Bimetallic thermometers: which mechanical element type is generally not used in their construction?

Introduction / Context:
Bimetallic thermometers translate temperature into motion using a bonded strip of two metals with different expansion coefficients. The strip is formed into shapes that magnify tip motion: common forms are flat spirals and helices. Bourdon tubes, by contrast, are elastic pressure elements used in pressure gauges and filled-system thermometers, not in pure bimetal devices.

Given Data / Assumptions:

• Bimetal element options: spiral, single helix, multiple helix.
• Bourdon tube is a hollow, curved tube that flexes with internal pressure.
• We are focusing on true bimetal designs (no fluid fill).

Concept / Approach:
A bimetal thermometer relies on solid-state thermal expansion; there is no internal pressure change. Hence, Bourdon tubes, which require a pressurised medium, are not part of bimetal thermometers. Spirals and helices are the standard mechanical geometries for bimetal elements because they compactly provide large angular deflection for small length changes.

Step-by-Step Solution:

Verification / Alternative check:
Instrument catalogs show bimetal dial thermometers built as spirals or helices; Bourdon tubes appear in pressure gauges and fluid-filled expansion thermometers.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing filled-system thermometers (which may use Bourdon tubes) with bimetal types.

Final Answer:
Bourdon tube