CHAPTER VI

OPTICAL PYROMETERS

=General Principles.=—When a solid is heated to 450° C., it commences to send out luminous radiations and appears a dull-red colour in a darkened room. As the temperature rises, the luminous radiations become more intense; the colour changes to a lighter red, then to orange, yellow, white, and finally to a dazzling white. Attempts have been made to assign temperatures to specified colours, and Pouillet, in 1836, introduced a table which purported to give the relation between colour and temperature. The following table, published by Howe in 1900, differs considerably from that of Pouillet, who had no accurate means of measuring the temperatures he assigned to the colours:—

HOWE’S TABLE.

───────────────────────────────┬───────────────┬────────────── Description. │ Temp. Deg. C. │ Temp. Deg. F. ───────────────────────────────┼───────────────┼────────────── Lowest red visible in darkness │ 470 │ 878 ” ” ” daylight │ 475 │ 887 Dull red │ 550 to 625 │ 1022 to 1157 Full cherry │ 700 │ 1292 Light red │ 850 │ 1562 Full yellow │ 950 to 1000 │ 1742 to 1832 Light yellow │ 1050 │ 1922 White │ 1150 │ 2108 ───────────────────────────────┴───────────────┴──────────────

If it were possible for all observers to detect exactly the colours to which these temperatures refer, the table would be of great utility; but in practice any two persons might differ in judgment to the extent of 50° C. below a yellow; and when the white is reached, and becomes dazzling, accurate discrimination is impossible. At the same time, a trained workman, used to quenching steel at a fixed temperature, say 850° C., acquires a high degree of judgment with constant practice, and may not vary by more than 20° C. at temperatures below a light yellow. The personal equation, however, is too great for colour judgment by the unaided eye to be taken as an accurate guide to temperature. A fairly close approximation, however, may be obtained by matching the colours against prepared standards, as will be referred to later.

The determination of the intrinsic brightness of the heated substance by a photometric method naturally suggests itself as a possible means of ascertaining temperatures by optical means, and it will be found that all the optical pyrometers used for industrial purposes are based on this procedure. The law connecting the intensity of the whole of the light waves emitted with temperature, for a given solid, is approximately given by Rasch’s formula:—

┌ ┐_x_ I_{1} │ T_{1} │ ───── = │ ───── │ I_{2} │ T_{2} │ └ ┘

where I_{1} and I_{2} are the intensities corresponding to absolute temperatures T_{1} and T_{2}; and the exponent

25000 _x_ = ───── . T_{1}

Hence at 1250° abs. the brightness increases as the 20th power, and at 2500° abs. as the 10th power of the temperature. This rapid increase in brightness for a small rise in temperature enables small increments to be readily observed; but a difficulty arises in practice owing to vast differences in brightness displayed by different substances at the same temperature. For example, the light emitted by an incandescent gas-mantle, which consists of thorium oxide, is vastly greater than that given out by a metal, such as platinum, at the same temperature; and it is therefore evident that the luminosity of a substance depends not merely upon its temperature, but also upon its nature. It is possible, however, to obtain indications for any substance in terms of a black body; thus if a heated solid possessed the same intrinsic brightness as a black body at a temperature of T, the “apparent” or “black-body” temperature of the solid would also be called T. All that this would signify would be that the condition of the solid was such that the light radiated was equal in intensity to that emitted by a black body at temperature T; and to obtain the true temperature of the solid, T must be multiplied by a factor which expresses the ratio of its emissive power to that of a black body.

In all photometric methods a standard light is employed, which should not vary in brightness, and with which the light from the source is compared. In optical pyrometers no attempt is made to measure the illumination in terms of candle-power; all that is necessary is to bring the standard and the source to the same degree of brightness by suitable adjustments. Amongst the standards employed are carbon-filament electric lamps, amyl-acetate lamps, and for higher temperatures the centre of an acetylene gas-flame; each of which is capable of producing a fixed degree of brightness when used under specified conditions. A black body, at known temperatures, is compared with the standard used, thus furnishing a scale of “black-body” temperatures to which the indications of a given source may be referred, as explained in the previous paragraph. Above 1000° C., however, the light becomes too dazzling to enable a proper comparison of the standard and source to be made, and absorbing glasses must then be used to reduce the brightness. Any coloured glass, taken at random, might not reduce the standard and source equally; but if a monochromatic glass be used—that is, a glass which transmits light of one wave-length only—a well-defined relation is found to exist between the intensity of the transmitted light and the temperature of the source. As optical pyrometers are used for temperatures above 1000° C. in most cases, involving the use of such glass, it will be necessary briefly to consider the relations between the wave-lengths of light and the temperature of the radiating substance, which in all cases will be assumed to be a black body.

