Micrometer Screw Gauge

Quick Answer: The Micrometer Screw Gauge is a hand instrument that measures small outside dimensions by turning a finely threaded spindle towards a fixed anvil and reading the movement on a sleeve scale and a rotating thimble scale. A micrometer screw gauge is the usual school instrument for wire diameters and sheet thicknesses.

Turning Rotation into Tiny Distances

One full turn of the thimble advances the spindle by one pitch of the screw. The sleeve carries a linear scale in whole and half millimetres, and the bevelled edge of the thimble is divided into equal parts, so a fraction of a turn corresponds to a fraction of the pitch. Least count equals pitch divided by the number of thimble divisions: a 0.5 mm pitch with 50 divisions gives 0.01 mm, ten times finer than a typical school vernier.

To find the pitch practically, give the thimble a set number of turns and note how far its edge moves along the sleeve. The ratchet at the end of the thimble slips once the anvil and spindle press on the object with a light, repeatable force, so every student applies the same pressure; tightening directly by the thimble can squash soft wire and flex the frame.

With the faces closed by the ratchet, the thimble zero should meet the reference line on the sleeve. If the zero has passed below the line, the error is positive; if it has not yet reached the line, it is negative. Wire diameter practicals commonly use a gauge like this on a length of steel wire or brass wire, with readings at several points and in two directions at right angles.

Specifications

Applications

• Measuring wire diameter for resistivity calculations with a metre bridge
• Finding the thickness of a glass slide, or of one sheet of paper by measuring a stack
• Measuring small ball bearings for density or falling-ball viscosity experiments
• Finding the wire diameter in a Young’s modulus experiment with Searle’s apparatus

Care & Handling

• Always close the spindle with the ratchet, never by forcing the thimble.
• Leave a small gap between anvil and spindle when storing, so the faces are not pressed together.
• Clean the measuring faces by closing them lightly on a strip of clean paper and drawing it out.
• Hold the frame only briefly while measuring, since hand heat expands it slightly.

Why Choose LabEquip

Screw gauges are a routine part of physics lab equipment for senior classes, usually one per bench for wire and sheet measurements. LabEquip lists this gauge in the General Lab Products range alongside digital and engineering versions; send your bench count through the contact page.

Frequently Asked Questions

What is the least count of a micrometer screw gauge?

It is the pitch of the screw divided by the number of divisions on the thimble. A common school design has a 0.5 mm pitch and 50 divisions, giving 0.01 mm. Check both figures on your own gauge before recording readings.

Why use the ratchet instead of the thimble?

The ratchet slips at a set light force, so the spindle closes on the object with the same pressure every time. Turning the thimble directly lets different users squeeze harder or softer, which changes readings and can deform thin wire.

How do I tell positive from negative zero error?

Close the faces with the ratchet. If the thimble zero has gone past, below the sleeve reference line, the error is positive and is subtracted. If the zero stops above the line, the error is negative and its size is added to readings.

What is backlash?

It is a small lost movement in the screw thread when the direction of turning reverses. Approach every reading by turning the spindle in the same direction, towards the object, so any slack is always taken up the same way.

Why measure a wire at several places?

Wire is rarely perfectly round or uniform. Taking readings at several points along its length, and in two directions at right angles at each point, then averaging them, gives a mean diameter that represents the whole wire.

Can it measure the inside of a tube?

No. This instrument measures outside dimensions between the anvil and the spindle. Inside diameters need the upper jaws of a vernier caliper, an inside micrometer or a bore gauge.

Last Updated: September 2026