REDUCTION OF BAROMETRIC HEIGHT TO STANDARD TEMPERATURE CORRECTIONS FOR RELATIVE EXPANSION OF MERCURY AND BRASS SCALE

Sub

by 20.6 we get .056 in. as the barometer correction. tracting this from 29.84 in. we get the barometer reading for mercury at 58.4°F. as 29.84 in. - .056 in. 29.784 in.

=

Corrections for Altitude and Latitude.-Since the height of a mercury column gives true pressure readings so long as it represents a definite force or weight and since the weight or force of gravity varies at different altitudes and latitudes over the earth's surface, it is necessary to enter such a correction when the extreme refinements of the work in hand demand it. The standard value of gravity is taken at 32.173. The following formula, in which Img is the correct reading, g is the gravity coefficient, A the latitude, and h the altitude at the point of pressure measurement, may be applied for this correction.

Thus in a certain engineering investigation in Berkeley, California, where the latitude is 38° and the elevation 50 ft., the condenser mercury column corrected for temperature read 28.473 in. What should its properly corrected reading be when gravity is taken into consideration?

Such refinements as the one for brass scale correction and especially for latitude and altitude readjustments are not necessary in most steam engineering tests. It is well, however, to bear in mind such computation in case investigations of extreme detail should arise.

CHAPTER IV

MEASUREMENT OF TEMPERATURES

W

HEN the finger is inserted into a cup of warm water and then again into water formed by the melting of ice a distinct sensation is felt in each case. Many years ago scientists and philosophers attempted to explain this sensation by assuming that a substance existed which they called "caloric" whose entrance into our bodies caused the sensation of warmth and whose egress therefrom gave the sensation of cold. But heat, if a substance at all, cannot be similar to those substances with which we are familiar, since a hot body weighs no more than one which is cold.

The discussion in this article is not concerned directly with FIG. 21.-A thermocouple for high heat but rather with one of its

temperature measurement.

effects, namely, that of change in temperature. From the above it is readily seen that temperature is an indicator of the physical effect of heat rather than a quantitative means of heat measurement. This statement is easily proved, for when we place our fingers alternately upon a piece of cold and hot iron at the temperatures mentioned for water in the opening paragraph of this discussion, the same physical sensation is experienced. Yet to transform the iron from a temperature of freezing water to that of boiling water takes far less heat than for the transfer of water under similar conditions.

Fixed Points for Thermometer Calibration.-Since water is the most generally distributed substance through out nature and one of the most convenient for handling in the laboratory its

freezing point and boiling point are used by common consent as two definite marks for temperature calibration. Thus in the Centigrade scale the freezing point of water is the zero point and the boiling point of water under standard conditions of atmospheric pressure is the one hundred unit point. Again, in the Fahrenheit scale the freezing point of water is the thirty-second division point and the boiling point of water the two hundred and twelfth division point. Similarly for the Reaumur scale, the freezing point of water is the zero division point and the boiling point of water, the eightieth division point.

The Various Temperature Scales Employed.-The Centigrade scale as described above has grown into rapid use in scientific investigation and now may be said to be universally adopted throughout the world for such practice. The Fahrenheit scale, on the other hand, has so ingrained itself into engineering practice that engineers are loath to part with it in spite of its cumbersome and unscientific divisions. In this work, then, we shall be compelled to express temperature measurement in the Fahrenheit scale. The Reaumer scale, mentioned above, finds slight application in this country and in such places where it is employed it is used for measurement in stills and breweries. All three of these scales are often for scientific purposes transformed to a socalled absolute zero which is 459.4°F. below the ordinary zero on the Fahrenheit scale. A free discussion of this absolute scale will be set forth in a discussion on thermodynamic laws of gases which will be found in another chapter.

Relationship of Fahrenheit and Centigrade Values. In order that transfers from one thermometer scale to another may be conveniently and rapidly accomplished, it now becomes necessary to develop some simple mathematical relationships whereby this may be done. Since all of the scales are graduated uniformly between the freezing and boiling point of water, their relationship may be said to be linear. In the study of analytical geometry we find that such relationships may be expressed by the straight line formula:

wherein x and y represent any simultaneous temperatures expressed in different scale readings and the subscripts 1 and 2 represent definitely known points in correspondence. In order then to find a relationship between the Fahrenheit and Centi