CHAPTER IV

RESISTANCE PYROMETERS

=General Principles.=—When a pure metal is heated, its resistance to electricity increases progressively with the temperature. Certain alloys, on the other hand, show a practically constant resistance at all temperatures, examples of such alloys being constantan, manganin, and platinoid. All the elementary metals, however, exhibit a tangible rise in resistance when the temperature is augmented; and Sir W. Siemens, in 1871, proposed to apply this principle to the measurement of high temperatures by determining the resistance, and deducing the corresponding temperature from a table prepared under known conditions.

The choice of a metal is in this case more greatly restricted than in the selection of materials for a thermal junction. A certain amount of external corrosion does not alter the E.M.F. of a junction; but an alteration in size produces a marked difference in the resistance of a wire, which varies directly as the length and inversely as the area of cross-section. To the necessity for the absence of any internal physical change affecting the resistance is therefore added the further condition of permanence of external dimensions. For temperatures above a red heat the only feasible metals to use are platinum or the more expensive metals of the platinum series—and hence platinum is universally employed for this purpose. The original Siemens pyrometer consisted of 1 metre of platinum wire, 1 millimetre in diameter, wrapped round a porcelain rod, and protected from furnace gases by an iron sheath An elaborate method of measuring the resistance, involving the electrolysis of acidulated water, was adopted for workshop use, but was too involved to become popular. Later, Siemens employed the differential galvanometer method, and finally the Wheatstone bridge, to measure the resistance. Both methods are still in use in connection with resistance pyrometers, and the principle of each will now be explained.

=Measurement of Resistance by the Differential Galvanometer.=—A differential galvanometer is one which possesses two windings, arranged so that a current passing through the one tends to turn the pointer in one direction, and through the other to cause a movement in the opposite direction. If the currents in each winding simultaneously be equal, the pointer remains at rest under the action of two equal and opposite forces. The experimental attainment of the condition of rest serves as a means of measuring resistance, the circuit being arranged as in fig. 30. Current from a battery B passes through a divided circuit, one branch containing the adjustable resistance R and one coil of the galvanometer G; and the other the unknown resistance P and the opposite coil. The resistance R is adjusted until on tapping the key K no deflection on the galvanometer is noted, when the current in each branch of the circuit will be the same. The resistances of each coil of the galvanometer being equal, it follows from Ohm’s law that P is equal to R when no deflection is obtained.

The accuracy of this method depends upon the sensitiveness of the galvanometer, and also upon the extent to which the two coils may be regarded as truly differential, as the measurement evidently assumes complete equality in resistance and effect on the moving part. With modern galvanometers of this pattern, it is possible to secure readings of sufficient accuracy for the purposes of pyrometry. The method, however, is less sensitive than the Wheatstone bridge, now to be described.

=Measurement of Resistance by the Wheatstone Bridge.=—The principle of this method is shown in fig. 31, where _a_ and _b_ are two fixed resistances of known value; _d_ is an adjustable resistance; _x_ the resistance to be measured; B a battery; and G a sensitive galvanometer. If, in this circuit, _d_ be adjusted until no deflection is shown on the galvanometer, then _a_/_b_ = _x_/_d_; or _x_ = (_a_ × _d_)/_b_. Hence, if _a_ = _b_, then _x_ will be equal to _d_.

It is not difficult to construct a portable apparatus, suitable for workshop use, by means of which the value of _x_ may be determined to 0·01 ohm; and in the laboratory, with a very delicate galvanometer, 0·001 ohm may readily be detected. The Wheatstone bridge method is the best for the accurate measurement of resistance; but in resistance pyrometers it is sometimes advisable to sacrifice extreme accuracy in order to gain advantages in other directions, as will be shown subsequently.

=Relation between the Resistance of Platinum and Temperature.=—As platinum is the only feasible metal to use in the construction of resistance pyrometers, it is essential that the effect of temperature on the resistance of this metal should be known. Difficulties were experienced, in the early days of resistance pyrometers, from the fact that different samples of platinum wire, of varying degrees of purity, gave widely differing results in this connection; and no certainty was attained until 1886, when Professor Callendar thoroughly investigated the subject, and evolved a formula from which the temperature of a given kind of platinum could be deduced with great accuracy from the resistance. In order to understand this formula and its application, it will be necessary to consider the underlying principles upon which it is founded.