=Wien’s Law.=—When the temperature of a substance increases, the enhanced brightness which results is shared by all parts of its spectrum; and if the substance were viewed through a glass prism, it would be noticed that every portion was brighter than before. Taking a ray of wave-length λ, the relation between its intensity and the temperature of the (black-body) source is given by Wien’s formula:—

J = _c_{1}_λ^{-5} × _e_^{-_c_{2}_/λT} (1)

where J = energy corresponding to wave-length λ; _e_ = the base of the natural system of logarithms; T = absolute (thermodynamic) temperature of the black-body source, and _c_{1}_ and _c_{2}_ are constants, the values of which may be found by measuring J at two known temperatures for light of a known wave-length. Experiment has shown that this formula is correct for wave-lengths which lie in the visible spectrum, but does not hold for longer waves; and modifications of Wien’s equation have been given by Planck and others which are of more extended application. For the purposes of optical pyrometry, however, using red light of wave-length about 65 millionths of a centimetre, Wien’s law may be applied with great accuracy; and a calibration based upon this law agrees closely with the values obtained by other pyrometric methods.

Wien’s formula may be written in the form—

log_{10}J = K_{1} + K_{2}(1/T) (2)

where K_{1} = log _c_{1}_-5 log λ and K_{2} = _c_{2}_(log _e_/λ). This simplified expression shows a linear relation between log J and 1/T; and hence if the temperatures corresponding to two intensities be known, the results may be plotted on squared paper in the form of a straight line connecting T and J, from which line intermediate or extraneous readings of temperatures may be obtained for any given intensity. Another useful form of Wien’s equation, referring to the ratio of two intensities J_{1} and J_{2}, is as under:—

┌ ┐ J_{1} _c_{2} log _e_ │ 1 1 │ log ───── = ─────────────── │───── - ───── │ (3) J_{2} λ │ T_{2} T_{1}│ └ ┘

where T_{2} and T_{1} are the absolute temperatures corresponding to J_{2} and J_{1} The value of _c_{2}_ is 1450000, when λ is expressed in millionths of a centimetre. Evidently, if the ratio J_{1}/J_{2} and the value of _c_{2}_, λ, and T_{2} be known, T_{1} may be calculated. When λ is not known, as in the case of a piece of red glass for which its value has not been determined, two readings at known temperatures will establish the value of (_c_{2}_ log _e_)/λ, and all other results may then be calculated. Examples illustrating the application of the formula will now be given.

_Example I._—A black body at an absolute temperature T_{1} is found to give twice the intensity observed at 1200° abs., the comparison being made with red glass transmitting wave-length 65 × 10^{-6} cms. To find the value of T_{1}.

Applying values to formula (3) ┌ ┐ 1450000 │ 1 1 │ log 2 = ─────── log 2·7183 × │ ──── - ──── │ 65 │ 1200 T_{1 │ └ ┘ and ┌ ┐ 1450000 × 0·4343 │ 1 T_{1} │ 0·3010 = ──────────────── × │ ───── - ────── │ 65 │ 1200 1 │ └ ┘ from which T_{1} = 1237° abs.

_Example II._—The intensity of the radiations from a black body at 2000° abs. are found to be equal to those from a given standard, taken as unity. To find the intensity at 3000 abs., compared with the same standard. λ = 65 × 10^{-6} cms.

Applying in (3) as before, ┌ ┐ J_{1} 1450000 × 0·43435 │ 1 1 │ log ──── = ───────────────── × │ ───── - ──── │ 1 65 │ 2000 3000 │ └ ┘

from which log J_{1} = 1·615, and J_{1} = 14·5.

In applying Wien’s law to the calibration of an instrument in which the intensity of a source may be measured photometrically against that of a standard, an electric furnace (fig. 29) may be used, with a piece of iron in the centre, coated with oxide, which gives black-body radiations. A thermo-electric pyrometer in contact with the oxide may be used to measure the standard temperatures, and brightnesses may then be compared with that of an amyl-acetate or other lamp giving a flame of constant luminosity. Temperatures corresponding to other intensities may then be deduced by calculation, as previously shown.

