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Discussion

Once the bottom edge of the scale is aligned with the meniscus of the mercury in the tube, the height of the mercury column can be read from the scale.

The movable scale piece usually is a Vernier scale-an auxiliary sliding scale used to more easily read the values on a fixed main scale. Its purpose is to allow accurate readings, rather than estimations, between the smallest graduations on the fixed scale. A Vernier scale commonly has ten graduation marks. Each division on the Vernier scale is nine-tenths of the size of the finest division on the main scale.

Narrative

The vernier scale is used to determine the last significant digit of a reading. As the sliding portion moves, marks on its surface align with the graduations on the fixed scale. When the sliding scale is in the correct position, the last digit of the reading is the mark on the sliding scale that aligns perfectly with the mark on the fixed scale.