If the resistance of a platinum wire be measured at a number of standard gas-scale temperatures, and the results depicted graphically by plotting resistances against corresponding temperatures, the curve obtained is part of a parabola, exhibiting a decrease in the rate at which the resistance increases at the higher temperatures. A second platinum wire, of different origin and purity, and of the same initial resistance as the foregoing, would furnish a curve which, although parabolic, would not overlap that obtained with the first wire. The advance made by Callendar was to deduce a formula from which the temperature of any kind of platinum wire could be deduced from its resistance, after three measurements at known gas-scale temperatures had been determined. The calibration of a resistance pyrometer was thereby reduced to three exact observations, instead of a large number distributed over the scale; and, moreover, the formula in question was found to give results of great accuracy over a wide range of temperature for any kind of platinum wire.

Before dealing with Callendar’s formula, the term “degrees on the platinum scale” will be explained. Such degrees are obtained by assuming that the increase of resistance of platinum is uniform at all temperatures; that is, that the temperature-resistance curve is a straight line, and not a parabola. For example, a piece of platinum wire of 2·6 ohms resistance at 0° C. will show an increase to 3·6 ohms at 100° C.—an addition of 1 ohm for 100°. We now assume that a further augmentation of 1 ohm, bringing the total to 4·6 ohms, will represent an increase of 100°, or a temperature of 200°. Similarly, a total resistance of 5·6 ohms would indicate 300°, and 12·6 ohms 1000°. The temperature scale obtained by this process of extrapolation is called the “platinum scale,” and differs considerably from the true or gas scale, the difference becoming greater as the temperature rises. This is indicated in fig. 32, in which A represents the true parabolic relation between resistance and temperature, and B the assumed straight-line relation. Reading from curve A, the temperature corresponding to 8 ohms resistance is 600° C.; but from B the same resistance is seen to represent only 545° C., which is the “temperature on the platinum scale” to which this resistance refers. An inspection of fig. 32 shows that at all temperatures, except between 0° and 100°, the platinum-scale readings for given resistances are less than those indicated on the gas scale.

Callendar’s formula is expressed in terms of the difference between the gas-scale and platinum-scale readings, and takes the form

_t_ - _p_ = δ{(_t_/100)^2 - (_t_/100)},

where _t_ = temperature on the gas scale, _p_ = temperature on the platinum scale. δ = a constant, depending upon the purity of the wire.

In order to determine the value of δ, it is necessary to measure the resistance of the wire at 0°, 100°, and a third temperature, which should be considerably above 100°. The readings at 0° and 100° are requisite to establish the platinum scale of temperatures; the third reading is required to calculate the value of δ, as _p_ and _t_ are equal at 0° and 100°, these points forming the basis of both scales. An example is appended to make this matter clear.

_Example._—A platinum wire has a resistance in ice of 2·6 ohms; in steam, 3·6 ohms; in boiling sulphur, 6·815 ohms. To find the value of δ, the boiling point of sulphur being 444·5 on the gas scale.

Since an increase of (3·6 - 2·6) = 1 ohm is produced by 100°, the increase observed in boiling sulphur, (6·815 - 2·6) = 4·215 ohms, will represent a temperature, on the platinum scale, of (4·215 × 100)/1 = 421·5° _p_.

Applying Callendar’s formula,

(444·5 - 421·5) = δ{(444·5/100)^2 - (444·5/100)}

the value of δ is found to be 1·5.

Callendar, in his experiments, employed the boiling point of sulphur for the third point, and determined this temperature on the gas scale with great accuracy, The necessity for extreme precision in applying this formula is made clear by noting the effects on the value of δ resulting from small differences in the figures chosen in the above example. If, for instance, the boiling point of sulphur on the gas scale were taken at 2° lower, or 442·5, the value of δ would work out to 1·37; and the error at 1200° C. thus caused would amount to 17°. The same discrepancy would be observed if the resistance in boiling sulphur were taken as 6·835 ohms, an error of 0·02 ohm; and a still greater error would result if the difference in resistance at 0° and 100° were measured as 0·99 ohm instead of 1 ohm. From an extensive experience of the difficulties attendant on correctly determining the value of δ, the author has found that no reliable result can be obtained unless measuring instruments of the highest precision are used, and elaborate precautions taken to ensure the exact correction for alterations in the boiling points of water and sulphur occasioned by changes in atmospheric pressure. Unless the necessary facilities are at hand, an operator would be well advised to standardize a resistance pyrometer by taking several fixed points and drawing a calibration curve, after the manner recommended for a thermo-electric pyrometer.