=Practical Forms of Optical Pyrometers.=—The instruments used in practice fall under the following heads:—

1. The standard light is constant, and the intensity of the light from the source varied in the instrument until equal to the standard. (Féry, Le Chatelier, Wanner, and Cambridge.)

2. The standard is varied until equal to that of the source, which may be reduced in intensity if this exceed that of the standard. (Holborn-Kurlbaum, made in commercial form by Siemens.)

3. The colour of the source is matched against a standard colour, made to agree with that obtained in a given operation (Lovibond); or the source may be made to produce a standard colour by a polarising device (Mesuré and Nouel); or the colour of the source is extinguished by suitable absorbents (various forms).

Examples of each type will now be described.

=Féry’s Optical Pyrometer.=—This instrument (shown in figs. 55 and 56) consists of a telescope furnished with a side-branch, in which a standard lamp E is placed. Light from E is focused upon a piece of transparent glass F, inclined at an angle of 45° to the axis of the telescope, from whence it is reflected into the eye-piece. To render the light received from the lamp monochromatic, a piece of red glass is interposed between E and the mirror. The telescope is sighted on the hot substance, rays from which pass through a piece of red glass D, and thence through two wedges of darkened glass, which diminish the intensity to a greater or less degree according to the thickness of absorbent glass interposed, which is reduced by sliding the wedges apart, and increased by the contrary movement. After passing through the wedges, the light proceeds through the inclined mirror to the eye-piece; consequently, the appearance presented to the eye is that of a field illuminated one-half by the standard lamp, and the other by the hot source. The adjustment consists in sliding the wedges, by a screw movement, until both portions of the field are equally illuminated. A temperature scale is provided on the moving piece which actuates the wedges, and is derived by Wien’s equation from the thickness of the wedges interposed when equality is obtained. Calibration is effected by noting the thickness of the wedges corresponding to two known temperatures, from which a straight line connecting thickness with the reciprocal of the absolute temperatures may be drawn, and a table formed giving values of T in terms of the thickness of the wedges. The calibration may be extended indefinitely, the accuracy of the readings depending upon the truth of Wien’s law. Féry’s optical pyrometer is a convenient instrument for occasional readings of high temperatures, combining simplicity with portability.

=Le Chatelier’s Optical Pyrometer.=—This pyrometer was the original form of instrument in which the temperature of a luminous source was deduced by photometric comparison with a standard light; and Féry’s apparatus, described above, is merely a convenient modification of the original. Instead of the absorbent glass wedges, Le Chatelier employed an iris diaphragm to reduce the quantity of light entering the telescope; the adjustment being carried out by altering the size of the opening in the diaphragm until the brightness of the source agreed with that of the standard. The intensity of the light received in the telescope will vary as the square of the diameter of the opening; and calibration at two known temperatures with a given monochromatic glass enables a temperature scale corresponding to diameter of opening to be computed by Wien’s law. Le Chatelier’s pyrometer is a valuable implement for research work in the laboratory, but is not so convenient for workshop purposes as Féry’s modification.

=Wanner’s Pyrometer.=—The principle of this pyrometer is the comparison of the brightness of a red ray from the standard with that of the ray of some wave-length obtained from the source, both rays being produced spectroscopically and therefore being truly monochromatic. The brightness is compared by the aid of a polarising device, resulting in a somewhat complicated optical arrangement, which is shown in fig. 57. Light from a standard electric lamp passes through the slit S_{1}, and from the hot source through S_{2}. Both beams are rendered parallel by means of an achromatic lens O_{1}, which is placed at a distance equal to its focal length from the slits. The parallel beams are dispersed by the direct-vision spectroscope P; and then pass through the polarising prism R, which separates each beam into two beams, polarised in planes at right angles. A biprism, B, placed in contact with a second achromatic lens, O_{2}, is made of such an angle that two fields of red light, polarised in planes at right angles, one from the source and the other from the standard, are focused on the slit D. These fields are viewed through an analyser A, and are brought to equal brightness by rotating the analyser, to which a graduated scale is attached, the temperature being deduced from the angle through which the analyser is turned. The calibration is effected by Wien’s law (equation (3) page 172), the intensities of standard and source being related to the angle of rotation as indicated by the equation. J_{2}/J_{1} = tan^2 Θ where J_{2} and J_{1} represent the intensities of source and standard respectively, and Θ = angle of rotation. Introducing this value into Wien’s equation (page 172), the relation between Θ and T may be shown to take the form log tan Θ = _a_ + _b_/T, where _a_ and _b_ are constants. Hence, if log tan Θ be plotted against 1/T a straight line is obtained, and hence by a few observations at known temperatures a calibration curve may be drawn from which intermediate and extraneous readings may be obtained. Messrs Hadfield have introduced a special chart, divided so that actual readings in degrees C. may be taken directly by observing the angle Θ. As sent out for use, the temperature scale is prepared beforehand, so that direct readings may be taken.

