Figure 8-11.—Expansion of a bar.length (LO) at some temperature and is heated, itwill expand (Lf). The amount of expansion is a function of the original length and thetemperature increase. The amount a materialchanges in length with temperature is called thelinear coefficient of expansion.The linear coefficient of expansion for amaterial is a physical property of that materialand describes its behavior with respect totemperature.BIMETALLIC EXPANSIONTHERMOMETERIf two materials with different linear coef-ficients are bonded together, as the temperaturechanges their rate of expansion will be different.This will cause the entire assembly to bend in anarc as shown in figure 8-12.When the temperature is raised, an arc isformed around the material with the smallerexpansion coefficient. Since this assembly isformed by joining two dissimilar materials, it isknown as a bimetallic element.A modification of this bimetallic strip servesas the basis for one of the simplest and mostcommonly encountered temperature-measuringinstruments, the bimetallic thermometer.Figure 8-13 shows a bimetallic thermometer.In it, a bimetallic strip is wound in the form ofa long helix. One end of the helix is held rigid.As the temperature varies, the helix tries to windor unwind. This causes the free end to rotate. TheFigure 8-12.—Effect of unequal expansion of a bimetallicstrip.free end is connected to a pointer. The pointeractually indicates angular rotation of the helix;however, since the rotation is linear and a functionof temperature, the scale is marked in units oftemperature.DISTANT-READING THERMOMETERSDistant-reading dial thermometers are usedwhen the indicating portion of the instrumentmust be placed at a distance from where thetemperature is being measured. The distant-reading thermometer has a long capillary, someFigure 8-13.—Bimetallic thermometer.8-7
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