If a resistance pyrometer be calibrated so as to read in platinum-scale degrees, and the value of δ be known for the wire, the correct gas-scale temperatures may be calculated from Callendar’s formula. The table on next page gives the results of a number of calculations made in this manner.

=Changes in Resistance of Platinum when constantly Heated.=—The resistance of platinum undergoes a gradual change when the wire is kept continuously above a red heat; and if the temperature exceed 1000° C. the change becomes very marked after a time, leading to serious errors in temperature indications when used in a pyrometer. The alteration under notice is due, as shown by Sir William Crookes, to the fact that platinum is distinctly volatile above 1000° C., and hence the diameter of the wire diminishes. This variation constitutes a serious drawback to the use of resistance pyrometers for temperatures exceeding 1000° C.

COMPARISON OF GAS AND PLATINUM SCALES. δ = 1·5. ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬┬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬┬▬▬▬▬▬▬▬▬▬▬▬▬ Platinum Thermometer│Air Thermometer Reading│ Difference Reading (Pt.). │ _t_ (deg. C.). │(_t_ ▬ Pt.). ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬┼▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬┼▬▬▬▬▬▬▬▬▬▬▬▬ -100 │ -97·1 │ +2·9 │ │ 0 │ 0 │ 0 50 │ 49·6 │ -0·04 100 │ 100 │ 0 200 │ 203·1 │ 3·1 300 │ 309·8 │ 9·8 │ │ 400 │ 420·2 │ 20·2 500 │ 534·9 │ 34·9 600 │ 654·4 │ 54·4 700 │ 779·4 │ 79·4 800 │ 910·7 │ 110·7 │ │ 900 │ 1049·4 │ 149·4 1000 │ 1197·0 │ 197·0 1100 │ 1355·0 │ 255·0 1200 │ 1526·7 │ 326·7 1300 │ 1716·0 │ 416·0 ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬┴▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬┴▬▬▬▬▬▬▬▬▬▬▬▬

=Terms used in Resistance Pyrometry.=—Following on the researches of Callendar and others, certain terms relating to resistance pyrometers have come into use, and will now be defined.

(1) _The Fundamental Interval_ is the increase in resistance between 0° C. and 100° C, or R_{100}-R_{0}. It should be remembered that the increases between 200° and 300°, or 800° and 900°, all temperatures being taken on the gas scale, differ from the fundamental increase.

(2) _The Fundamental Coefficient_ is that fraction of the resistance at 0° C. by which it increases per degree between 0° and 100°, on the average, or

(R_{100} - R_{0}) ───────────────── . (R_{0} × 100)

This figure is in reality the average temperature coefficient between 0° and 100°. For pure platinum the value is or 1/260 or 0·003846.

(3) _The Fundamental Zero_ is the temperature, on the platinum scale, at which the resistance would vanish; it is evidently the reciprocal of (2), prefaced by a minus sign, or

(R_{0} × 100) ─ ───────────────── . (R_{100} - R_{0})

For pure platinum this temperature would be -260_p_, since it is assumed that the average increase or decrease per degree holds throughout; that is, for every degree the metal is cooled the loss of resistance is taken to be 1/260 the resistance at 0°. Hence at -260_p_ the resistance, on this assumption, would vanish.

(4) _The Difference Formula_ is the expression which gives the relation between gas-scale and platinum-scale temperatures, or

_t_ - _p_ = δ{(_t_/100)^2 - (_t_/100)}

This formula has already been fully dealt with.

(5) _The Platinum Constant_ is δ in the above expression. The value for pure platinum is about 1·5, but small quantities of impurities may alter the figure considerably. The truth of the formula (4), however, is unaffected by changes in δ, as _p_ would be correspondingly altered.

=Practical Forms of Resistance Pyrometers.=—A typical form of resistance pyrometer, made by the Cambridge and Paul Instrument Company, is illustrated in fig. 33. The coil of platinum wire is wound round the edges of a mica framework, made of two strips of mica fastened at right angles so as to form a + in section. This method of winding is due to Callendar, who discovered that mica was chemically inert towards platinum, even at high temperatures. The leads, also of platinum wire, pass from the coil through mica washers to terminals fastened to the boxwood head. A second wire, not connected with the coil, but identical in length and diameter with the ordinary leads, is bent into two parallel branches, which are passed through the mica washers side by side with the leads, and are brought to a second pair of terminals in the head. The function of this wire is to compensate for changes in the resistance of the leads when heated, by opposing the compensating wire to the pyrometer in the measuring arrangement, when the resistance of the leads and wire, being equal, will cancel, the resistance actually measured being in consequence that of the coil only. Fig. 34 shows the connections for a Wheatstone bridge when this method of compensation is employed, _a_ and _b_ representing two equal fixed resistances, P the pyrometer coil, _x_ the leads, L the compensating wire, and _d_ the adjustable resistance. When no deflection is observed on the galvanometer,

