📏 Vernier & Screw Gauge

📖 The Physics

A plain ruler can read to about a millimetre — but what if you need tenths of that? A vernier calliper gets you there with a clever second scale.

You read the whole millimetres from the main scale, then look along the sliding vernier scale for the one line that exactly lines up with a main line (shown in red) — that line number gives the fraction.

A screw gauge works on the same idea but uses a rotating thimble (pitch ÷ 100), giving an even smaller least count of 0.01 mm — perfect for wires and thin sheets.