As the standard electric lamp will vary in brightness with repeated use, means must be provided to restore it to its proper value. This can be done by placing a rheostat in the circuit of the lamp, and adjusting the current until the brightness, as viewed through the pyrometer, exactly agrees with that of a ground-glass surface illuminated by a standard amyl-acetate lamp. The flame of this lamp really constitutes the standard; but as it would be blown about by air-currents when used in a workshop, the electric lamp, lighted by a portable battery, is brought to equality and used for general measurements.

=Cambridge Optical Pyrometer.=—During the recent war the manufacture of pyrometers of this type was taken up by the Cambridge and Paul Instrument Company. The external form of the Cambridge optical pyrometer is shown in fig. 58, in which an observer is shown using the instrument, the accessories consisting of a 4-volt accumulator, an ammeter, and an adjustable resistance for regulating the current through the electric lamp used for comparison; and a standard amyl-acetate lamp for adjusting the electric lamp to the correct brightness. The scale is marked on a circular disc, and direct readings are obtained from the position of a pointer which rotates with the analyser. By interposing a monochromatic glass to dim the source, the range of the pyrometer can be modified; and instruments are provided in four ranges: 700°-1400° C.; 900°-2000° C.; 1200°-2500° C., and 1400°-4000° C.

The Cambridge optical pyrometer has proved a useful instrument in skilled hands, and has been found of great service in the steel, glass, and pottery industries. Trained observers have found it possible to detect a difference of 10° C. at the region of 1900° C. The adjustment of the two fields to equality, however, involves a judgment which varies with different observers, and in practice it is advisable for one individual to be entrusted to take all readings.

=Holborn-Kurlbaum Pyrometer.=—In the optical pyrometers previously described a constant standard is used, and the brightness of the light from the source varied until equality is obtained. The idea of varying the brightness of the filament of an electric lamp until its colour matched that of the source, and deducing the temperature from the current taken by the lamp, was due to Morse, who used a filament in the form of a flat spiral, heated by a battery of E.M.F. 40 volts. This spiral was placed in a metal tube and interposed between the eye and the heated object. The Holborn-Kurlbaum pyrometer, as made by Siemens, is a refinement of that of Morse, and capable of reading over a more extended range. In fig. 59, L is a small electric lamp with a hairpin filament, as shown at A. This lamp is placed in a telescope, so that the filament is in the focus of the eye-piece and is lighted by a 4-volt accumulator, in series with which is a rheostat, R, and a milliammeter, M. The heated source is focused by moving the object-glass of the telescope, and both lamp and source are viewed through red glass placed in front of the eye-piece, D. The rheostat, R, is then adjusted until the tip of the filament is indistinguishable from the background, which is illuminated by the source. If the lamp be too bright, the filament will appear as a bright line; if duller than the source, as a dark line; and when equal to the source it will merge into the background. When equality is obtained, the milliammeter is read, and the temperature deduced from the current taken by the lamp.

The relation between current and the temperature of the filament varies with each lamp, but is in all cases represented by a formula of the type

C = _a_ + _bt_ + _ct^2_

where C = current, _t_ = temperature in degrees C., and _a_, _b_, and _c_ are constants depending upon the lamp used, and which can be determined by making a number of observations at known temperatures. The instrument is calibrated in this manner by the makers, and a scale affixed from which temperatures may be read corresponding to observed currents.

