The thermometer is used to measure the temperature. There are several scales to determine the temperature, which are Celsius, Fahrenheit and kelvin. The range of measurement scales are different for each of the scales or unit of temperature. For Celsius scale earlier the lowest temperature was 0°C, but after the invention of absolute zero which is 0 K (K = Kelvin). The lowest temperature of Celsius scale is -273°C. Thus the smallest unit of Celsius scale is -273°C.
