on whether the scale is for a Fahrenheit or for a centigrade thermometer.
9. Fahrenheit Thermometer. — The Fahrenheit thermometer is made by dividing the distance on the scale between the freezing and boiling points into 180 equal divisions, called degrees. The freezing point is marked 32, and the boiling point 212; 32 divisions are marked off from the freezing point downwards, and the lowest one is marked 0; this is called the zero point of the scale. The graduations may be extended above the boiling point, or below the zero point, as desired.
Instead of writing the word degrees after each reading of temperature it is customary to represent it by a symbol — a small circle placed above and to the right of the figures. Also, the word Fahrenheit is usually represented by the letter F. Thus, 32° F. means the same as though it were written "32 degrees Fahrenheit, " and 8° F. means the same as "8 degrees Fahrenheit." In this thermometer, there are 212 divisions, or degrees, between the zero point and the boiling point.
10. Centigrade Thermometer. — In the centigrade thermometer, the freezing point is marked and the boiling point 100, the distance between the two being divided into 100 equal divisions. As in the Fahrenheit scale, the divisions may be carried above the boiling point and below the zero point. The word centigrade is usually abbreviated and written C, as, for example, 10° C, 28° C, etc.
11. Thermometer Readings. — Beginning with 0°, the divisions on both the Fahrenheit and the centigrade scales are numbered 1, 2, 3, 4, etc., both above and below the zero point. Therefore, in giving the lower readings of a thermometer, it is necessary to state the number of degrees and whether they are above or below zero. To distinguish temperatures below zero from those above, the sign of sub- traction is always placed before the former. Thus, on either scale, 12° means 12° above zero, while —12° means 12° below zero.
