How to Find Least Count and Take Accurate Screw Gauge Readings
A screw gauge, also called a micrometer screw gauge, is a precision instrument widely used in Physics laboratories for measuring the diameter or thickness of very small objects, such as wires or metal sheets. Its importance in mechanics and practical physics arises from its ability to provide measurements more accurately than ordinary rulers or even vernier calipers.
The screw gauge works on the principle of a fine-threaded screw. When the thimble is rotated, the screw moves forward or backward through a fixed nut, allowing precise linear displacement. This leads to a measurable shift, which is read using calibrated scales. The device mainly consists of two parts: a stationary frame with an anvil, and a spindle that moves towards or away from the anvil through rotation of the thimble.
Certain parts of a screw gauge are stationary (such as the sleeve and anvil), while others (such as the spindle, thimble, and ratchet) are movable. Before measuring, it is good practice to clean the spindle and anvil using a clean cloth or paper. This ensures accuracy by removing any dust or residual material that could affect readings.
To use the screw gauge, carefully place the object to be measured between the anvil (fixed) and the spindle (movable). Rotate the ratchet or thimble clockwise until the spindle just makes contact with the object. The ratchet ensures that no extra force is applied, ensuring measurement consistency. Once tight, a lock can be applied to fix the spindle during reading.
Measurement is obtained using two scales: the main (sleeve) scale and the circular (thimble) scale. The main scale typically measures in millimetres (mm). The thimble scale, having fine divisions, allows reading fractions of a millimetre. By combining these readings, precise measurements are achieved.
To calculate the final measurement, follow these steps:
- Note the reading on the main scale adjacent to the reference line.
- Record the number that aligns on the circular (thimble) scale.
- Use the least count to convert the thimble reading into millimetres.
- Add the main scale reading and the converted thimble reading for the total measurement.
The key formula for the least count (L.C.), which determines the smallest measurable value, is:
In practical terms, "pitch" is the distance the spindle moves forward in one full rotation of the thimble. For instance, if the pitch is 1 mm and the thimble is divided into 100 parts, the least count will be 0.01 mm.
| Term | Definition | Typical Value |
|---|---|---|
| Main Scale | Fixed ruler, measures in mm | 0.5 mm/div |
| Circular Scale | Rotating scale, divided into fine parts | 50 or 100 divisions |
| Pitch | Advance per one rotation | 1 mm |
| Least Count | Smallest measurable value (Pitch/Divisions) | 0.01 mm |
Let us look at an example for better understanding. Suppose you have a screw gauge with a pitch of 1 mm and 100 divisions on the circular scale.
- First, calculate the least count:
L.C. = 1 mm / 100 = 0.01 mm - Suppose the main scale shows 3 mm, and the thimble or circular scale's 45th division coincides with the reference. Multiply the circular scale reading (CSR) by the least count:
CSR = 45 × 0.01 mm = 0.45 mm - Add both readings to get the measurement value:
Total Reading = 3 mm + 0.45 mm = 3.45 mm
To avoid errors, do not force the spindle during rotation. Use the ratchet stop for consistent pressure. Always clean the instrument for accuracy and recalibrate if you observe unusual readings.
| Step | Action | Purpose |
|---|---|---|
| 1 | Clean anvil and spindle | Ensures accuracy |
| 2 | Place object between anvil and spindle | Correct positioning |
| 3 | Rotate ratchet to tighten | Prevents over-tightening |
| 4 | Read scales and calculate | Obtains measurement |
If you want to explore more, practice with experiments involving screw gauge to measure thickness and diameter of wires and plates. For a structured step-by-step approach, visit the following:
- Diameter Measurement Using a Screw Gauge
- Sheet Thickness Measurement Lab
- Volume of Irregular Object with Screw Gauge
In summary, mastering the screw gauge enables you to carry out high-precision measurements that are essential in physics laboratories and for practical exams. Practice stepwise procedures frequently, pay attention to details like the least count, and always ensure your readings are accurate and reliable.
