
projector or microscope.
i.
Which there is indirect application of opticals.
i.
SOURCE OF LIGHT-: It is a high power electric bulb,
which should be placed at the principle focus of the
collimating lens.
ii.
COLLIMATING LENS -: A collimator lens is used to
make the light beams parallel to each other.
iii.
PROJECTION LENS -: It is a combination lenses.
forms a real image on screen and is placed between
projection lens and collimator lens
Horizontal
projector
Vertical projector
Cabinet projector

Pneumatic measuring instruments use to apply a jet of
air to the surface being measured and utilized a
controlled air pressure as an amplifying medium.

The jet orifice is placed very closed to the surface of
the test piece.

The variation in the dimension of work piece affects
the escape of air from the aperture and back pressure
can be measure.

The mechanical parameters can be measured by the
use of electrical means.

This is based of the fact that the use of the electrical
parameter are physically related to mechanical
parameters of length, mass, time.

Simple operation.

Quick response.

Used under severe working condition.

Having high sensitivity and accuracy.

The electronic instrument have a number of
advantage over the mechanical type.

They have little or on moving parts and hence can
retain their accuracy over long period.

The accuracy of these instrument is likely to be
affected by the temperature and humidity.

Measuring probe

Amplifier and indicating unit

Power unit

Base and stand unit
To reduce the error of measurement the following
mechanical magnification are usually used-:
i.
Lever method
ii.
Vernier method

The lever is supported on a knife edge support. When a
distance ‘l’ is measured by the indicator the magnification
ratio is given by:
a/b = Reading on the scale/b sinα= a. α/b sinα
Where α= angle of displacement corresponding to the
measured distance l.

The friction can be reduced by means of specially
designed pivots & bearings & making use of knife edges

In this method a vernier is used. The vernier principal
can also be applied for increasing the accuracy of
angular measurement.

It is an additional scale which is used in place of a
pointer or indication line on the moveable member & it
enables the main fixed scale to be read to a smaller value