Measuring the Diameter of a Wire Using a Screw Gauge

🎯 Aim

To measure the diameter of the given wire using a screw gauge.

🧰 Apparatus Required

• Screw gauge
• Wire whose diameter is to be measured
• Magnifying glass

📘 Theory

A screw gauge is a precision instrument used to measure very small lengths (up to 0.01 mm). It works on the principle of a micrometer screw, where rotational motion of the screw is converted into linear motion.

Linear Scale Reading (L.S.R.) = N x pitch

If the nth division of circular scale coincides with reference line, circular scale reading (C.S.R.) = n x Least Count (L.C.) in mm.

Total Reading (T.R.) = L.S.R. + C.S.R. = (N x pitch) + (n x L.C.)

Example Screw Gauge Image

🧭 Procedure

1. Determine the pitch
   Rotate the screw head five full turns and measure the distance moved.
   Divide by 5 to get pitch.

2. Find the least count
   Least Count (L.C.) = Pitch / Number of divisions on circular scale.

3. Find the zero error
   Check whether the zero of the circular scale coincides with the reference line when the jaws just touch.

4. Measure the diameter

   • Place the wire between jaws A and B.
   • Rotate the ratchet until it slips—this ensures correct pressure.
   • Note the linear scale and circular scale readings.

5. Rotate the wire by 90°
   Measure again to check uniformity of thickness. This completes one set.

6. Repeat steps (4–5)
   Take at least three sets of readings at different points along the wire.

📊 Observations

✔️ Precautions

• Always rotate the screw using the ratchet to avoid excessive force.
• Move the screw in one direction to avoid backlash error.
• Take readings at different points of the wire.
• Avoid parallax error while noting scale readings.

⚠️ Sources of Error

1. The screw gauge may have backlash error.
2. The wire may not have a uniform diameter.
3. Friction in the screw may affect accuracy.