Installations of Resistance Pyrometers.—The resistance method cannot be so readily applied to the purpose of a centrally controlled installation as the thermo-electric, owing to the difficulty of producing a set of pyrometers exactly equal in resistance. The introduction of the ohmmeter method of measuring resistances, as in the Harris indicator ([page 122]), has, however, rendered this project feasible, as it is possible in this arrangement to bring a set of pyrometers to a common resistance by adding the requisite amount in the form of a wire of negligible temperature coefficient. Several instruments, brought thus to a zero resistance of 3 ohms, for example, may then be wired up to a Harris recorder, and will give closely identical results. For various reasons, however, a thermo-electric installation is preferable.
Management of Resistance Pyrometers.—It is not advisable to use resistance pyrometers continuously above 900° C. (1650° F.), although an occasional reading may be taken up to 1200° C. (2190° F.). Great care must be taken that metallic vapours or furnace gases do not find access to the interior, and for this reason a cracked or defective sheath should immediately be replaced. As the resistance gradually changes, even when 900° C. is not exceeded, a reading should be checked at a fixed point in the neighbourhood of the working temperature, and allowance made for the observed error. Another method of correction recommended by some makers is to measure the resistance in ice, and to note how much this differs from the zero resistance noted when the indicator was marked, and to correct by simple proportion. Thus, if the observed resistance in ice were 10·2 ohms, the original having been 10·0 the reading on the indicator would be multiplied by 10·0/10·2 = 0·98, a correction which assumes a linear relation between resistance and temperature, and is therefore only approximate. Generally speaking, any serious defect entails the sending of the instrument to the maker, as a special degree of skill is required to execute the necessary repairs.
As the indicators are usually not automatic in action, care should be taken in the manipulation not to damage any part, particularly the galvanometer; and it is advisable not to trust the instruments to unskilled observers. The remarks applying to recorders and protecting sheaths in relation to thermo-electric pyrometers ([page 92]) apply equally in this case.
Special Uses of Resistance Pyrometers.—In all cases in which an exact reading is required, and a steady temperature can be secured, the resistance pyrometer can be used to advantage. Thus for accurate determinations of melting points and boiling points, or for exact readings of temperatures in experimental furnaces, a resistance pyrometer is superior to appliances of other kinds. On the other hand, it is not capable of responding to changes with the same rapidity as a thermal junction, and is therefore inferior for such purposes as the determination of recalescence points, or the temperature of exhaust gases from an internal combustion engine. The resistance method may be applied to atmospheric and very low temperatures (liquefied gases, etc.), to measure steady conditions with accuracy, nickel wire being sometimes used instead of platinum below 400° C. Many cold stores are fitted with resistance thermometers, the temperature being read directly on the galvanometer, which is placed across a Wheatstone bridge, and shows a deflection which depends upon the amount by which the bridge is thrown out of balance. Changes in the temperature of the resistance element may thus be read accurately. Whether the resistance method is suitable to a given purpose must be decided by the three factors: (1) temperature to be measured, which must not exceed 1000° C. continuously; (2) degree of accuracy required (a thermo-electric pyrometer giving results to 10° C.); (3) stability of the temperature measured, rapid changes not being readily shown by resistance pyrometers.
One advantage of resistance pyrometers is that the readings are independent of the resistance of the wires used to connect the pyrometer with the indicator, as such wires are duplicated and opposed to each other in the measuring device, their resistance being thereby cancelled. Hence the same reading is obtained at any distance, and, in addition, the head of the pyrometer may vary in temperature to any extent without altering the reading. These are points of superiority over the thermo-electric method; but, on the other hand, resistance pyrometers and indicators are more costly, more fragile, more difficult to repair, require more skilled attention, and are more liable to get out of order when used for industrial purposes. These drawbacks have resulted in restricting the use of resistance pyrometers to special purposes, the general run of observations being conducted by means of thermo-electric pyrometers.
CHAPTER V
RADIATION PYROMETERS
General Principles.—It is a common experience that the heat radiated by a substance increases as its temperature rises; and it would obviously be an advantage if the temperature of a hot body could be deduced from the intensity of its radiations, as the measurement could then be made from a distance, without the necessity of placing a pyrometer in contact with the heated substance. At temperatures above 1000° C., when difficulties are experienced either with the metals or protecting sheaths of thermo-electric or resistance pyrometers, the advantage gained would become more conspicuous as the temperature increased. A brief survey of our knowledge of the relations between radiant energy and temperature will indicate how this desired end may be achieved.
Any substance at a temperature above absolute zero (-273° C.) radiates energy to its surroundings by means of ether waves. Below 400° C. these waves produce no impression on the retina of the eye, and the radiating body is therefore invisible in a dark room. Above 400° C., however, a proportion of visible waves are emitted; and as the temperature rises the effect on the retina is enhanced, and the body increases in brightness. The difference between the non-luminous and luminous waves is merely one of wave-length, the shorter wave-lengths being visible to the eye; and both represent radiant energy. In addition to giving out radiant energy, a substance receives waves from its surroundings, which it absorbs in greater or less degree, and which when absorbed tend to raise the temperature of the receiving substance. A number of objects in a room, all at the same temperature, are therefore radiating energy to one another, and equality of temperature is established when each object receives from its surroundings an amount of energy equal to that which it radiates. A hot substance radiates more energy than a cold one; thus if a hot iron ball be hung in a room it will radiate more energy to its surroundings than it receives from them, and will therefore cool until the outgoing energy is balanced by the incoming, when its temperature will be equal to that of the other objects in the room.