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"Liquid in glass thermometers show different readings for temperatures other than the fixed points because of differing expansion properties.A thermometer which uses a gas gives the same readings regardless of which gas is used".

This is what my book says.

I can understand that different liquids will give different temperature readings due to different expansions,but I can't understand why are the fixed points an exception.

If I take the fixed point as the boiling point of water,then a thermometer with water in might show a different reading than a thermometer with mercury in it for the boiling point of water.Right?

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  • $\begingroup$ I disagree with the book with respect to the note on gases. It might be true if all gases were ideal gases, but sadly they are not. $\endgroup$
    – Jon Custer
    Commented Jun 13, 2017 at 19:46
  • $\begingroup$ I think it is just assuming that the gases are ideal. $\endgroup$
    – Pragati Joshi
    Commented Jun 13, 2017 at 19:48

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Every individual thermometer of any type is calibrated to give the correct temperature reading at the fixed points. This is why they are called fixed points. There is no guarantee that the thermometer will read the correct temperature at other points.

All thermometers of any type which use the same fixed points will give the same temperature reading at these fixed points. If they use different fixed points, there is no guarantee that they will read the same at any temperature.

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Book is right.

Fixed points are calibrated so that they are in accordance with the expansion of the thermometric liquid. For example, the fixed points of a Mercury thermometer will differ form that of an alcohol thermometer for sane temperature readings.

In between the fixed points, the thermometer may not give correct readings. That's why fixed points are marked.

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