# Property of thermometric substance

1. Oct 20, 2009

### songoku

1. The problem statement, all variables and given/known data
For the construction of a thermometer, one of the essential requirements is a thermometric substance which

a. remains liquid over the entire range of temperature to be measured
b. has a property that varies linearly with temperature
c. has a property that varies with temperature
d. obey Boyle's Law
e. has a constant expansivity

2. Relevant equations

3. The attempt at a solution
I think the answer can be (a) and (b). The substance must remains liquid so that it can measure the temperature and it must varies linearly with temperature to obtain good measurement. But the answer can be only 1...I don't know how to choose the right one

Thanks

2. Oct 20, 2009

### CompuChip

There are thermometers which rely on the expansion / contraction of a piece of metal to measure the temperature: nothing liquid there.
Also, if you have some property which is quadratic in temperature, does that make it impossible to use in a thermometer?

Why do you think c) , d) and e) are not correct?

3. Oct 20, 2009

### songoku

Hi CompuChip

(c) is wrong because I think it should be linear (which is wrong after I read your post).

(d) is wrong because it's for ideal gas only

(e) is wrong because I think it''s really difficult to obtain a substance that has constant expansivity. The best would be "almost" constant expansivity.

So, I think the answer is (c). But, I don't know whether the reasons for (d) and (e) are correct.

Thanks

4. Oct 21, 2009

### CompuChip

Why did you think that was an invalid reason for c?
Varying linearly with temperature is simply a special case of varying with temperature, isn't it?

5. Oct 21, 2009

### songoku

Hi CompuChip

Yes. I mean that my reason for (c) is wrong. (c) should be the right answer, I think.
And are my reasoning for (d) and (e) correct?

Thanks