Temperature instrumentation: in a vapor-pressure thermometer, which component functions as the secondary element that converts pressure into mechanical motion?

Introduction / Context:
Filled-system thermometers (liquid-in-glass, gas-filled, vapor-pressure) use a primary sensing bulb connected by a capillary to a pressure-responsive element that drives an indicator. Understanding which part is “secondary” helps classify instruments by function.

Given Data / Assumptions:

• Vapor-pressure thermometer uses a volatile fluid in bulb and capillary.
• Temperature changes vapor pressure in the bulb.
• Pressure is converted to motion for indication.

Concept / Approach:
The primary element is the bulb where temperature–pressure conversion occurs. The Bourdon tube (or bellows) is the secondary element: it translates pressure into mechanical displacement. The pointer is an indicating element attached to the linkage, not the secondary transducer itself.

Step-by-Step Solution:
Temperature rise → vapor pressure increases in bulb.Pressure transmits through capillary to Bourdon tube.Bourdon tube flexes proportionally, driving the pointer via a linkage.

Verification / Alternative check:
Instrument schematics show bulb (primary), capillary (transmission), Bourdon/bellows (secondary), and pointer/scale (indication) in sequence.

Why Other Options Are Wrong:

• Pointer: indicates only; it does not convert energy.
• Bulb: primary sensor, not secondary element.
• None of these: incorrect because Bourdon tube is correct.

Common Pitfalls:
Confusing “secondary” with “indicator.” Secondary elements perform energy conversion (pressure → displacement); indicators simply display.

Final Answer:
Bourdon tube