While all temperature measurements in American and English shops are expressed according to the Fahrenheit scale, it is often necessary to change the Fahrenheit into the Centigrade readings. Below is given a comparison of the two at the boiling and freezing points, together with conversion formulas for use in changing readings from one standard to the other. (Figure 61.) Fahrenheit is denoted by the letter F. and Centigrade by the letter C.

 Boiling Point Freezing Point Fahrenheit Scale........ 212° 32° Centigrade Scale............. 100° 0°

5(F - 32) / 9 = C

9 / 5 C + 32 = F

Fig. 61. - Centigrade and Fahrenheit Scales.

* For a description and illustration of the pyrometer see Chapter XV (Magnetism And Electricity. 196. Nature Of Magnetism).

This rule may be stated as follows:

To convert Fahrenheit to Centigrade subtract 32° from the Fahrenheit degree and divide by 1.8, or take 5/9 of it.

5 (212 - 32) / 9 = 5 (180) / 9 = 900 /9 = 100˚ C or 212 - 32 = 180

180 ÷ 1.8 = 100° C.

Example 2. - Convert 100° C. to Fahrenheit reading.

9 (100) / 5 = +32 = 900 / 5 + 32 = 180 + 32 = 212° F.

or 100 X 1.8 = 180

180 + 32 = 212° F.