_a_ _x_ + P ──── = ──────── , _b_ L + _d_

and since _a_ = _b_ and _x_ = L, it follows that P = _d_.

The protecting tube used by the Cambridge and Paul Instrument Company is made of porcelain, which is found to shield the platinum completely from the furnace gases, but is extremely fragile, and for workshop use should be protected by an outer iron sheath.

Resistance pyrometers made by other firms differ in detail from the foregoing. In the Siemens pyrometer the coil is wound on special fireclay, and protected by an iron sheath, the space between the coil and the sheath being filled with magnesia, which effectively prevents the corrosion of the platinum; and compensation is effected by means of a single wire passing down the centre and connected to one end of the coil, a special form of Wheatstone bridge being used to take the measurement. In the instruments made by R. W. Paul the coil is made of flat strip rolled out from wire, wound on mica, and protected by a silica tube and outer iron sheath. The Leeds-Northrup Company of Philadelphia employ a rod of obsidian on which to wind the coil, and also make a form in which the coil is wound so as to be self-sustaining, thus dispensing with the support. In all cases the coil is wound non-inductively, _i.e._ the wire is doubled before making into a spiral.

The zero resistance of a given instrument depends upon the accuracy of the measuring appliances used, and upon the degree of precision it is desired to attain. If, for example, it is intended to read to 1°C., with appliances capable of measuring to 1/100 of an ohm, a convenient zero resistance is 2·6 ohms; it being found that with pure platinum the resistance rises from 2·6 ohms at 0° to 3·6 ohms at 100° C., an increase of 1/100 of an ohm for 1° C. With coarser measuring arrangements, for the same degree of precision, a correspondingly higher zero resistance will be required; thus if 1/25 ohm be the least amount detectable by the measuring device, a zero resistance of 10·4 ohms would enable 1° C. to be observed. It is evident that a suitable zero resistance may be calculated similarly in all cases when the limit of the measuring appliance is known, and the minimum temperature interval specified.

For work above a red heat, the leads from the coil should always be made of platinum. Copper leads, when heated, give off vapour in sufficient quantity to attack the platinum; and the same applies to a greater degree to all kinds of solder. For low temperature work, however, copper leads may be used, thus reducing the cost of the instrument. Mica, above 1000° C., tends to crumble; and most forms melt at 1300° C. or lower; hence a mica-wound instrument should not be used continuously above 1000° C. The fireclay winding used by Siemens permits of occasional readings being taken up to 1400° C., and the same applies to wires wound on obsidian (melting point = 1550° C.), or those in which the coil is self-sustaining. As previously mentioned, however, alterations in the platinum itself render continuous readings above 1000° C. inaccurate after a short time.

It has been pointed out that with accurate measuring devices, a resistance corresponding to a change of 1° C. can be measured; and it might appear at first sight that the resistance method is considerably more accurate in practice than the thermo-electric. If a perfectly constant temperature were to be measured, a resistance pyrometer would undoubtedly give a closer indication; but constancy to 10° C. is seldom possible with gas-fired or coal furnaces or other hot spaces in which pyrometers are used. The accuracy of a pyrometer under workshop conditions therefore depends upon the rapidity with which it responds to temperature fluctuations, which condition will evidently be influenced by the thermal conductivity of the sheath. As it is necessary to protect a resistance pyrometer with a porcelain or silica sheath, both of which are poor conductors of heat, this instrument is in consequence not capable of following a rapidly changing temperature. The same applies to the magnesia packing used in the Siemens form; whereas a thermo-electric pyrometer is often sufficiently shielded by an iron tube, which transmits heat with a fair degree of freedom. The superior delicacy of the resistance method is therefore nullified by the sluggishness of its indications; and for reading changing temperatures the thermo-electric pyrometer is at least equally accurate. If, however, a constant temperature can be obtained, as in the determination of melting points, or when using experimental furnaces capable of exact regulation, the steady temperature reading may be secured with greater precision by using the resistance pyrometer.

