**Describe the construction and working of the screw gauge? **

Difficulty: Hard

**Screw gauge:**

A screw gauge is an instrument that is used to measure small lengths with accuracy greater than a Vernier Caliper. It is also called a micrometer screw gauge.

**Construction:**

A simple screw gauge consists of a U-shaped metal frame with a metal stud at its one end. A hollow cylinder (or sleeve) has a millimeter scale over it along a line called the index line parallel to its axis. The hollow cylinder acts as a nut. It is fixed at the end of the U-shaped frame opposite the stud. A Thimble has a threaded spindle inside it. As the thimble completes one rotation, the spindle moves 1 mm along the index line. It is because the distance between consecutive threads on the spindle is 1 mm. This distance is called the pitch of the screw on the spindle.

**Least count of screw gauge:**

Least count = pitch of the screw gauge Least count ÷ no. of division on circular scale = 1mm ÷ 100 => 0.01 mm = 0.001 cm

Thus, the least count of the screw gauge is 0.01 mm or 0.001 cm.

**Working of a screw gauge:**

The first step is to find the zero error of the screw gauge.

** Zero error:**

To find the zero error, close the gap between the spindle and the stud of the screw gauge by rotating the ratchet in the clockwise direction. If zero of the circular scale coincides with the index line, then the zero error will be zero.

**Positive zero error:**

Zero error will be positive if zero of the circular scale is behind the index line. In this case. Multiply the number of divisions of the circular scale that has not crossed the index line with the least count of screw gauge to find zero error.

**Negative zero error:**

Zero error will be negative if zero of the circular scale has crossed the index line. In this case, multiply the number of divisions of the circular scale that has crossed the index line with the least count of screw gauge to find the negative zero error.

**Tidbits**

**Relative sizes of molecules and micro-organisms.**

**MINI EXERCISE**

**1. What is the least count of a screw gauge?**

** Ans:** The least count of the screw gauge is 0.01 mm or 0.001 cm.

2. What is the pitch of your laboratory screw gauge?

**Ans: **The pitch of our laboratory screw gauge is 1mm.

**3. What is the range of your laboratory screw gauge?**

**(a) **Vernier Calipers ** (b) **Screw Gauge

**Ans:** The range of our laboratory screw gauge is 100 mm.

**4. Which one of the two instruments is more precise and why?**

**(a)** Vernier Calipers** (b) **Screw Gauge

**Ans:** The least count of Vernier calipers is 0.01cm while the least count of screw gauge is 0.001 cm. The Vernier calipers measure the length with an accuracy of 0.01 cm. The screw gauge measures the length with an accuracy of 0.001 cm. therefore screw gauge is a more precise instrument.

**USEFUL INFORMATION**

**The least count of the ruler is 1mm. It is 0.1mm for Vernier Calipers and 0.01mm for micrometer screw gauge. Thus, measurements taken by a micrometer screw gauge are the most precise than the other two. **

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