A Celsius thermometer will read 100 degrees for the boiling point of water and 0 degrees for the freezing point; a Fahrenheit thermometer will read 212 degrees for the boiling point of water and 32 degrees for the freezing point.
The thermometer features a scale in both Fahrenheit and Celsius . . The melting point of ice is 32 degrees Fahrenheit and the boiling point of water is 212 degrees Fahrenheit on the Fahrenheit scale. Thus, the Fahrenheit scale ranges from 32 to 212 degrees Fahrenheit, with 32 degrees F representing the freezing point of water and 212 degrees F being the boiling point of water. The Celsius scale works similarly, ranging from 0 to 100 degrees Celsius with 0 degrees C representing the freezing point of water and 100 degrees C being the boiling point of water.
As mentioned, the thermometer feature a dual-scale for measuring temperatures in either Fahrenheit or Celsius. This dual-scale design allows users to measure temperatures that cannot be represented by single numbers on the standard 7-10/11-13 digit Fahrenheit / Celcius dials. For example, if someone tells you that the temperature outside is -20 degrees F, they are saying that it is 20 degrees below zero. However, a normal person would not be able to tell whether this was 20 degrees below zero F or 200 degrees below zero F due to the inability of humans to sense cold temperatures past about -60 degrees F (or 60 degrees below zero F).
However, using the dual-scale design of the thermometer, people can say that the temperature is -20 degrees F even though it's really only -7 degrees F because the number -7 is closer to zero than -20 is.
The Celsius scale is used in most scientific domains to measure temperature. Water has a freezing point of zero degrees Celsius and a boiling temperature of 100 degrees Celsius. The Kelvin scale is also used in some scientific domains to measure temperature. It is based on the ideal gas law that states that the volume of one molecule of gas at a given pressure is constant, regardless of the temperature. Water has a melting point of 0 degrees Kelvin and an evaporation rate of 100 percent at the triple point of water (at a temperature of 231 degrees Celsius and a pressure of 1 atmosphere).
Although it is not widely used today, the Fahrenheit scale was once commonly used by Americans to measure temperature. It is based on the definition of the thermometer as a device that measures heat; thus, it should be no surprise that it uses the degree symbols from the Mercator map projection system. At its inception in 1724, the American Medical Association's guide to health suggested using the Fahrenheit scale instead of the Celsius scale because it thought that people would not assume that temperatures outside of the range of 32 degrees Fahrenheit to 212 degrees Fahrenheit were safe. However, this definition was not adopted by all scientists at the time, so today both scales are used interchangeably by scientists.
Water has a melting point of 32 degrees Fahrenheit and a boiling temperature of 212 degrees Fahrenheit on the Fahrenheit scale (at standard atmospheric pressure). Water's boiling and freezing points are now 180 degrees apart. As a result, a degree on the Fahrenheit scale equals 1/180 of the distance between the freezing and boiling points. The number of degrees in a right angle is also important in explaining why 212 degrees F is used for the boiling point of water in the Fahrenheit scale.
As we know, heat is measured in degrees Kelvin. So how many degrees are in a Fahrenheit degree? It turns out that there are not really any good reasons for using 100 degrees F as the reference point for the scale, other than historical accuracy. The United States adopted the Fahrenheit scale in 1724, so it's natural that they would use something related to the boiling point of water as the reference point. The original scale was based on the boiling point of water at sea level which is 0 degrees F, but since then the boiling point of water has been shown to be higher when the altitude is taken into account. So today the reference point for the Fahrenheit scale is actually closer to 98.6 degrees F.
In conclusion, one degree on the Fahrenheit scale equals 1/180 of the distance between the freezing and boiling points of water.