Let A and B be the lower fixed point and upper fixed point of a thermometer respectively.
Three more thermometers graduated in Celsius, Fahrenheit and Kelvin scale are kept side by side. The readings of the point P of the thermometer AB in other three scales are C, F and K respectively. Therefore the readings of distance PA in these three scales are C-0, F-32 and K-273 respectively. Since PA/BA is a constant it can be written,
PA/BA = C-0 / 100-0 = F-32 / 212-32 = K-273 / 373-273
or, C/100 = F-32 / 180 = K-273 / 100
or, C/5 = F-32 / 9 = K-273 / 5
This equation indicates the relation among Celsius, Fahrenheit and Kelvin scale. The convenient relation between Celsius and Kelvin scale is — by adding 273 to Celsius scale reading the Kelvin scale reading can be obtained. For example, 1°C temperature = (1+273) K = 274 K temperature. But if the temperature difference is 1K, then it is equal to 1°C.