Temperature & Thermometers

therm-digitalTemperature is the degree of hotness of a body. A thermometer tells us how hot by assigning a number to a temperature based on fixed points.

A fixed point is a physical property that doesn’t change, such as the freezing point of pure water or its boiling point at standard atmospheric pressure. We then assign an equal scale between these two points, such as the Celsius scale (0 to 100)

There are lots of things that change with temperature – the length of a thin capillary tube filled with mercury, the resistance of a piece of wire, the pressure of an ideal gas – the list goes on and on. The nice thing about thermometers is that we can scratch the numbers on the glass like the numbers on a ruler – they are evenly spaced…

We don’t use mercury thermometers much these days because mercury is highly toxic and can’t be gotten rid of easily since it’s almost as unreactive as gold. Also, it might cause a little problem if you bit the end off accidentally while taking your temperature. Instead, we use digital thermometers which use resistors – called thermistors – whose resistance changes with temperature. A little processor measures the change in resistance and converts it to a digital display for us.

If temperatures change rapidly, we can’t use liquid-in-glass thermometers, they can’t respond quickly enough. [exam question alert…] Instead we can use a thermocouple. A thermocouple is made from two dissimilar metals ( not the same), twisted together at both ends with a meter in the loop.

Not a great picture - but you get the idea
Not a great picture - but you get the idea

If one end is hotter than the other a small voltage exists between the joined ends – this is called the Seebeck Effect. The bigger the temperature difference, the bigger the voltage measured by the voltmeter in the loop. We can calibrate the meter to read temperature.

These things are very versatile. Dependent on the choice of metals, they can measure rapid changes across very large ranges, also both high and low temperatures can be measured.

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