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MISCELLANEOUS TABLES 599
Thermometer Scales
There are two thermometer scales in general use in this country at the present time, the Fahrenheit and the Centigrade. On the Fahrenheit scale the melting point of ice is 320 and the boiling point of water at sea-level is 2120. On the Centigrade scale 0° is the melting point of ice and ioo° the boiling point of water. Another scale, the Absolute, is sometimes used. This takes its zero at a point assumed to be the lowest temperature that can exist. This point was calculated from the contraction of gases when cooled and found to be — 2730 C, i. e., 2730 below zero Centigrade. The size of the degrees of the Centigrade and Absolute scales is the same, so to convert degrees Centigrade to Absolute all that is necessary is to add 273.
To convert degrees Centigrade to Fahrenheit multiply by 1.8 and add 32.
To convert degrees Fahrenheit to Centigrade, subtract 32 and divide the result by 1.8. Care should be taken that the sign of the result is correct when the temperature is below the freezing point of water.
(The constant 1.8 is obtained as follows: Between the freezing and boiling points of water there are ioo° C and 2120— 320 = 1800 F. Therefore, i° C = 1.8° F. The factor 32 arises from the fact that o° C corresponds to 320 F.)
Temperature Coefficients of Electrical Resistance
Metal
Temperature Coefficient (approximately) for i° C.
Aluminium (commercial) ....
Copper (annealed)
German silver
Gold (annealed) .... . . .
Iron (wrought)
Mercury
Platinum
0.00435 O.00400 0.00036 0.00365 0.00463 0.00072 0.00247 0.00377 0.00570 0.00365
Silver
Tungsten
Zinc (pressed) . . . .'
Note. The temperature coefficient of a material is its increase in resistance for each degree Centigrade rise in temperature, and it is expressed as a decimal fraction of the resistance at o° C.