TEMPERATURE AND TEMPERATURE SCALES

 a. liquid-crystal thermometer

b. liquid in glass thermometer

With both of the thermometers shown in figure, it is important to wait for a minute or two if you want to see the correct reading. This is because the thermometer has probably been stored somewhere relatively cool, perhaps in a drawer at 20 degree celsius. The patient's temperature will be approximately 37 degree celsius, and it takes a short while for the thermometer to reach this  temperature.
      This gives us idea of what we mean by temperature. The thermometer is placed in contact with the patient's body. It has to warmup until it reaches the same temperature as the patient. Energy from the patient is shared with the thermometer until they are at the same temperature. Then you will get the correct reading. [So the thermometer does not tell you the patient's temperature - it tells its own temperature! However, we know that the patient's temperature is the same as the thermometer's.]
     

 Figure shows a thermometer measuring the temperature of some hot water. The molecules of the water are rushing above very rapidly, because the water is hot. They collide with the thermometer and share their energy with it. The bulb the thermometer gets hotter. Eventually, the thermometer bulb is at the same temperature as the water. [We say that the water and the thermometer bulb are in the thermal equilibrium with one another . Energy is not being transferred from one to the other.]

Temperature and internal energy
You can see from this that it can be important to make a careful choice of thermometer. How could you measure the temperature of small container containing hot water? If you chose a large, cold thermometer and poked it into the water, it might absorb a lot of energy from the water and thus make it much cooler. You would get the wrong answer for the temperature. A better solution might be to use an electronic thermometer with a very small probe. This would absorb less of the energy of the water.
         A thermometer thus tells us about the average energy of the particles on the object whose temperature we are measuring. It does this by sharing the energy of the particles. If they are moving rapidly, the thermometer will indicate a higher temperature. Placing a thermometer into an object to measure its temperature is rather like putting your finger into some bath water to detect how hot it is.
Your finger does not have a scale from 0 to 100 but it can tell you how cold or hot water is, from uncomfortably cold to comfortably warm to painful hot.    
        Thus the temperature of an object is a measure of the average kinetic energy of its particles. Because it is the average kinetic energy of a particle, it does not depend on the size of object. We compare internal energy and temperature:

• internal energy is the total energy of all of the particles
• temperature is measure of the average kinetic energy as the individual particles.

So a bath of water at 50 degree celsius has more internal energy than a cup of water at the same temperature, but its individual molecules have the same average kinetic energy as the molecules of the water in the cup.

The Celsius scale
Galileo is credited with devising the first thermometer, in 1593. The air inside the flask expanded and contracted as the temperature rose and fell. This made the level of the water in the tube change. This could only indicate changes in the temperature over a narrow range, and proved unsatisfactory because water evaporated from the reservoir. Galileo knew that air expands as its temperature increases. Modern liquid-in-glass thermometers use mercury or alcohol instead of air. These are also substances that expand when they are heated.

       Anders Celsius, working in Sweden, devised a more successful thermometer than Galileo's. It had a volume of mercury in an enclosed and evacuated tube, with no chance of liquid loss by evaporation. It was like the much more modern Celsius thermometer shown in figure b. Celsius also devised a scale of temperature, now known as the Celsius scale. This had two fixed points:

• 0 degree celsius - the freezing point of pure water at atmospheric pressure
• 100 degree celsius - the boiling point of pure water at atmospheric pressure.

Each time he had a new thermometer, Celsius could calibrate it quite simply by putting it first into melting ice and then into boiling water, marking the scale each time. Then he could divide the scale into 100 equal divisions. This process is known as calibration of the thermometer.