Page 13 - Personal Study Notes (Engineering Metrology - 22342)
P. 13
Page 13 of 176
Q. Write short note on End Bar.
End Bars or Length Bars.
These are used for the measurement of larger sizes of
work. These consist of carbon steel round bar about 20
mm in diameter and made in sizes varying from 10 mm to
1200 mm. These are hardened only at ends upto 800 HV
and supported at ‘Airy’ points so that end surfaces are
parallel to each other. For bars above 150 mm size, the
airy points are indicated by pairs of circumferential lines
inscribed around the bars.
Such bars are used for standardising the normally used
one inch bars in combination. These are, therefore,
generally not found in majority of engineering works but in
standardising laboratories etc.
These are also available in four grades of accuracy (i)
reference, (ii) calibration, (Hi) inspection, (iv) workshop.
Q. Define Wavelength Standard. What are its advantages?
It is very clear from the methods discussed earlier that comparison and verification of
the sizes of the gauges pose considerable difficulty. This difficulty arises because the
working standard used as a reference is derived from a physical standard and successive
comparisons are required to establish the size of a working standard using the process
discussed earlier, leading to errors that are unacceptable. By using wavelengths of a
monochromatic light as a natural and invariable unit of length, the dependency of the
working standard on the physical standard can be eliminated. The definition of a
standard of length relative to the metre can easily be expressed in terms of the
wavelengths of light.
The use of the interference phenomenon of light waves to provide a working standard
may thus be accepted as ultimate for all practical purposes. However, there were some
objections to the use of the light wavelength standard because of the impossibility of
producing pure monochromatic light, as wavelength depends upon the amount of
isotope impurity in the elements. However, with rapid advancements in the field of
atomic energy, pure isotopes of natural elements have been produced. Cadmium 114,
krypton 86, and mercury 198 are possible sources of radiation of wavelengths suitable
for the natural standard of length. There is no need to preserve the wavelength standard
as it is not a physical one. This standard of length is reproducible, and the error of
reproduction can be of the order of 1 part in 100 million.
Finally, in 1960, at the 11th General Conference of Weights and Measures held in Paris,
it was recommended and decided that krypton 86 is the most suitable element if used in
a hotcathode discharge lamp maintained at a temperature of 68 K. According to this
standard, metre is defined as 1,650,763.73 × wavelengths of the red–orange radiation
Engineering Metrology (Only For Personal Use) 5
