> Given that different materials often have different coefficients of linear
> expansion, how about suspending a block of a metal in a liquid. As the
> temperature changes, the density of each will change and this will be seen
> as a change in weight of the metal block.

Fascinating idea.
Some issues include the temperature dependency od the suspended block, the 
liquid, the container and the suspension system* and the thermal inertia of 
the sysem. Compared to all those a PT100 resistor or a precision thermistor 
look most attractive.

* The suspension system also features as the change in length of the 
suspending element with temperature will raise or lower the suspended object 
slightly.

A variation of this idea would be to place a mass on an oscillating arm. 
Frequency of oscillation would change as arm length changed. If the mass was 
centre suspended and aligned along the axis of the the centre of mass (but 
not the moment of inertia) would remain constant as the mass expanded or 
contracted (but not a sthe arm did). If the mass was suspended at one end on 
the arm then it's expansion would increase the centre of mass disatnce from 
the centre. While this system could be a horror to calculate accurately 
theoretically it would probably behave very well in practice. Variations 
would be tiny but measurement accuracy of frequncy of oscillation can be 
measured with great accuracy. This does not seem likely to be attractive 
when compared to eg PT100s but it may have a niche application.


  Russell McMahon



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