=Indicators for Resistance Pyrometers.=—All existing indicators for resistance pyrometers are in reality outfits for measuring resistance, either by the Wheatstone bridge, differential galvanometer, or other method, the resistance being translated on the dial into corresponding temperatures. Typical examples will now be described.

=Siemens’ Indicator.=—This instrument is based upon the Wheatstone bridge principle, and is shown in fig. 35. The galvanometer is mounted in the centre of the dial, round the edge of which is fixed a ring on which the adjustable resistance is wound in spiral form. Suitable terminals are provided, duly labelled, to which the battery, pyrometer leads, and compensator are attached. A brass arm, movable about the centre of the dial, terminates in a tapping-key which moves over the adjustable resistance; the key being placed in the battery circuit. The fixed known resistances are located in the interior of the indicator. The adjustment consists in moving the key round the circumference until, on tapping, no deflection is obtained on the galvanometer. The pointed end of the movable arm then indicates the temperature of the pyrometer on the dial, which is marked in temperatures corresponding to the resistance opposed to the pyrometer for different positions of the key. In taking a reading, the operator is guided by the fact that when the temperature indicated is too high, the movement of the galvanometer needle will be in one direction; whereas if too low an opposite deflection will be given. The intermediate position of no deflection must then be found by trial; and the procedure should not occupy more than two minutes if the observer possess an approximate notion of the temperature to be measured.

=Whipple’s Indicator.=—This instrument (fig. 36) is employed by the Cambridge and Paul Instrument Company, and is also a form of Wheatstone bridge. The pyrometer leads and compensator are connected to properly labelled terminals T, and the battery to other terminals at the opposite side of the box. The pointer of the galvanometer is visible through the small window B, and a battery of two dry cells is placed at the side of the box. The fixed resistances are contained in the interior, and the adjustable resistance consists of a continuous wire wound on a drum, which may be rotated by the handle H. The shaft connecting H with the drum is screwed, and works in a nut, so that the turning of H produces a spiral movement of the drum. The adjustment consists in rotating H until, on tapping the key F, no deflection of the galvanometer pointer is observed. The temperature of the pyrometer is then read off directly from a paper scale wound round the drum and rotating with it, visible through the window A, the reading being indicated by a fixed pointer. This arrangement forms a compact and convenient indicator.

=The Harris Indicator.=—In the Siemens and Whipple indicators it is necessary, before a reading can be taken, to adjust a resistance until the galvanometer shows no deflection—an operation which takes up time and requires a fair amount of skill. This is obviated in the Harris indicator, made by R. W. Paul, and shown in fig. 37. This instrument is a special form of ohmmeter, which automatically indicates the resistance of the pyrometer by the movement of the pointer; the scale, however, being divided so as to read corresponding temperatures. In this indicator the scale may be made to notify an excess temperature—say 100°—above a given fixed number, and hence is capable of yielding an exact reading over the working range for which it is used. It may also be connected so that the whole scale represents the complete range—say 0° to 1000° C.—or other specified interval. The advantage possessed by this instrument is that the manipulation is much simpler than in the indicators previously described.

=The Leeds-Northrup Indicator.=—In this apparatus the Wheatstone bridge principle is employed, but the galvanometer is provided with a scale divided or temperatures. Coils are provided which correspond to an increase of resistance due to a rise of 100° C. on the part of the pyrometer, and by inserting these coils in the circuit the temperature is obtained to the nearest 100°. If the temperature were exactly at an even hundred—say 700°—the pointer of the galvanometer would be at zero on its scale; but if now the temperature rose, the system would no longer be balanced, and the galvanometer pointer would move over its scale by an amount depending upon the potential difference at its terminals. A very sensitive galvanometer would give a movement to the end of its scale with a slight alteration from the correct balance of the system; but by using a coarser instrument the pointer would remain within bounds; and the greater the increase of resistance, the larger would be the deflection. It is possible, in such a case, to divide the galvanometer scale to read temperatures corresponding to a given increase above that of the coils placed in the circuit. In one form of the Leeds-Northrup indicator, the whole scale is thus divided to read 100°, and the reading is obtained by adding the figure shown on the galvanometer to the hundreds represented by the coils inserted. In another form the galvanometer has a central zero, and its scale is divided both right and left, one side giving the number of degrees above, and the other below, the nearest hundred. The observations are thus much simpler than in the case where adjustment to the condition of no deflection is requisite.

