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Units, Prefixes & Measuring Tools

Basic Science & Engineering · Measurement, Drawing & Mechanics

Useful for: Railway RRB

Overview

Workshop measurement uses SI units, prefixes and precision tools such as vernier callipers and micrometers.

Key points

SI prefixes: kilo 10³, milli 10⁻³, micro 10⁻⁶ (1 micron = 10⁻⁶ m), nano 10⁻⁹.
Vernier callipers: least count usually 0.1 mm (0.01 cm).
Micrometer (screw gauge): least count = pitch ÷ number of circular divisions, usually 0.01 mm.
Steel rule, try square, feeler gauge and dial indicator are common workshop tools.
Zero error must be corrected in every reading.

Explanation

Positive zero error is subtracted; negative zero error is added.
Measure at several places and take the average.
Use the right tool for the accuracy needed.
Keep instruments clean and calibrated.

Step by step

Identify the concept, tool or rule involved.
Recall the definition, value or procedure.
Check units, safety logic and the direction of the effect.

Examples

Vernier: main scale 2.1 cm, 6th division coincides, least count 0.01 cm → 2.1 + 0.06 = 2.16 cm.
Screw gauge: pitch 0.5 mm, 50 divisions → least count 0.01 mm.
1 micron = 10⁻⁶ m = 0.001 mm.

Traps and advanced points

Trap: Ignoring zero error.
Trap: Wrong least count for the screw gauge.
Trap: Mixing micro and milli.

Quick revision

LC vernier 0.01 cm; micrometer 0.01 mm.
Correct zero error.
Micron = 10⁻⁶ m.

Practice MCQs

ALP Basic Science & Engineering: Complete the statement: “First aid for severe electrical shock begins with ____.”
1. making the scene electrically safe before touching the victim
2. 1/R = 1/R₁ + 1/R₂
3. greenhouse gases
4. watt
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Answer: (a) making the scene electrically safe before touching the victim. Do not directly touch a person still in contact with live supply.
ALP Basic Science & Engineering: Complete the statement: “Velocity differs from speed because velocity includes ____.”
1. load/effort
2. metre
3. direction
4. protect a circuit from excessive current
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Answer: (c) direction. Velocity is a vector; speed is scalar.
How many base units are there in the SI system
1. 5
2. 7
3. 6
4. 9
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Answer: (b) 7. Metre, kilogram, second, ampere, kelvin, mole and candela.
The prefix 'micro' stands for
1. 10⁻⁶
2. 10⁻³
3. 10⁻⁹
4. 10⁶
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Answer: (a) 10⁻⁶. Milli is 10⁻³ and nano is 10⁻⁹.
The least count of a common screw gauge is
1. 0.1 mm
2. 0.01 mm
3. 1 mm
4. 0.001 mm
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Answer: (b) 0.01 mm. Pitch 0.5 mm divided by 50 circular divisions gives 0.01 mm.
Which instrument is used to measure the internal diameter of a pipe
1. Screw gauge
2. Vernier calliper
3. Ammeter
4. Hydrometer
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Answer: (b) Vernier calliper. Its internal jaws measure inside diameters.
The SI unit of electric current is the
1. ohm
2. volt
3. ampere
4. coulomb
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Answer: (c) ampere. Current is one of the seven base quantities.
1 kilometre is equal to
1. 100 metres
2. 1,000 metres
3. 10,000 metres
4. 1,00,000 metres
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Answer: (b) 1,000 metres. Kilo means 10³.