What temperature is the same on both Celsius and Fahrenheit scales?
Fahrenheit the value in Celsius: This is also an exact conversion making use of the identity -40 °F = -40 °C. Again, is the value in Fahrenheit and the value in Celsius: Fahrenheit proposed his temperature scale in 1724, basing it on two reference points of temperature. In his initial scale (which is not the final Fahrenheit scale), the zero point was determined by placing the thermometer in a mixture of ice, water, and ammonium chloride ("salis Armoniaci"). This is a frigorific mixture which stabilizes its temperature automatically: that stable temperature was defined as 0 °F (−17.78 °C). The second point,
Fahrenheit a degree on the Fahrenheit scale is of the interval between the freezing point and the boiling point. On the Celsius scale, the freezing and boiling points of water are 100 degrees apart. A temperature interval of 1 °F is equal to an interval of degrees Celsius. The Fahrenheit and Celsius scales intersect at −40° (i.e., −40 °F = −40 °C). Absolute zero is −273.15 °C or −459.67 °F. The "Rankine" temperature scale uses degree intervals of the same size as those of the Fahrenheit scale, except that absolute zero is 0 °R — the same way that the "Kelvin"