Fig. 13
Stem correction = 0.000085 × 200 × (400 - 85) = 5.3 degrees.
As the stem is at a lower temperature than the bulb, the thermometer will evidently read too low, so that this correction must be added to the observed reading to find the reading corresponding to total immersion. The corrected reading will therefore be 405.3 degrees. If this thermometer is to be corrected in accordance with the calibrated corrections given above, we note that a further correction of 0.5 must be applied to the observed reading at this temperature, so that the correct temperature is 405.3 - 0.5 = 404.8 degrees or 405 degrees.
Fig. 12 shows how a stem correction can be obtained for the case just described.
Fig. 13 affords an opportunity for comparing the scale of a thermometer correct for total immersion with one which will read correctly when submerged to the 300 degrees mark, the stem being exposed at a mean temperature of 110 degrees Fahrenheit, a temperature often prevailing when thermometers are used for measuring temperatures in steam mains.
Absolute Zero —Experiments show that at 32 degrees Fahrenheit a perfect gas expands 1 ⁄ 491.64 part of its volume if its pressure remains constant and its temperature is increased one degree. Thus if gas at 32 degrees Fahrenheit occupies 100 cubic feet and its temperature is increased one degree, its volume will be increased to 100 + 100 ⁄ 491.64 = 100.203 cubic feet. For a rise of two degrees the volume would be 100 + (100 × 2) / 491.64 = 100.406 cubic feet. If this rate of expansion per one degree held good at all temperatures, and experiment shows that it does above the freezing point, the gas, if its pressure remained the same, would double its volume, if raised to a temperature of 32 + 491.64 = 523.64 degrees Fahrenheit, while under a diminution of temperature it would shrink and finally disappear at a temperature of 491.64 - 32 = 459.64 degrees below zero Fahrenheit. While undoubtedly some change in the law would take place before the lower temperature could be reached, there is no reason why the law may not be used within the range of temperature where it is known to hold good. From this explanation it is evident that under a constant pressure the volume of a gas will vary as the number of degrees between its temperature and the temperature of -459.64 degrees Fahrenheit. To simplify the [Pg 82]
[Pg 83] application of the law, a new thermometric scale is constructed as follows: the point corresponding to -460 degrees Fahrenheit, is taken as the zero point on the new scale, and the degrees are identical in magnitude with those on the Fahrenheit scale. Temperatures referred to this new scale are called absolute temperatures and the point -460 degrees Fahrenheit (= -273 degrees centigrade) is called the absolute zero. To convert any temperature Fahrenheit to absolute temperature, add 460 degrees to the temperature on the Fahrenheit scale: thus 54 degrees Fahrenheit will be 54 + 460 = 514 degrees absolute temperature; 113 degrees Fahrenheit will likewise be equal to 113 + 460 = 573 degrees absolute temperature. If one pound of gas is at a temperature of 54 degrees Fahrenheit and another pound is at a temperature of 114 degrees Fahrenheit the respective volumes at a given pressure would be in the ratio of 514 to 573.
Ninety-sixth Street Station of the New York Railways Co., New York City, Operating 20,000 Horse Power of Babcock & Wilcox Boilers. This Company and its Allied Companies Operate a Total of 100,000 Horse Power of Babcock & Wilcox Boilers