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| Micrometer Screw-Gauge |
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| Micrometer screw-gauge is another instrument used for measuring accurately the diameter of a thin wire or the thickness of a sheet of metal. |
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| It consists of a U-shaped frame fitted with a screwed spindle which is attached to a thimble. |
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| Screw-gauge |
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| The screw has a known pitch such as 0.5 mm. Pitch of the screw is the distance moved by the spindle per revolution. Hence in this case, for one revolution of the screw the spindle moves forward or backward 0.5 mm. This movement of the spindle is shown on an engraved linear
millimeter scale on the sleeve. On the thimble there is a circular scale which is divided into 50 or 100 equal parts. |
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| When the anvil and spindle end are brought in contact, the edge of the circular scale should be at the zero of the sleeve (linear scale) and the zero of the circular scale should be opposite to the datum line of the sleeve. If the zero is not coinciding with the datum line, there will be a positive or negative zero error as shown in figure below. |
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| Zero error in case of screw gauge |
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| While taking a reading, the thimble is turned until the wire is held firmly between the anvil and the spindle. |
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| The least count of the micrometer screw can be calculated using the formula given below: |
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Least count  |
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| = 0.01 mm |
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| The wire whose thickness is to be determined is placed between the anvil and spindle end, the thimble is rotated till the wire is firmly held between the anvil and the spindle. The rachet is provided to avoid excessive pressure on the wire. It prevents the spindle from further movement. The thickness of the wire could be determined from the reading as shown in figure below. |
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| Reading = Linear scale reading + (Coinciding circular scale x Least count) |
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| = 2.5 mm + (46 x 0.01) |
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| = (2.5 + 0.46) mm |
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| = 2.96 mm |
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| Relationship in the Metric system of length |
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| 1 kilometer (km) = 103
m |
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| 1 centimeter (cm) = 10-2 m |
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| 1 millimeter (mm) = 10-3 m |
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| Mass is the quantity of matter contained
in a body. |
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| If you push a book, it moves faster than
if you push a car with the same force. This is because the car has more mass
than the book. If you had two identical boxes, one containing iron and the
other containing cotton we could identify them by pushing the boxes. We can
say that the car and iron box are more reluctant to move than the book and
the cotton box. We call this reluctance to move "inertia". Larger the mass
of an object, larger is its inertia. Hence mass of a body is a measure of
its inertia. |
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| Moving objects have inertia too. A moving object needs force to make it stop. A moving car has more inertia than a moving book. It needs more force to make it stop. |
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| Measurement of Mass |
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| Mass of an object can be determined by
comparing the mass of it with a standard mass. For this we can use a lever
balance or a common balance. |
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| Common Balance |
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| This balance consists of a beam and two scale pans (shown in figure below), the beam being balanced at its mid point on a knife-edge. The scale pans also hang on knife edges and rest on the base board. When the balance is not in use the beam rests on the beam support. |
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| A Laboratory Balance |
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| How to use a balance? |
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 Use the
leveling screws, attached beneath the base board to make sure that the beam is horizontal. It can be verified with the help of the plumb- line provided shown in the diagram. |
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Use the arrestment knob to raise the beam and the adjusting screw at the two ends of the beam, to bring the pointer to the middle or zero mark on the scale. |
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Lower the beam using the arrestment knob again. |
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Place the body to be weighed on the left scale pan and put weights on the |
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| right hand scale pan to balance the beam (when pointer is at zero). |
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