 Welcome back. Let us continue our discussion of the Kelvin scale of temperature. It is an ideal gas scale of temperature and we notice that the reference temperature that of the triple point of water was defined to be 273.16 Kelvin. The question is why did we define it to be so? And the reason is the same, uniformity standardization. This particular number 273.16 Kelvin as the ideal gas Kelvin temperature for the triple point of water was selected because when you select that it turned out that the temperature of a system in Kelvin approximated temperature of the same system on the Celsius temperature scale plus 273.15. So, 0 degree Celsius was approximately 273.15 Kelvin, steam point which was 100 degree Celsius was approximately 373.15 Kelvin. But again we cannot have this approximation. We cannot say Celsius is defined this way, Kelvin is defined this way and the relation is approximately this. So today's definition is that the Celsius scale is defined in terms of the Kelvin scale. So today we define that the Celsius temperature of any system is defined to be not approximately equal to the temperature of the same system on the Kelvin scale minus 273.15. So that today the Celsius temperature is defined in terms of Kelvin temperature and the Kelvin temperature is defined using the ideal gas scheme that we saw earlier. Because of this on a scale of temperature we should note the following. Let us say this is the ice point. Let us say this is the steam point. Let us say this is not really to scale, but just to be clear this is triple point of water. Which one of these is exactly defined? This one. This one is defined to be 273.16 Kelvin. I am putting it in the block indicating this is exact. And because of this definition on the Celsius scale this turns out to be 0.01 degree Celsius again exact because of the definition of the Celsius scale as it stands today. Because of this the ice point needs to be measured and it turns out to be I will not write approximately, but because it is not in blocks and I am not going to write exact. This is approximately 273.15 Kelvin which is approximately 0.00 degree Celsius. Similarly the steam point now turns out to be when measured 373.15 Kelvin and on the Celsius scale by the definition of the Celsius scale this will be almost exactly, but not really exactly 100 degree C. And now you will notice the reason for selecting this value to be 273.15 is that the difference between the ice point and the steam point is difference delta T between ice point and steam point is 100 degree C and which is also 100 Kelvin as a temperature difference. In this schematic one should remember that there are only two exact values. This temperature it is exact because it is defined to be so and this temperature on the Celsius scale is also exact because it is defined to be so using this equation. Whereas all other values ice point at 0 degree C steam point at 100 degree C are excellent approximations, but they are not really exact. Thank you.