Plain English Meaning

How the instrument actually works inside — what it senses and how it turns that into the number you see on the dial.

Full Definition

The underlying physical mechanism by which an instrument senses a quantity and converts it into an indication on its display. For the sensitive altimeter, the principle of operation is that a stack of sealed aneroid wafers expands as outside (static) air pressure decreases with altitude and contracts as pressure increases, and this mechanical movement is geared to pointers on the altimeter face to show height above a selected pressure reference.

Context Anchor

Seen in instrument handbook sections that explain how pitot/static instruments, including the sensitive altimeter, create their indications.