Chapter III: Thermo-Electric Pyrometers (2)

where E = electromotive force in microvolts, _t_ = temperature in Centigrade degrees, and A and B are constants depending upon the junction. With certain junctions this formula may be applied over the working part of the scale with an error not exceeding 2° C., but with others the discrepancy is greater. In order to determine the constants A and B, it is necessary to measure the E.M.F. at two known temperatures, which should be chosen as far apart as possible in the working region. When these constants are known, a measurement of E enables the temperature _t_ to be found by calculation.

_Example._—Le Chatelier found that a junction at the
temperature of melted aluminium (657° C.) gave 6200
microvolts; at the melting point of copper in air (1062° C.)
the figure was 10580. Applying in the above formula
log 6200 = A log 657 + B
and log 10580 = A log 1062 + B,
the value of A is 1·2196 and of B 0·302, as may be found
by taking logarithms and solving for A and B.

The values of the constants A and B vary for different junctions, and also for different melts of what are reputed to be the same materials. When once determined for a quantity of homogeneous wires, to which the formula applies with sufficient accuracy, it is evident that an indicator with a millivolt scale may be made to read temperatures directly without any necessity for further experiment, although it is always advisable to take one check reading at a fixed point in the working range.

In order to determine the E.M.F. of a junction at different temperatures, the potentiometer method is used, in which the E.M.F. of the test-couple is balanced against the known E.M.F. furnished by a constant cell. The circuit is shown in fig. 16, in which B is an accumulator which sends a current through the resistances R_{1}, R_{2}, and the calibrated wire DE. The cold ends of the couple are attached at P so as to be in opposition to B, and in this branch of the circuit are included a sensitive galvanometer G and a portion of the wire DE. A standard cadmium cell, S, is connected between R_{1} and R_{2} at one end, and may be put in circuit with the galvanometer through the switch A. In commencing, S is connected to the galvanometer and R_{1} adjusted until no deflection is obtained on G. The switch A is now moved over to the circuit of the couple, and the terminal F moved along the wire until zero deflection is again obtained. The E.M.F. of the couple is determined from the relation

E of junction Resistance of DF
────────────────── = ───────────────── .
E of standard cell R_{2}

By exposing the hot end of the junction to successive standard temperatures, and maintaining the cold ends at a known constant temperature, the necessary data for inclusion in a formula may be obtained.

In fixing a permanent temperature scale, calculated from the formula, to a millivoltmeter, it must be remembered that the values given by the experiment are absolute, and independent of the resistance of the circuit composed of the thermo-element and galvanometer. On the other hand, a millivoltmeter is marked to read difference of potential at its terminals; and if in series with a junction and leads of notable resistance, its indications will not be the E.M.F. of the junction. An example will make this point clear.

_Example._—A millivoltmeter has a resistance of 100
ohms, and is marked to read P.D. at its terminals. A
thermocouple and leads connected to the millivoltmeter
have a resistance of 5 ohms. To find the relation between
the true E.M.F. of the junction and the readings of the
indicator.

If E = the E.M.F. developed by the junction, and V, the
reading of the millivoltmeter, = P.D. at its terminals,
then the current in the circuit = E/105 = V/100; and V =
(100/105)E. That is, the readings are lower by 5 per cent.
than the true E.M.F. of the junction. In the same way a
low resistance voltmeter, if applied to a cell of high
resistance, shows a lower reading than the E.M.F. of the
cell.

This example indicates how a table connecting true E.M.F.’s with reading in millivolts may be calculated when the resistances concerned are known. It is presumed, in preparing a scale in this manner, that the resistance of the couple will not be subject to such alterations as to affect the reading.

The advantages of this method of calibration are manifest when a number of junctions are being made from a given batch of wires, as it is only necessary to divide the scale of the indicator so as to represent millivolts—a simple operation—and then to attach a temperature scale. This procedure is much more expeditious than standardizing each indicator at several fixed points when a number are concerned, but for a single junction the fixed point method is easier. The potentiometer method of measuring E.M.F. may also be used to determine temperatures in place of an indicator, and is of great service in cases where very accurate readings are specially required, being far more delicate in detecting small differences of temperature than an indicator. Special potentiometers for thermo-electric work are made by the Cambridge and Paul Instrument Company, Siemens, and others, and are useful in conducting accurate research, but are too elaborate for workshop or ordinary laboratory practice.