When the temperature of the source exceeds that of the standard at maximum current, an absorbing device, E, consisting of two prisms of darkened glass, with their reflecting faces parallel, is placed over the end of the telescope, so as to reduce the intensity of the source below that of the lamp. A separate calibration is performed with the absorber in position, and a second temperature scale provided, from which readings are taken when the absorbing device is used. Fig. 60 represents the instrument as made by Messrs Siemens, for use in a fixed position, the telescope, milliammeter, and rheostat being mounted on an upright supported by a tripod, and the current obtained from a portable accumulator. A second form (fig. 61) is designed for use in cases when observations at a number of different places are required, the rheostat being mounted on the telescope, and the milliammeter contained in a leather case provided with shoulder-straps.

The adjustment in this pyrometer is simple, and the condition of equality sharply defined. Whereas, in matching the colours of two contiguous fields, separate observers may disagree to an extent representing 40° C. or more, a divergence of 10° C. is seldom exceeded when different operators adjust the tip of the filament to extinction. In a special test to decide this point, the author compared the observations of five persons, some trained and others untrained, with the result that all agreed to within 10° at a steady temperature in the vicinity of 1200° C.; and in this respect the Holborn-Kurlbaum pyrometer is superior to other forms of optical pyrometer. The continuous accuracy of the readings depends upon the permanence of the standard lamp, which is ensured by over-burning for 20 hours, after which the lamp may be used at its proper voltage for a long period without further change. As used for occasional readings in the workshop, such a lamp will last for a year or more without varying in brightness by an amount representing 10° C. at a temperature of 1800° C. When a new lamp is used, a fresh calibration is necessary; the makers, however, in such case send out a new temperature scale with the lamp.

=Lovibond’s Pyrometer.=—It is possible, by the use of coloured glasses superposed, to match closely any given colour; and Lovibond, whose tintometer for this purpose is well known, has applied this method to temperature measurement. Taking the case of a block of steel in a furnace, it is possible to arrange combinations of glasses which, when illuminated by a standard light, will give the same tint as the steel at any specified temperature. If it be desired to work the steel at 850° C., for example, glasses are provided which, when viewed by the light transmitted from a 4-volt glow-lamp, using a constant current, represent the tint of steel at 840°, 850°, and 860° respectively. The image of the steel is reflected by a mirror through one hole in a brass plate, which forms the end of a wooden box, at the opposite end of which an eye-piece is placed. A second hole in the brass plate receives light from the standard lamp, after passing through the glasses; and the appearances of the two lights are then compared. A skilled eye can readily detect a disagreement in the two fields corresponding to 10° C.; and by introducing the glasses in turn it can be observed whether the steel is within 10° C. of the temperature required. This instrument is cheap and simple, but is obviously only useful in deciding a pre-arranged temperature, as to take a measurement at an undefined temperature would involve an unwieldy number of glasses, and absorb a considerable time. The correct glasses to use for a given operation are decided under working conditions at temperatures measured by a standard pyrometer; after which any number of instruments may be made from glasses of the same colour and absorptive power as those used in the calibration. Correct matching is difficult below 700° C.

=Mesuré and Nouel’s Pyrometer.=—This instrument, shown in fig. 62, consists of two Nicol prisms, between which is placed a piece of quartz cut perpendicularly to its axis. Light from the source, in passing through the first Nicol prism, is all polarised in the same plane; but on passing through the quartz is polarised in various planes, according to the wave-length. The colour seen after passing through the second prism, used as analyser, will depend upon the angle between this and the first or polarising prism. The analyser is connected to a rotating disc, divided into angular degrees; and on viewing the heated source the colour will appear red if the analyser be turned in one direction, and green if rotated in the opposite. The intermediate colour is a lemon-yellow; and the adjustment consists in rotating the analyser until this tint is obtained. The angular reading is then taken, and the temperature read off from a table prepared by making observations at known temperatures. Observers may disagree by as much as 100° C. in using this pyrometer, owing to differences in eyesight and judgment of the lemon-yellow tint; but a given operator, who has trained himself to the use of the instrument, may obtain much closer results with practice. The chief use of this device is to enable a judgment to be formed as to whether a furnace is above or below an assigned temperature, within limits of 25° C. on either side at the best; and hence it is convenient for a foreman or metallurgist to carry about for this purpose when other pyrometers are not in use. A great advantage is that the instrument is always ready for use, and has no accessories.

