Absolute Temperature Scale

Absolute Temperature Scale

There are different types of thermometers we can use to measure temperature, e.g. mercury in glass, resistance, thermocouple. Each of these thermometers has a property that changes with temperature, e.g. length of mercury thread, resistance, and voltage.

We can measure this property at ice and steam points, and then divide the quantity of this property between the two points into 100 intervals - like drawing 100 marks next to the glass tube of a mercury thermometer.

The process of doing this to find the temperature is called calibration. The exact way it is done is called temperature scale. The way just described - dividing the temperature between ice and steam points into 100 intervals - is called the Centigrade scale.

The ice and steam points above are used as reference points for the scale. When used in this way, they are called the fixed points of the scale. Instead of ice and steam points, we can choose two other temperatures for the fixed points.

Suppose we put plenty of salt in ice, lower its temperature as much as possible, and use this as one fixed point - 0 degree. Next, we look people having fever, find the person running the highest temperature, and use that as the other fixed point - 100 degrees. We mark the lengths of the thread in a mercury thermometer, and make 100 intervals in between. This scale is called the Fahrenheit scale. (Note: This just is a parody of the historical origin of the Fahrenheit scale.)

There are other types of temperature scales apart from Centigrade and Fahrenheit. For now, let us use the Centigrade scale. If I measure the temperature of a cup of hot tea using a mercury, a resistance and a thermocouple thermometer, would I get the same reading? (Assuming that we can keep the tea is kept at the same hotness while we carry out all three measurements.)

The answer is - most likely not. Not exactly anyway. The reason is that the length of the mercury thread, the resistance of the wire, the voltage from the thermocouple, can change at different rates with temperature. They would all measure 0 degree at ice point, and 100 degrees at steam point, because this is defined by the calibration. If temperature is increased from 0 degree, the resistance may at first increase more slowly than the mercury thread, and later more quickly, finally arriving at 100 degrees together.

From the above imaginary experiment, we arrive at an important conclusion: the measured temperature depends on the material of the thermometer.

That is not really a big problem. As long as we know which thermometer is used to measure the temperature of my perfect cup of tea, I can always use the same thermometer the next time I need to brew my perfect cup.

What is really interesting is that a temperature scale exists that is independent of material properties. This was discovered by Lord Kelvin about 100 years ago, and is named after him.

Unfortunately, it is well beyond the scope of this article to discuss how Lord Kelvin made this discovery. However, the Kelvin scale is very widely used today, and we shall look at how to use it next.

The Physics Notes