=Cold Junction Compensators.=—The necessity for paying attention to the cold junction has led to various attempts to compensate automatically for changes of temperature at this part of the pyrometer. A thermometer located near the cold junction, as in fig. 6, is all that is needed to correct a two-junction circuit; but when a three-junction circuit is used a correct reading is not secured by adding the excess temperature of the thermometer over the calibration temperature to the reading on the indicator. In Bristol’s arrangement a mercury thermometer, with a large bulb and wide stem, is stationed at the cold junction, and participates in any temperature change. In the stem is placed a loop of thin platinum wire, which forms part of the pyrometer circuit. When the mercury is heated it expands up the stem and short-circuits a portion of the loop, thereby diminishing the resistance of the pyrometer circuit, and tending to increase the deflection on the indicator. Simultaneously the cold junction will be heated, tending to diminish the current, and so to cause a less deflection. By adjustment these two tendencies may be counterbalanced, so that the reading is unaffected, but such adjustment will only apply to a given E.M.F., and therefore to one temperature of the hot junction. Hence this method fails in general application.

Peake’s compensated leads are intended to remedy cold-junction errors by transferring this junction, in effect, to the galvanometer. They are used for pyrometers in which the platinum metals are employed, and consist of wires of two different alloys of copper and nickel, which connect the cold end to the indicator. These alloys are such that the electromotive forces set up at the junctions in the head—Pt and Cu-Ni 1, and Pt-Ir with Cu-Ni 2—are equal and opposite at all working temperatures, and hence changes at the cold junctions do not affect the reading. At the indicator, however, temperature changes would cause an alteration in deflection; but as the indicator is generally placed well away from the furnace, and is not liable to notable heating or cooling, the possible errors are greatly reduced by the use of these leads. They are obviously of no value for use with base-metal pyrometers, as the wires used in such may be prolonged to the indicator, with an identical result.

An automatic compensator for use with base-metal pyrometers has been devised by the author, and is illustrated in figs. 17 and 18. A spiral made of a compound strip of two metals is attached to the needle of the indicator, and coils or uncoils when cooled or heated, thereby moving the pointer over the scale. The length of the spiral is such that an alteration of a given number of degrees in its temperature moves the pointer by the same number of degrees on the scale—or, in other words, the temperature scale of the pyrometer is identical with that of the spiral. The metals forming the junction are continued, in the form of wires, to the interior of the galvanometer, where a cold junction is formed, which will always possess the same temperature as the spiral. The scale is constructed to represent differences of temperature between the hot and cold junctions, and before coupling up the pyrometer the pointer indicates the temperature of the spiral; that is, of the cold junction. On connecting the thermocouple the pointer is moved by the coil of the indicator through an amount represented by the difference in temperature between the two junctions, and therefore finally indicates the temperature of the hot junction.

_Example._—If the cold junction were at 20°, the pointer,
before connecting the couple, would indicate 20° on the
scale. If the hot junction were 580° hotter than the cold,
then on completing the circuit the pointer would move 580
additional degrees along the scale, so that the figure
indicated would be (20 + 580) = 600°, the temperature of
the hot junction. If now the indicator were heated by 10°,
the spiral would tend to augment the deflection by 10°,
but simultaneously the deflection due to the junctions
would fall off by 10°, and the reading would still be 600°.

This method of compensation renders the readings independent of the cold junction, and, in addition to its use for high temperatures, enables ordinary and low temperatures to be read simply and correctly, as will be shown later. The spiral is located in the tower rising from the top of the indicator in fig. 18.

