Melting Points of Metals —The approximate temperature of a furnace or flue may be determined, if so desired, by introducing certain metals of which the melting points are known. The more common metals form a series in which the respective melting points differ by 100 to 200 degrees Fahrenheit, and by using these in order, the temperature can be fixed between the melting points of some two of them. This method lacks accuracy, but it suffices for determinations where approximate readings are satisfactory.
The approximate melting points of certain metals that may be used for determinations of this nature are given in [Table 8] .
Radiation Pyrometers —These are similar to thermo-electric pyrometers in that a thermo-couple is employed. The heat rays given out by the hot body fall on a concave mirror and are brought to a focus at a point at which is placed the junction of a thermo-couple. The temperature readings are obtained from an indicator similar to that used with thermo-electric pyrometers.
Optical Pyrometers —Of the optical pyrometers the Wanner is perhaps the most reliable. The principle on which this instrument is constructed is that of comparing the quantity of light emanating from the heated body with a constant source of light, in this case a two-volt osmium lamp. The lamp is placed at one end of an optical tube, while at the other an eyepiece is provided and a scale. A battery of cells furnishes the current for the lamp. On looking through the pyrometer, a circle [Pg 91] of red light appears, divided into distinct halves of different intensities. Adjustment may be made so that the two halves appear alike and a reading is then taken from the scale. The temperatures are obtained from a table of temperatures corresponding to scale readings. For standardizing the osmium lamp, an amylacetate lamp, is provided with a stand for holding the optical tube.
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Determination of Temperature from Character of Emitted Light —As a further means of determining approximately the temperature of a furnace, [Table 9] , compiled by Messrs. White & Taylor, may be of service. The color at a given temperature is approximately the same for all kinds of combustibles under similar conditions.
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FOOTNOTES
[ [2] ] When one temperature alone is given the “true” specific heat is given; otherwise the value is the “mean” specific heat for the range of temperature given.
[ [3] ] For variation, see [Table 13] .