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ing body, and as a consequence the temperature is thus ascertained.
Thus, the whole range of temperatures met with in engineer
FIG. 27.-Principle of operation of the thermo-couple.
ing practice is covered by some form of accurate temperature indicating device. The Bureau of Standards at Washington is ready to calibrate for a small fee any thermometer sent to them.
At least one carefully calibrated thermometer should be kept for reference or comparison in the laboratory of any one interested in steam engineering testing.
Standardization and Testing of Thermometers.-The testing of thermometers is of utmost importance. All thermometers should be carefully calibrated for refined steam engineering tests. The Bureau of Standards has issued in its circular No. 8, an excellent guide for such work. All thermometers are calibrated when completely immersed in the substance whose temperature is being ascertained.
The Stem Correction.-In engineering practice temperatures of steam and water are usually ascertained by setting the thermometer into a well which is sunk into the pipe conveying the
steam or water. This well is filled with mercury or oil and the heat transferred to the thermometer by conduction. As a consequence, however, a portion of the thermometer protrudes in the atmosphere above the well and is consequently at a lower temperature. A so-called stem correction is hence necessary to ascertain the correct reading of the thermometer.
This correction is large if the number of FIG. 29.-Well for degrees emergent and the difference of temthermometer inserperature between the bath and the space above it are large. It may amount to more than 35°F. for measurements made with a mercurial thermometer at 750°F.
The stem correction may be computed from the following formula:
Stem correction = Kn(t1
K = factor for relative expansion of mercury in glass; 0.00015 to 0.00016 for Centigrade thermometers; 0.000083 to 0.000089 for Fahrenheit thermometers, at ordinary temperatures, depending upon the glass of which the stem is made.
n = number of degrees emergent from the bath.
temperature of the bath.
= mean temperature of the emergent stem.
Thus suppose that the observed temperature was 100°C. and the thermometer was immersed to the 20° mark on the scale, so that 80° of the mercury column projected out into the air and the
mean temperature of the emergent column was found to be 25°C., then
As the stem was at a lower temperature than the bulb, the thermometer read too low, so that this correction must be added to the observed reading to find the reading corresponding to total immersion.
THE ELEMENTARY LAWS OF THERMODYNAMICS
S pointed out in the discussion on temperatures, scientists in former times conceived that the phenomena accompanying the addition or subtraction of heat could only be explained by the existence of a fluid which they called "caloric."
But these scientists or calorists, as they were called, had to give a hitherto unknown property to their substance and maintained that "caloric" was a weightless fluid. This substance also had the property of filling the interstices of bodies and of passing between bodies over any intervening space. To illustrate, they said, "caloric" would fill the interstices of a body as Now, when we squeeze a sponge some of the water oozes out and wets our hands. The calorists assumed that the friction or rubbing of a body with the hand for instance, made the hand warm because friction was supposed to decrease the capacity of a body for holding "caloric," and as in the squeezing of the sponge, water oozes out, so caloric oozed out and made the hand feel warm.
FIG. 30.-The establishment of
water enters a sponge.
The Irrefutable Experiments of Davy.-Davy, however, exploded this theory in 1799, when by rubbing two pieces of ice together, he actually caused the ice to melt. This evidently would be impossible under the caloric theory above stated, according to which friction caused capacity for caloric to be decreased. Yet here was evidenced the reverse. From time immemorial, men have considered that the force of truth is almighty, and yet how slow the human race is to overthrow an
imperfect but well-established theory. For instance, so powerful was Sir Isaac Newton's grip on the scientific world that because he announced that no successful correction could ever be made for the uneven refraction of light rays in lenses, the whole world for fifty years thoroughly abandoned the idea of ever being able to use refractive telescopes, and consequently, during that period we find telescopic reflective mirrors used entirely.
FIG. 31.-The furnace gases and entering air obey rigid but simple thermodynamic laws. (Boiler fronts at Long Beach Plant of the Southern California Edison Company.)
Joule's Complete Demonstration of the Mechanical Equivalent of Heat. And so it was in the case of the theory of heat. Notwithstanding the all-powerful demonstration of Davy in 1799, it remained for Joule, nearly fifty years later to finally put forth the finishing data to forever overthrow the caloric theory and introduce the modern idea of heat. This eminent scientist constructed a machine in many respects similar to an ice-cream freezer, the essential difference being, however, that the machine was used to increase the heat in the liquid instead of cooling the same. Joule conceived the idea that heat was one form of energy. Should this be true, it should be mutually convertible. One of the easiest methods of measuring energy is the well known pile driver. Energy is definitely computed by weighing the hammer in pounds and multiplying this weight by the distance in feet through which the weight falls. The result is foot-pounds energy. By a clever contrivance constructed somewhat on this principle, Joule measured the amount of energy absorbed in his