Filled-system (vapor pressure) thermometers: In a vapor pressure thermometer, which component acts as the main functioning (pressure-sensing/force-developing) element that converts pressure into pointer motion?

Introduction / Context:
Filled-system thermometers (liquid, gas, or vapor pressure types) contain a bulb, capillary, and an elastic element that converts internal pressure into motion. Recognizing the role of each element aids in troubleshooting and selection.

Given Data / Assumptions:

• Vapor pressure thermometer: temperature changes alter saturated pressure in the bulb/capillary.
• Mechanical link translates pressure into dial indication.

Concept / Approach:
The bulb is the sensing element that generates pressure changes. However, the functioning pressure-to-motion transducer is the Bourdon tube (or other elastic element) which deflects under pressure, driving the pointer via linkages. Thus, the Bourdon tube is the functioning element converting pressure to mechanical displacement.

Step-by-Step Solution:
Temperature rise → vapor pressure increase in bulb/capillary.Pressure applied to Bourdon tube → elastic deflection.Deflection transmitted to pointer → temperature reading.

Verification / Alternative check:
Instrument cutaways show C-shaped or helical Bourdon tubes connected to the dial mechanism in filled-system thermometers.

Why Other Options Are Wrong:

• Pointer: indicates but does not convert pressure to motion.
• Bulb: primary sensing element; it does not provide mechanical amplification.
• Capillary: transmits pressure; not the functioning conversion element.
• None of these: incorrect because the Bourdon tube is correct.

Common Pitfalls:
Equating “sensing” (bulb) with “functioning element” (Bourdon tube) that moves the pointer.

Final Answer:
Bourdon tube