Measurement of Lower Temperatures by the Thermo-electric Method.—Many cases arise in practice in which a thermal junction and a sensitive galvanometer are preferable to a mercury thermometer; and below -39° C., at which temperature mercury freezes, a thermal junction is frequently better to employ than an alcohol or pentane thermometer. A number of practical examples of the use of thermal junctions for ordinary and low temperatures will now be considered.
Measurement of Surface Temperatures.—A mercury thermometer, when laid on a hot surface, only touches along a line, and does not show the true surface temperature. The construction of a thermal junction suitable for this purpose is described on page 41, and for steam-pipe surfaces, hot plates, and the exterior of furnaces, a specially calibrated millivoltmeter, giving a full-scale deflection with 20 millivolts, may be used. In making the temperature scale, boiling water (100°C.), boiling aniline (184° C.), and melting tin (232° C.) are convenient standards. If the surface temperature be less than 100° C. a mirror galvanometer should be used, and the junction standardized in paraffin wax (freezing point usually about 50° C., but should previously be determined with an accurate thermometer), absolute alcohol at boiling point (79° C.) and boiling water. The author has found that this method yields excellent results in the case of steam-pipes, the exterior of rotary cement kilns, and hot surfaces generally.
Measurement of Low Temperatures.—Junctions of iron and constantan, Hoskin’s alloys, copper and German silver, or copper and constantan, are suited to these measurements. In a laboratory the cold junction may be kept in ice in a Dewar vessel, the mechanically protected form known as the “Thermos” flask being very useful for this purpose. With a good mirror galvanometer precise readings may be secured, 1/10 of a degree C. being easily detected. Calibration between -40° and +40°C. may be effected by comparison with a standard mercury thermometer, a water-bath being used above 0°, and alcohol surrounded by a freezing mixture of ice and calcium chloride crystals below zero. For very low temperatures (-200°C. or less) the junction maybe calibrated in solid carbon dioxide (-78°C.) and liquid air (-184°C.). Dewar has found that copper and German silver form a reliable junction for very low temperatures, and the author has successfully used a couple of Hoskin’s alloys for special work down to -200°C., a pivoted indicator being employed. No couples tested show a linear relation between E.M.F. and temperature at these low ranges.
Owing to the magnitude of the error caused by changes in the cold junction, the thermo-electric method is not suited to the measurement of atmospheric temperatures, or for explosive magazines or cold stores. In such cases instruments of the resistance type, to be described later, are used.
Temperature of Steam, Exhaust Gases, etc.—For measuring the temperature of ordinary or superheated steam, the exhaust gases from internal combustion engines, etc., iron-constantan junctions, with suitable indicators, are satisfactory. When placed in a pipe the junction should be as nearly as possible in the centre, so as to avoid the cooling effect of the walls. Several junctions, situated in different parts of the pipe, may be used with a single indicator and suitable switchboard. The above remarks also apply to the hot-blast for blast furnaces, and similar instances where the temperature does not exceed 900° C.
Measurement of Differences of Temperature.—Cases frequently arise in practice in which the difference in temperature between two points is required, and if this difference be subject to rapid changes, a mercury thermometer, from its large mass, would not respond with sufficient rapidity to indicate these changes. In such cases a circuit is made after the manner of [fig. 2], one junction being located at each point; thin wires of iron and constantan being used. For small differences—1° C. or less—a mirror galvanometer should be used. Calibration may be performed by placing one junction in hot water and the other in cold, the water temperatures being read with an accurate thermometer.
Advantages of the Thermo-electric Method of Measuring Temperatures.—Compared with other methods, the thermo-electric possesses the following points of superiority:—(1) Simplicity, no special experiment being necessary to obtain a reading; (2) cheapness of outfit; (3) adaptability to a variety of purposes; (4) ease of repair in case of damage; (5) robustness, not being liable to get out of order under workshop conditions; and (6) suitability to the purpose of a centrally controlled installation. The drawbacks are:—(1) Liability to error owing to fluctuations in the cold junction (which may be avoided with care); and (2) lack of sensitiveness at very high temperatures compared with the resistance method—a point seldom of great practical importance, as the limit of accuracy is usually within the amount by which an ordinary furnace fluctuates in temperature under working conditions.