=Siemens’ Differential Indicator.=—This form of indicator is still in use, and consists of a differential galvanometer and box of resistance coils, connected as shown in fig. 30. By adjusting the coils until no deflection is produced, the resistance of the pyrometer is obtained, and the corresponding temperature read off from tables provided. This form of indicator is preferred by some users, but it is less sensitive than the more recent Wheatstone bridge indicator made by this firm (fig. 35), and equally difficult to manipulate.

=Recorders for Resistance Pyrometers.=—The value of records in high-temperature work has led to the invention of recording mechanisms for use with resistance pyrometers. The form in common use in Britain is that devised by Callendar, shown in fig. 38, and consists of a mechanism for restoring automatically the balance of the resistances in a Wheatstone bridge circuit, in such a manner as to indicate the existing resistance on a chart. To this end the moving coil of the galvanometer carries a boom, or contact-arm, which, on swinging to the right or left, completes one of two electric circuits. The closing of either circuit brings into action a clockwork mechanism, which causes a slider carrying a pen to move over the bridge wire until the balance is restored, and incidentally to produce a mark in ink on a paper wound on a drum, which rotates at a known speed. When the resistance of the pyrometer is balanced, the galvanometer boom will be in a central position, and the slider at rest; whereas a rise in temperature causing an increase in the resistance of the pyrometer, will result in the boom swinging over and completing the circuit, which introduces more resistance in opposition to the pyrometer. A fall in temperature will similarly result in the liberation of the second mechanism, owing to the boom swinging in the opposite direction, with the result that the slider moves so as to oppose a less resistance to the pyrometer. If the chart be divided horizontally into equal spaces, representing equal increments or decrements of resistance, they may be marked to represent degrees on the platinum scale, which may be translated into ordinary degrees by reference to a conversion table. In careful and skilled hands this recorder gives excellent results, and the value of the records obtained is clearly shown by an inspection of the example shown in fig. 39, which represents the fluctuations of an annealing furnace during a period of nine hours. It will be noted that during the period covered by workman A the furnace has received constant and careful attention; but workman B has evidently neglected his duty conspicuously at two separate times.

=The Leeds-Northrup Recorder.=—In the Callendar recorder the boom which completes the electric circuits is pressed against the contact-surface merely by the small force due to the axial twist of the galvanometer coil, which necessitates the use of delicate mechanism if certainty of action is to be secured. A surer contact is secured in the instrument made by the Leeds-Northrup Company of Philadelphia, by means of an intermittent action which will be understood from the annexed drawing (fig. 40). The boom from the galvanometer terminates in a platinum tip, P, which moves between two blocks, the upper of which consists of two pieces of silver, A and B, separated by a strip of ivory, I, whilst the lower block, C, is another piece of silver, which is moved periodically up and down by an electro-magnetic contrivance not shown in the drawing. When the galvanometer is at the position of balance, the tip of the boom is beneath the ivory piece I; and when C ascends the tip P is then squeezed on to the ivory, and no current will then pass from the battery through either of the circuits E or F. If, however, the point of the boom be beneath A, owing to an alteration in the temperature of the pyrometer, then on C rising the circuit through E will be completed; and, similarly, if beneath B the circuit through F will be established. The result in either case is to bring into action a mechanism which moves a slider, carrying a pen, over a resistance wire opposed to the pyrometer in such a manner as to restore the balance. Certainty of contact is thus secured, which enables all the parts to be strongly made. The actual recorder is shown in fig. 41, in which it will be seen that the slider carries an ordinary stylographic pen in contact with the chart. This recorder is worked on the differential galvanometer method; and the adjusting resistance, over which the slider moves, consists of a manganin wire wound on a tapered core, such that horizontal movements represent equal changes of temperature, and not of resistance, thus obviating the necessity of translating platinum-scale readings into ordinary degrees. Concordant and accurate results, coupled with robust construction, are claimed for this instrument by the makers. The other type of recorder made by this firm (fig. 26) may also be used in conjunction with a resistance pyrometer. In this case the movements described introduce or cut out resistance opposed to the pyrometer in a Wheatstone bridge circuit, until the balance is restored.

=Paul’s Recorder.=—This instrument, as used for thermo-electric pyrometers, has already been described. By replacing the galvanometer by a Harris indicator, and using a suitable chart, the same mechanism serves to record the indications of a resistance pyrometer.

=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.