FAQs on Screw Gauge (Micrometer Screw Gauge): Principle, Working & Calculations
1. What is a screw gauge?
A screw gauge, also called a micrometer screw gauge, is a precision instrument used for measuring small dimensions with high accuracy. It is commonly used to measure the diameter or thickness of wires, sheets, and other small objects in Physics experiments. The instrument consists of a screw spindle, main scale, circular scale, anvil, sleeve, thimble, and ratchet, providing readings up to 0.01 mm precision.
2. What is the principle of a screw gauge?
The screw gauge works on the principle of the screw, i.e., rotational motion is converted into linear motion. When the thimble is rotated, the spindle moves forward or backward by a fixed distance (pitch) per revolution, allowing precise linear measurement of small objects.
3. How do you find the least count of a screw gauge?
The least count (L.C.) of a screw gauge is the smallest measurement it can read and is calculated using:
Least Count = Pitch / Number of divisions on circular scale
For example, if the pitch is 1 mm and there are 100 divisions, L.C. = 1 mm / 100 = 0.01 mm.
4. How do you read a screw gauge?
To read a screw gauge, follow these steps:
- Note the Main Scale Reading (MSR).
- Note the Circular Scale Reading (CSR).
- Multiply CSR by Least Count (L.C.).
- Add MSR and (CSR × L.C.); adjust for zero error if present.
Total Reading = MSR + (CSR × L.C.) ± Zero Error
5. What is zero error in a screw gauge? How is it corrected?
Zero error is the error when the screw gauge does not read zero even when its jaws are in contact. There are two types:
- Positive zero error: Zero is below reference line (subtract the error from final reading).
- Negative zero error: Zero is above reference line (add the error to final reading).
Zero correction is: Final value = Actual reading ± Zero error.
6. What is the difference between a screw gauge and a vernier caliper?
The main differences are:
- Screw gauge: Measures very small thickness (least count usually 0.01 mm), used for wires, thin sheets.
- Vernier caliper: Measures both internal and external dimensions (least count usually 0.1 mm), less precise than screw gauge.
Screw gauge offers higher accuracy for micro-measurements.
7. What are the main parts of a screw gauge and their functions?
Main parts include:
- Anvil: Stationary surface for holding the object.
- Spindle: Movable screw for pressing the object.
- Sleeve (Barrel): Has the main scale.
- Circular scale (thimble): Shows fractional readings.
- Ratchet: Ensures consistent pressure.
- Lock: Fixes the spindle position during reading. Each part contributes to the device's high-precision measurement.
8. What is pitch in a screw gauge and how is it determined?
Pitch is the distance advanced by the spindle per one complete rotation of the thimble.
To determine pitch:
- Note the initial reading.
- Rotate the thimble for 10 rotations and note the final reading.
- Pitch = (Total distance moved) / (Number of rotations). Typically measured in millimeters (mm).
9. How do you measure the diameter of a wire using a screw gauge?
To measure wire diameter:
1. Place the wire between anvil and spindle.
2. Rotate the thimble to hold the wire gently.
3. Note Main Scale Reading (MSR).
4. Note Circular Scale Reading (CSR).
5. Calculate: Diameter = MSR + (CSR × Least Count) (adjust for zero error).
10. Why is a ratchet used in a screw gauge?
The ratchet ensures that a uniform, gentle force is applied while measuring with a screw gauge. It prevents excessive pressure that could deform the object or affect measurement accuracy by producing a clicking sound at the correct force.
11. What are common errors when using a screw gauge and how can they be minimized?
Common errors include:
- Zero Error (corrected as explained above)
- Parallax Error (reading at wrong angle)
- Dirt or Dust between anvil and spindle
Minimize errors by:
- Cleaning the instrument
- Reading with eye exactly perpendicular to scale
- Using the ratchet properly
- Applying zero correction to all measurements
12. How do you calculate the final reading with positive and negative zero error in a screw gauge?
To calculate the final measurement:
- With positive zero error: Subtract the zero error value from the calculated reading.
- With negative zero error: Add the zero error value to the calculated reading.
Formula: Final Reading = Observed Reading ± Zero Error (add for negative, subtract for positive).
