Measurement of Temperature

For everyday purposes, the measurement of temp is a simple and routine procedure in which the expansion and contraction of the fluid in a thermometer is noted. The most accurate thermometers contain mercury, the only metal that exists as a liquid at normal earth temps. Less expensive models are available using dyed alcohol.

It is often useful to know the daily maximum and minimum temps. A maximum thermometer is very similar t a regular thermometer, but with 2 differences. Unlike a regular thermometer, it must contain mercury, it cannot use dyed alcohol. The other difference is that in the tube just beyond the bulb is a very narrow constriction that allows the mercury to expand outward when the temp increases but prevents it from contracting back into the bulb when the temp decreases. The temp shown on the max thermometer indicates the highest temp experienced since he last time it was reset. Resetting the thermometer is easy; the mercury can be forced down simply by shaking the thermometer downward.

A minimum thermometer is also similar to a regular thermometer, except it can only contain dyed alcohol and has within it a small index shaped like a weightlifter's dumbbell. If the index is at the end of the alcohol and the temperature is decreasing, surface tensions pulls the index toward the bulb. When the temp increases, the index remains at its present position as the alcohol expands away from the bulb. Min thermometers are mounted horizontally with a latch that can be released to allow the instrument to be inverted. The instrument is reset by turning t upside down, allowing the index to slowly slide down to the end of the alcohol.

Another instrument for the measurement of temp is the bimetallic strip which consists of two thin strips of different metals bonded together. Because all metals have different rates of expansion and contraction with temp, one undergoes a greater change in length than the other, causing the strip to bend. A pointer and scale are attached to the bimetallic strip, whose bending is amplified by a lever. When this mechanism is coupled with a rotating drum and a pen, the resulting thermograph gives a continuous record of temp. Thermostats for the heating and air conditioning units of many homes use bimetallic strips to determine the room temp.

Considerably more sophisticated instruments for measuring temp also exist. Among these are the resistance thermometers, instruments that send an electrical current trough a very thin filament made of conductor or semiconductor material exposed to the air. The temp of the filament is the same as that of the surrounding air and its temp influences its resistance to the electrical current. The instrument registers the amount of resistance and uses the reading to determine the air temp. A thermistor is a particular type of resistance thermometer that uses a ceramic semiconductor instead of a metallic wire for a filament.

Resistance thermometers are