In Paul’s method of compensation the thermocouple and indicator are placed across a Wheatstone bridge, two arms of which contain resistances of copper, whilst the resistances in the other two arms are of manganin. Any change in temperature at the cold junction is shared by these four resistances, and, whilst affecting the resistance of the copper parts, no change is caused in the manganin parts, as this alloy has a negligible temperature coefficient. If, therefore, the bridge were initially balanced at 20° C., and the temperature rose to 30°, the increased resistance of the copper would destroy the balance, and permit of a small current passing through the indicator. A fall to 10°, by diminishing the resistance of the copper, would cause an equal current to pass through the indicator in the opposite direction. The amount of this current is arranged so as to add the rise in temperature of the cold junction to the reading of the indicator in the one case, and to subtract the fall in the other, thus retaining true readings for the cold-junction temperature at which the couple was standardized.

=Constant Temperature Cold Junctions.=—If the cold junction can be kept at a steady temperature, compensators are unnecessary, but no good practical means of achieving this end has yet been devised. Water-cooled heads have already been referred to; but in many situations the connecting-pipes entailed would be objectionable, and hence this arrangement is not greatly used. An alternative method, suggested by Prof. A. Zeleny, is to bury the cold junction in the ground. Recent experiments, conducted at Cambridge by R. S. Whipple, showed that a junction buried 10 feet deep did not vary in temperature by more than 2° C. over a period of three years. This has led to the adoption of buried junctions in special cases; but it is probable that much greater variations would be experienced in the ground beneath large furnaces, in which case the advantages of this procedure would be lost. A common workshop method is to locate the cold junction in a thermos flask filled with oil, when a temperature constant to 2° C. may be secured, although the changes in the temperature of the surrounding atmosphere may be as great as 150 C. For special work, ice may be used in the thermos flask, thus securing absolute constancy; but this procedure is not feasible in ordinary works practice.

=Special-Range Indicators.=—When the working range of a pyrometer is from 600° C. upwards, it is evident that the part of the scale occupied by the first 600° is useless, and that it would be an advantage if the whole scale could be utilised for the special working range, so as to secure more exact readings. This may be accomplished by a “set-up” against the movement of the pointer caused by the thermocouple, so as to prevent any motion over the scale until an assigned temperature is reached. For example, a junction developing 12 millivolts at 1000° C. may be coupled to an indicator in which the full-scale deflection of the pointer is produced by 6 millivolts. If an E.M.F. of 6 millivolts be opposed to the junction, no deflection will occur until the temperature at which the couple develops 6 millivolts is reached—when the opposing E.M.F. will be overcome. This temperature may be 500° C., so that the whole scale may be divided up between 500° and 1000°. The length of the indicator scale is thus effectively doubled; and by using different values for the set-up, it is evident that any desired range may be obtained within the limits of sensitivity of the indicator. The method of procuring the opposing E.M.F. varies with different makers. The Cambridge and Paul Instrument Company employ a dry cell and a series resistance, connected so as to oppose the thermocouple; and by adjusting the resistance any desired set-up may be obtained, the value of which, in degrees, may be read off by connecting the cell and resistance to the indicator, the couple having been switched out of the circuit. Thus, to adjust for a range of 500°-1000° on an indicator giving full-scale deflection for 500°, the resistance is regulated so that the cell alone causes the pointer to move to the end of the scale. The method adopted by Paul consists of suitable resistances inserted in a Wheatstone bridge, which may be thrown off the balance, and thus cause an opposing E.M.F. of the correct amount at the terminals of the indicator.

A mechanical set-up has been introduced by the Cambridge and Paul Instrument Company, the indicator in this case having a suspended coil. By turning a milled-head a twist may be given to the suspending strip, and by the turning of a second head the pointer may be brought back to zero, retaining the initial twist, which is opposed to that produced by the current due to the couple. Thus, if the imposed twist were such as to move the pointer to the 400° mark on the scale, the temperature indicated by the junction would be the observed reading plus 400. By this method it is possible to obtain any desired range within the limits of the indicator. The danger of producing errors due to “creeping” is said to be negligible.

