Part 8 (2/2)

When the sign of the correction is positive (+) it n is a negative (-) the correction must be subtracted The formula for the stem correction is as follows:

Stem correction = 0000085 n (T-t)

in which T is the observed teent coluent, and 0000085 is the difference between the coefficient of expansion of the lass in the sterees and the therrees of the mercury coluent colu another therent12 Suppose this rees, then

Sterees

As the stem is at a lower temperature than the bulb, the thermometer will evidently read too low, so that this correctioncorresponding to total irees If this thermometer is to be corrected in accordance with the calibrated corrections given above, we note that a further correction of 05at this temperature, so that the correct terees

[Illustration: Fig 12]

[Illustration: Fig 13]

Fig 12 sho a stem correction can be obtained for the case just described

Fig 13 affords an opportunity for co the scale of a thermometer correct for total ied to the 300 degrees rees Fahrenheit, a te when ther temperatures in stearees Fahrenheit a perfect gas expands 1/49164 part of its volume if its pressure reree Thus if gas at 32 degrees Fahrenheit occupies 100 cubic feet and its teree, its volume will be increased to 100 + 100/49164 = 100203 cubic feet For a rise of two degrees the volume would be 100 + (100 2) / 49164 = 100406 cubic feet If this rate of expansion per one degree held good at all temperatures, and experias, if its pressure remained the same, would double its volurees Fahrenheit, while under a diminution of temperature it would shrink and finally disappear at a terees below zero Fahrenheit While undoubtedly soe in the laould take place before the lower temperature could be reached, there is no reason why the law e of teood From this explanation it is evident that under a constant pressure the volurees between its terees Fahrenheit To simplify the 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 nitude with those on the Fahrenheit scale

Temperatures referred to this new scale are called absolute terees centigrade) is called the absolute zero To convert any terees to the terees Fahrenheit will be 54 + 460 = 514 degrees absolute terees Fahrenheit will likewise be equal to 113 + 460 = 573 degrees absolute terees Fahrenheit and another pound is at a terees Fahrenheit the respective voluiven pressure would be in the ratio of 514 to 573

[Illustration: 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]

British Thermal Unit--The quantitative measure of heat is the British thermal unit, ordinarily written B t u This is the quantity of heat required to raise the terees Fahrenheit; that is, frorees In the metric system this unit is the _calorie_ and is the heat necessary to raise the terees to 16 degrees centigrade These two definitions lead to a discrepancy of 003 of 1 per cent, which is insignificant for engineering purposes, and in the following the B t u is taken with this discrepancy ignored

The discrepancy is due to the fact that there is a slight difference in the specific heat of water at 15 degrees centigrade and 62 degrees Fahrenheit The two units may be compared thus:

1 Calorie = 3968 B t u 1 B t u = 0252 Calories

_Unit_ _Water_ _Teree Fahrenheit 1 Calorie 1 Kilograraree Fahrenheit

Hence 1 calorie = (22046 9/5) = 3968 B t u

The heat values in B t u are ordinarily given per pound, and the heat values in calories per kilogram, in which case the B t u per pound are approxiray has a certain definite relation to work, one British ther equivalent from his deterator, found that 778 foot pounds were a ations indicate that the correct value for a B t u is 77752 foot pounds or approxiy to work as determined is a demonstration of the first law of thery are mutually convertible in the ratio of 778 foot pounds for one British therebraically expressed, is W = JH; W being the work done in foot pounds, H being the heat in B t u, and J being Joules equivalent Thus 1000 B t u's would be capable of doing 1000 778 = 778000 foot pounds of work

Specific Heat--The specific heat of a substance is the quantity of heat expressed in thermal units required to raise or lower the teiven teree This quantity will vary for different substances For example, it requires about 16 B t u to raise the terees or 05 B t u to raise it one degree, while it requires approximately 180 B t u to raise the terees or one B t u for one degree

If then, a pound of water be considered as a standard, the ratio of the amount of heat required to raise a siree, to the aree is known as the specific heat of that substance Thus since one pound of water required one B t u to raise its teree, and one pound of ice requires about 05 degrees to raise its teree, the ratio is 05 which is the specific heat of ice To be exact, the specific heat of ice is 0504, hence 32 degrees 0504 = 16128 B t u would be required to raise the terees For solids, at ordinary temperatures, the specific heat may be considered a constant for each individual substance, although it is variable for high teases a distinction must be made between specific heat at constant volume, and at constant pressure

Where specific heat is stated alone, specific heat at ordinary temperature is ie value of this quantity between the temperatures named