=Colour-extinction Pyrometers.=—Various attempts have been made to produce superposed glasses, or cells of coloured fluids, which will have the effect of extinguishing the colour of a heated source. As an example, three cells containing various dyes in solution may be prepared which, when looked through, will extinguish the colour at 840°, 850°, and 860° C. respectively. If it be desired to work at 850°, a difference of 10° on either side may be detected by a trained eye; but to follow a changing temperature a large number of cells would evidently be necessary. Heathcote’s extinction pyrometer, in its early form, consisted of an eye-shade in front of which two pairs of cells containing coloured fluid were mounted. In bringing a furnace to an assigned temperature, observation was made from time to time until a faint red image was perceived through one pair of cells, when the heat supply was regulated so as to maintain the existing temperature. When viewed through the second pair of cells, which contained a slightly darker fluid, no red image was to be seen at the correct temperature. With training, a workman could control a furnace to a fair degree of accuracy by this means, but the operation was tedious, and useful only for the attainment of a single temperature. In a later instrument, known as the “Pyromike” (fig. 63), Heathcote employs a single cell with flexible walls, so that by turning the screw-end, the length of the column of fluid interposed between the eye and the furnace can be altered. In taking a reading, the furnace is sighted and the screw turned so as to increase the length of the column of coloured fluid, until the image is no longer visible. A direct reading of the temperature is then obtained on a spiral scale marked on the cylindrical body of the instrument, over which the screwed portion rotates. This forms a simple and convenient temperature gauge for workshop use.

The “Wedge” Pyrometer, designed by Alder and Cochrane (fig. 64), consists of a small telescope through which a prism of darkened glass may be moved, and which is focused on the heated object. By turning a head the wedge may be moved so as to interpose a thicker layer of dark glass between the eye and the furnace, and the same operation causes a temperature scale to pass in front of a fixed pointer. When the image of the hot source is just extinguished, the temperature is read from the mark opposite the fixed point. Training is needed to enable an observer to judge the exact point of extinction, when it becomes possible to obtain results of 20° C. in the region of 1300° C. On the other hand, when used by one unaccustomed to the instrument, the reading may be wrong by 50° C. or more. As an aid to the judgment near the extinction point, the hand may be interposed between the telescope and furnace, when, if extinction be complete, no alteration should be observed in the field of view. The simple construction of this pyrometer is an advantage, no accessories being needed; and when used with the precautions stated above, readings sufficiently close for many processes can easily be obtained.

=Management of Optical Pyrometers.=—Careful usage is essential with optical pyrometers, which are liable to get out of adjustment with rough handling; and for this reason a trained observer should be in charge of such instruments. Skilled attention is equally requisite in taking readings, as the matching of tints correctly is an operation which demands a high degree of judgment. Careful attention must be paid to the standard lights; if flames, regulation to the standard height is essential; if electric lamps, care must be taken not to use them for a longer period than necessary, in order to increase the useful life. Accumulators should be recharged regularly—say once in two weeks—to keep in good order. Separate parts, such as absorption glasses, should be kept in a place of safety, as their destruction may involve a new calibration. It should be kept in mind that the temperatures indicated by optical pyrometers are “black” temperatures; that is, they correspond to the readings that would be given by a black-body of the same degree of brightness. In consequence, readings should always be taken under black-body conditions, the precautions in this respect being identical with those necessary for total-radiation pyrometers, given on page 163. In some special cases the connection between the apparent and true temperatures has been worked out for a given type of pyrometer, but, owing to the different emissive powers of different substances, no general relation can be given.

=Special Uses of Optical Pyrometers.=—The advantageous use of optical pyrometers is restricted to observations at temperatures beyond the scope of instruments which have the working part in the furnace; or to cases in which occasional readings of temperature suffice. To follow a changing temperature continuous adjustment is necessary, involving labour, and therefore costly. Amongst workshop uses may be mentioned: (1) ascertaining the temperature of pottery kilns and glass and steel furnaces; (2) in the treatment of steels at very high temperatures, to which end the pyrometer may be set to a given reading, and the process carried out when the steel is observed to attain such assigned temperature; (3) to take casual readings when a number of furnaces are in use, or when a number of sighting-holes are provided, as in large brickmaking furnaces; and (4) for occasional observations of the firing end of rotary cement kilns. As an instrument of research in the laboratory, a good form of optical pyrometer is very useful, as, for example, in investigating the working temperatures of electric lamps, and taking observations in electric furnaces. It is a great drawback that records cannot be taken by optical pyrometers, as much valuable information can be gathered from an accurate knowledge of temperature fluctuations in most operations. This disadvantage must always militate against the general use of these instruments.