PRINCIPLES OF NAVAL ENGINEERING 



be negligible for most purposes because only a 

 very small amount of mercury is contained in 

 the very narrow bore. 



The minimum indicating thermometer shown 

 in figure 7-5 is an alcohol-in-glass thermometer 

 with an unusually large bore. The upper part of 

 the bore is filled with air underpressure to help 

 prevent evaporation of the alcohol. The ther- 

 mometer is mounted with the bulb a few degrees 

 below the horizontalposition. A dumbbell-shaped 

 piece of black glass (called an index ) is the 

 device that makes possible a reading of the mini- 

 mum temperature that has occurred since the 

 thermometer was last set. As the temperature 

 increases, the alcohol readily flows upward past 

 the index without moving it. As the temperature 

 decreases, the retreating alcohol column flows 

 past the index until the top of the column touches 

 the upper end of the index. With a further de- 

 crease in temperature, the alcohol retreats still 

 more and surface tension causes the index tobe 

 carried along down with the column. If the tem- 

 perature increases again, the index is left un- 

 disturbed at its lowest point while the alcohol 

 column rises again. Thus the top of the index 

 always indicates the lowest temperature that has 

 occurred since the thermometer was last set. 



BIMETALLIC EXPANSION THERMOM- 

 ETER.— Bimetallic expansion thermometers 

 make use of the fact that different metals have 

 different coefficients of linear expansion. The 

 essential element in a bimetallic expansion 

 thermometer is a bimetallic strip consisting of 

 two layers of different metals fused together. 

 When such a strip is subjected to temperature 

 changes, one layer expands or contracts more 

 than the other, thus tending to change the cur- 

 vature of the strip. 



The basic principle of a bimetallic expansion 

 thermometer is illustrated in figure 7-6. When 

 one end of a straight bimetallic strip is fixed in 

 place, the other end tends to curve away from the 

 side that has the greater coefficient of linear 

 expansion when the strip is heated. 



The coefficient of linear expansion is defined as the 

 change In length per unit length per degree change in 

 temperature. As is apparent from this definition, the 

 numerical value of the coefficient of linear expansion 

 is independent of the units in which the length is ex- 

 pressed but is not independent of the temperature scale 

 chosen. 



COLD 



THIS METAL HAS GREATER 

 COEFFICIENT OF LINEAR 

 EXPANSION 



THIS METAL HAS- 

 SMALLER COEFFICIENT 

 OF LINEAR EXPANSION 



147.53 

 Figure 7-6.— Effect of unequal expansion of 

 bimetallic strip. 



For use in thermometers, the bimetallic 

 strip is normally wound into a flat spiral (fig. 

 7-7), a single helix, or a multiple helix. The 

 end of the strip that is not fixed in position is 

 fastened to the end of a pointer which moves 

 over a circular scale. Bimetallic thermometers 

 are easily adapted for use as recording ther- 

 mometers; a pen is attached to the pointer and 

 is positioned in such a way that it marks on a 

 revolving chart. 



Bimetallic thermometers used aboard ship 

 are normally used in thermometer wells. The 

 wells are interchangeable with those used for 

 mercury-in-glass thermometers. 



FILLED-SYSTEM THERMOMETERS.-In 

 general, filled-system thermometers are de- 

 signed for use in locations where the indicating 

 part of the instrument must be placed some 

 distance away from the point where the tem- 

 perature is to be measured.' For this reason 

 they are often called distant- reading thermom- 

 eters. 



A filled-system thermometer (fig. 7-8) con- 

 sists essentially of a hollow metal sensing bulb 



This is not true of all filled-system thermometers. 

 In a few designs the capillary tubing is extremely short 

 and in a few it is nonexistent. In general, however, 

 filled-system thermometers are designed to be dis- 

 tant-reading thermometers, and most of them do in 

 fact serve this purpose. Some distant-reading ther- 

 mometers may have capillaries as long as 125 feet. 



132 