=Potentiometer Indicators.=—The advantage of measuring E.M.F. by the potentiometer method is that the result is independent of the resistance of the circuit under test, whereas an indicator is affected by changes in the resistance of the circuit in which it is inserted. When long leads are used to connect a couple to its indicator, notable errors may be caused by the varying resistance of the leads, due to changing temperature; and, in addition, the resistance of the couple-wires varies according to temperature and depth of insertion in the furnace. Attempts have therefore been made to produce indicators based on the potentiometer principle, suitable for workshop use, and one form, known as Northrup’s “Pyrovolter,” is arranged as shown in fig. 19, A. A cell D sends a current through a rheostat R, a copper coil C, and a manganin coil S. The copper coil has the same resistance as the copper winding of the indicator G. The couple is connected, with G in circuit, across the manganin coil S, the resistance of this material being unaffected by temperature. By adjusting R until no deflection is shown on G, the drop of volts across S is made equal to the E.M.F. of the couple. To measure this drop, a key is pressed, altering the circuit as shown in B, the indicator being now in series with S and the couple detached. The value of the current passing through S is unchanged, as the indicator coil has the same resistance as the copper coil C, which it now replaces. The deflection on G indicates the value of this current, and, as the drop of volts across S is proportional to the current, G may be marked off to read E.M.F. and the corresponding temperature of the junction. The advantages claimed are that the indicator may be used with any type of junction, and is unaffected by temperature changes in the circuit. A similar instrument is made by the Brown Company of Philadelphia. Up to the present potentiometer indicators have not been adopted to any extent in Britain, and the adjustments necessary to obtain a reading must be accounted a distinct drawback from a workshop standpoint.

=Recorders for Thermo-electric Pyrometers.=—It is frequently of importance to know not only the existing temperature of a furnace, but also the fluctuations to which it is subject. Continuous observation of a pyrometer would involve too much labour, and it is therefore evident that an automatic recorder would possess many advantages in such cases. A continuous record shows whether the attendant has maintained the temperature between the prescribed limits, and furnishes a permanent history of a given operation, which often serves as a guide to future procedure.

The first successful recorder, suggested by Sir W. Roberts-Austen and designed by Gen. Holden, F.R.S., was used in conjunction with a mirror galvanometer. In its original form, the spot of light from the mirror was made to fall on a sensitized plate, to which a gradual vertical motion was conveyed by connecting the dark slide to a water-float by means of a chain and pulley. The float was placed in a tank of water, which was gradually emptied through a tap, causing the float to sink and the plate to rise. If the deflection of the spot of light remained steady, a vertical straight line was traced on the plate, fluctuations producing a sinuous line. Trials at known temperatures enabled a standard plate to be obtained, divided into degrees, which could be superposed on a trial plate, and the temperatures thus determined. Much valuable work was accomplished with this recorder by Roberts-Austen for the Alloys Research Committee of the Institution of Mechanical Engineers.

In its modern form (fig. 20) the photographic plate is replaced by a sheet of sensitized paper wound round a drum which rotates at a known rate—say, once in 12 hours—by means of internal clockwork, shown to the left of the figure. The galvanometer is placed at the opposite end, and the mirror is illuminated by means of an electric lamp placed externally, the rays from which are reflected from a prism in the interior on to the mirror. The ray of light leaving the mirror is broken into two portions, one of which passes through a narrow slit on to the sensitized paper, whilst the other portion is reflected on to a ground-glass scale on the lid, divided so as to read temperatures. In this manner the arrangement serves not only as a recorder, but also indicates the existing temperature without necessitating the examination of the sensitized paper. The whole arrangement is made impervious to light, so that it may be used in daylight. A dark room is necessary for fixing the records. When desired, records of two or more pyrometers may be taken on the same sheet, a clockwork device being used to switch each instrument in turn on to the galvanometer for a given period, an external dial indicating which pyrometer is for the time being in circuit.

Whilst it is a drawback to the use of this recorder that the record is not visible, the use of a mirror galvanometer confers a high degree of sensitiveness to the instrument, not possessed by the recorders to be described subsequently.

