MEASURING INSTRUMENTS

• Measuring instruments are used to measure physical quantities.

• Scientists, engineers and other humans use a vast range of instruments to perform their measurements.

• Measuring instruments:

METRE RULE MEASURING TAPE VERNIER CALLIPERS SCREW GUAGE

METRE RULE:

• A metre rule is used to measure length of an object or distance between two points.

• It is one metre long which is equal to 100 centimetres and each centimetre is divided into 10 small divisions called millimetre (mm).

• Metre rule is made up of different materials and in a wide range of size.

MEASURING TAPE:• Measuring tapes are used to measure length in metres

and centimetres.• It is a common measuring tool.• It is used by blacksmith and carpenters.• It consists of a ribbon of cloth, plastic, fiber or glass .• Its design allows for a measure of great length.

VERNIER CALLIPERS:• A vernier calliper is an instrument used to measure the

internal or external diameters of an object.

VERNIER CALLIPERS:• Vernier callipers consists of two jaws.• One is a fixed jaw with main scale. It has centimetre and

millimetre marks on it.

VERNIER CALLIPERS:• The other jaw is a moveable jaw having vernier scale on

it. A vernier scale has 10 divisions on it and each division is 0.9 mm.

LEAST COUNT OF VERNIER SCALE:

• The difference between one small division on main scale and one vernier scale division is 0.1 mm.

• It is called least count of the vernier callipers.• To find the least count of vernier callipers, we use this

method:Least count of Vernier Callipers = smallest reading on main scale / no. of divisions on vernier scale = 1mm / 10 divisions = 0.1 mm

Hence, Least Count = 0.1 mm = 0.01 cm

ZERO ERROR AND ZERO CORRECTION• What is zero error?If there is any error in the measuring instrument then it is called zero error of the instrument.

• What is zero correction?If zero line of vernier scale is not coinciding with main scale the zero error will exist. Knowing the zero error necessary correction can be made to find the correct measurement. Such a correction is called zero error.

ZERO ERROR:• There is no zero error as zero line of vernier scale is

coinciding with the zero of main scale.

POSITIVE ZERO ERROR:• Zero error is (0+0.03) cm as 3rd line of vernier scale is

coinciding with main scale.

• Zero error is positive if the zero line of vernier scale is on the main scale.

NEGATIVE ZERO ERROR:• Zero error is (-0.1+0.05) cm as 5th line of vernier scale is

coinciding with main scale.

• Zero error is negative if the zero line of vernier scale is outside the main scale.

SCREW GAUGE:• A Screw gauge is an instrument used to measure small

lengths such as diameter of a wire, thickness of a sheet, etc.

• It is also called as micrometer screw gauge.

CONSTRUCTION:• SCREW:A simple screw gauge consists of a U-shaped metallic structure attached to a hollow cylinder on one end.

• Hollow cylinder:A hollow cylinder has millimetre scale over it along a line called index line parallel to its axix.

• Spindle:A spindle is a very accurately machined screw and the object is measured between the spindle and the anvil.

CONSTRUCTION:• Thimble:The thimble is the end of the cylindrical tube and is turned to move and adjust the spindle. The thimble carries the vernier or secondary scale.Ratchet; Ratchet it itself a small device which is used to provide a limited applied force, it is installed at the right end of screw gauge, ratchet acts as a safety device for instruments and also adds more precision in measurement.

• Locking device:Some screw gauge also consists of locking device which is usually used to hold the scale at a particular position for prevent any kind of error while taking readings.

Presentation by: Hooria Shahzad