Class 9 > Unit # 01: Physical Quantities & Measurement > Length Measuring Instruments

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Chapter # 1: Physical Quantities and Measurements — Length Measuring Instruments

Lecture Overview

This comprehensive lecture from Talha's Physics Academy covers the fundamental instruments used to measure length in physics, detailing the construction, working principles, least counts, and reading calculation methods for the Meter Rule, Vernier Calipers, and Screw Gauge.

Source Video: Length Measuring Instruments | Chapter 01 | Physical Quantities and Measurements | Class 9

1. Meter Rule

A meter rule is a common length-measuring instrument used in laboratories and daily applications.

• Length & Construction: Typically 1 meter long (\(100 \text{ cm}\) or \(1000 \text{ mm}\)), made of wood or metal (steel).
• Least Count: The smallest reading that can be measured accurately with a meter rule is 1 \text{ mm} (or 0.1 \text{ cm}).
• Measurement Technique: Align the zero mark of the rule with one end of the object, keeping the rule parallel to the length being measured, and read the position on the scale corresponding to the other end.

$$\text{Least Count of Meter Rule} = 1 \text{ mm} = 0.1 \text{ cm}$$

2. Vernier Calipers

A Vernier Calipers is a precision instrument used to measure small lengths, internal diameters, external diameters, and depths with higher accuracy than a meter rule.

• Key Components: Main scale (fixed), Vernier scale (sliding), lower jaws (for external measurements), upper jaws (for internal measurements), and a depth rod.
• Least Count: Typically 0.1 \text{ mm} or 0.01 \text{ cm}.
• Reading Formula: \text{Total Reading} = \text{Main Scale Reading (MSR)} + (\text{Vernier Coincidence} \times \text{Least Count}).

$$\text{Total Reading} = \text{MSR} + (\text{VC} \times \text{LC})$$

3. Screw Gauge (Micrometer)

A Screw Gauge (or micrometer screw gauge) is an extremely precise instrument used to measure tiny lengths, wire diameters, and sheet thicknesses.

• Working Principle: Operates on the principle of a screw, where a complete rotation of the circular scale moves the spindle forward or backward by a distance equal to its pitch.
• Key Components: Stud, spindle, main scale (sleeved scale), circular scale (thimble scale), and ratchet.
• Least Count: 0.01 \text{ mm} or 0.001 \text{ cm}.
• Reading Formula: \text{Total Reading} = \text{Main Scale Reading (MSR)} + (\text{Circular Scale Reading} \times \text{Least Count}).

$$\text{Total Reading} = \text{MSR} + (\text{CSR} \times \text{LC})$$

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