VIDEO LECTURE
THEORY NOTES
1. Meter Rule
A meter rule is a device which is used to measure length of different objects. A meter rule of
length 1m is equal to 100 centimeters (cm). On meter rule each cm is divided further in to 10 divisions
which are called millimeters (mm). So, a meter rule can measure up to 1mm as smallest reading. It is
made up of a long rigid piece of wood or steel
The zero-end of the meter rule is first aligned with one end of the object and the reading is
taken where the other end of the object meets the meter rule
2. Vernier Caliper
The Vernier Caliper is a precision instrument that can be used to measure internal and
external distance extremely accurate.It has both an imperial and metric scale. A Vernier caliper has
main jaws that are used for measuring external diameter, as well as smaller jaws that are used for
measuring the internal diameter of objects. Some models also have a depth gauge. The main scale is
fixed in place, while the Vernier scale is the name for the sliding scale that opens and closes the jaws.
Reading a Vernier Caliper
Step 1 Place the object between the jaws of the Vernier caliper
Step 2 Note the main scale reading by counting lines before the zero line of Vernier scale
Step 3 Count the next line of Vernier scale after zero coinciding main scale
Step 4 Add the two reading for total
Zero Error
The error which arises due to the incorrect gradation of the scales of the verneir calipers
is called Zero Error.
i. No Zero Error: If the zero of main scale coincide with the zero of the Vernier scale then the
instrument has no zero error.
ii. Positive Zero Error: If the zero of the venier scale on the right of zero of the main scale then the
zero error will be positive, but the zero correction will be negative. To find the positive zero error, look
at the Vernier scale and not the number of divisions of the venier scale which coincide with one the
main scale divisions and multiply this number by least count.
iii. Negative Zero Error: If the zero of the Vernier scale is one the left of the main scale zero, then the
zero error will be negative and zero correction will be positive. To find the zero error note the number
of Vernier scale division which coincide with one of the main scale and multiply with least count.
3. Micrometer Screw Gauge
Screw gauge in extensively used in engineering field for obtaining precision measurements.
Micrometer screw gauge is used for measuring extremely small dimensions.A screw gauge can even
measure dimensions smaller than those measured by a Vernier Caliper.
Micrometer Screw gauge works on the simple principle of converting small distances into
larger ones by measuring the rotation of the screw. This “screw" principle facilitates reading of smaller
distances on a scale after amplifying them.
Reading A Micrometer Screw Gauge
Step 1 Turn the thimble until the anvil and the spindle gently grip the object. Then turn the ratchet
until it starts to click.
Step 2 Take the main scale reading at the edge of the thimble
Step 3 Take the thimble scale reading opposite the datum line of the main scale. Multiply this reading
with least count i.e., 0.01mm
Step 4 Now add main scale reading to thimble reading. This will be the diameter of the object.
Zero Error of a Screw Gauge:
When the plane surface of the screw and the opposite plane stud on the frame are brought into contact, if the zero of the head scale coincides with the pitch scale axis, there is no zero error.
Positive Zero:Error When the plane surface of the screw and the opposite plane stud on the frame
are brought into contact, if the zero of the head scale lies below the pitch scale axis, the zero error is
positive.
Negative Zero Error:
When the plane surface of the screw and the opposite plane stud on the frame are brought into
contact, if the Zero of the head scale lies above the pitch scale axis,the zero error is negative.
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