=The Thread Recorder.=—In this instrument an intermittent record is secured in ink, possessing the advantages of visibility during the period over which readings are taken, and of permanence without subsequent treatment of the chart. The principle is shown in fig. 21, where A is a boom terminating in a V-shaped piece of ivory, and attached to the galvanometer suspension B. By means of a cam E, rotated by clockwork, a bar D is made to descend at stated intervals, pressing the end of A on to an inked thread G, and causing the thread to touch a paper wound round the drum C. This drum rotates on its axis once in 25 hours by the action of internal clockwork. The continued rotation of the cam E alternately raises and depresses the boom A, leaving it free for a sufficient time to enable it to attain the position it would occupy if the mechanism were absent. The thread G is passed over pulleys, and is wound round through an ink-well, so that the portion opposite A is always moist. With the bar D descending every two minutes, the successive dots form a nearly continuous line. The paper on C is divided horizontally into temperatures, and vertically into time units, so that the temperature existing at any given time may readily be ascertained. The front of the bar D, or a separate strip parallel to it, is divided so as to enable temperatures to be read without reference to the chart. The actual instrument is shown in fig. 22. When several simultaneous records are required, the drum C is extended, and other galvanometers introduced, to which the separate pyrometers are connected. Several records can be taken on one chart by introducing a clockwork mechanism to couple each pyrometer in turn to the one galvanometer.

=The Siemens Recorder.=—In this instrument (fig. 23) the boom from the galvanometer terminates in a knife-edge, and moves over a thin horizontal rail, the top of which is rounded. Between the rail and the boom are placed an inking ribbon and a paper chart, which is moved forward by clockwork. A chopper-bar, also actuated by clockwork, descends at about half-minute intervals, and depresses the end of the galvanometer boom, thus producing a small dot on the chart. The paper is 12 cms. wide and 40 yards long; it is divided into time and temperature units, and moves forward at the rate of 2 cms. per hour. Levelling screws are fixed to the base of the recorder.

=Foster’s Recorder.=—Foster’s recorder (fig. 24) is designed for use with base-metal couples of the nickel-chromium type, known as Hoskin’s alloys, which yield an E.M.F. about five times as large as a platinum-rhodioplatinum couple. The force available in this case enables the coil of the galvanometer to be pivoted in a horizontal position, the pointer being vertical, and yet to be sufficiently sensitive. The chart is mounted on a vertical plate which rotates on its axis, the time ordinates taking the form of concentric circles, which are cut at an angle by the temperature ordinates. At the terminus of the pointer is placed a small capillary tube, fitted with an inked wick, which, when pressed upon the chart, makes a mark. The presser-bar is curved to the same radius as the pointer, and carries a pad wetted with ink, so that at each depression the supply of ink to the wick is replenished by an amount equal to that imparted to the chart. This recorder is sometimes fitted with special contacts, so that when the correct temperature exists an electric lamp with a white bulb remains lighted; whereas when too low or too high a green or red lamp is lit up, and an alarm thus given. Such an addition involves the use of a relay circuit, but is advisable in cases where expensive articles might suffer if overheated. It can be modified to permit of several simultaneous records being taken, and possesses the advantage that the whole chart is visible at any time. On the other hand, the circular coordinates may be accounted a drawback by some, as not being quite so familiar to read as charts in which the lines are straight. Robust construction is a feature of this recorder.

=Paul’s Recorder.=—In the recorders previously described, the motive power is furnished by clockwork. R. W. Paul has introduced an instrument in which all the moving parts are actuated by a motor driven with power from the mains. This recorder is shown in fig. 25. The motor is furnished with a special type of governor to ensure constant speed, and is connected by suitable gearing to the mechanisms moving the chart, presser-bar, and inking ribbon, provision being made to vary the speeds of these movements by changing the gear. The galvanometer is of uni-pivot pattern, and the pointer is pressed at intervals on to a typewriter ribbon which lies above the chart. Immediately beneath the ribbon is placed a thin metal rod over which the paper passes, and the result of the contact is to produce a small dot. As in the thread recorder, the chart is divided into rectilinear coordinates, the ribbon in this case serving the same purpose as the thread in the former instrument. The lower part of the recorder is prolonged so as to display a considerable length of the chart, which is in the form of a roll, and is drawn forward by the mechanism. When two records are taken simultaneously the ribbon consists of two strips, one moistened with black ink, and the other with red; and it is arranged that each strip in turn is over the thin rod on to which the pointer is pressed, so that the records appear in separate colours. This recorder can also be arranged for multiple records, or fitted with a scale-control. With a view to workshop use, all the covers are fitted with faced metal joints, which are much better for keeping out dust than wooden ones. A further useful feature is that the various units in the recorder—galvanometer, motor, feed and record mechanism, and reducing gear—are all separate and interchangeable. By introducing a suitably divided chart this recorder will also serve for a radiation pyrometer, or, as will be shown later, for a resistance pyrometer.

