25. For measuring ordinary temperatures, the mercurial thermometer described in Art. 3 is used. For measuring low temperatures, alcohol is used in the thermometer tube instead of mercury. Mercury becomes a solid at about 38° below zero, but alcohol remains fluid at -200°. As alcohol boils at 173°, these instruments must not be exposed to heat exceeding 150°.

For measuring high temperatures, the air thermometer may be used.

26. Recording Thermometers

Recording Thermometers. When it is desired to have a continuous record of the changes of temperature at any certain point, the object may be attained by means of instruments called thermographs, or recording thermometers.

In measuring atmospheric temperature, the thermometer should be exposed to unobstructed circulation, and should be protected from the direct rays of the sun and from radiation from all warm bodies in its vicinity. It must be kept strictly dry. If there is any moisture on the bulb, it will evaporate and cause the mercury to fall to a lower point than the true temperature of the air.