=The Leeds-Northrup Recorder.=—The Leeds and Northrup Company, of Philadelphia, manufacture a recorder which is largely used in the United States. As in Paul’s recorder, all the mechanism is motor driven; but the other arrangements are entirely distinct. Instead of measuring the deflection of the pointer, a zero deflection method is used. The pyrometer forms part of a potentiometer circuit, and the function of the mechanism is to oppose an E.M.F. equal to that of the pyrometer, from which the temperature is known. This has the advantage that the measurement is independent of the resistance of the leads, and is capable of great accuracy. The manner in which the adjustment of the opposing E.M.F. is controlled may be understood from fig. 26, in which the galvanometer coil is shown at the top of the figure. The shaft from the motor carries four cams, B, C, D, D, and at each revolution the cam B raises the bar (5) so as to press it against an arm attached to the galvanometer coil. At the same moment the cam C pushes against the bar (3), and thereby releases a clutch (2) from the disc beneath. As shown, the boom from the coil is to the right of the central position, and is gripped between a bar (5) and the lever (4) when the former rises, causing an angular movement of the clutch-arm (2). As the rotation continues the cam C leaves the bar (3), which then springs back and engages the clutch on the disc. The cam D then descends and presses on the projection of the clutch-arm to the left, causing the disc to rotate. The movement of the disc is conveyed to an arm which moves over the slide wire of the potentiometer; and this movement continues until the galvanometer boom is in the central or zero position, when neither of the levers 4, 4 is gripped, and consequently the disc is not fed in either direction. If the boom swing to the left, the movement of the disc will evidently be in the converse direction to that described.

In this recorder considerable power is available to drive printing or other mechanisms. The arm moving over the potentiometer wire carries a pen which marks the moving chart, or, when several records are taken simultaneously, a stamping machine is used which impresses the number of the pyrometer on the chart. The same galvanometer mechanism serves also for use with resistance pyrometers, as will be explained later.

=Control of Furnace Temperatures.=—Many attempts have been made to secure the automatic regulation of furnace temperatures by means of mechanisms controlled by an indicator or recorder. In the arrangement employed by the Brown Company of Philadelphia, movable stops are provided, which may be brought to any part of the scale, the mark between the stops representing the temperature it is desired to maintain. The indicator (or recorder) is provided with a presser-bar which descends periodically; and if the temperature be too low the depressed pointer completes a circuit through the inner stop, whilst if too high the circuit is through the outer stop. Both circuits contain a relay which brings a mechanism into operation, the result being to increase the supply of electricity or gas if the temperature be too low, or to diminish the supply when too high. When correct, the depression of the pointer fails to complete either circuit, and in this manner control between small limits may be ensured. In the case of large furnaces the relay circuits are employed to light lamps of different colours, the adjustment then being made by the man in charge of the furnace. Arrangements of this kind effect a considerable saving in fuel by preventing unnecessary heating, and are particularly valuable in cases where overheating would be deleterious to the articles in the furnace. The future will probably witness considerable developments along these lines.

=Contact-Pen Recorders.=—The force with which the pointer of an indicator is urged over the scale is relatively small, particularly in the case of pyrometers in which the platinum series of metals are used, as these furnish only a low E.M.F. If, therefore, the pointer terminate in a pen which is in continuous contact with the record-paper, the friction thus occasioned interferes considerably with the free movement of the pointer. When cheap-metal pyrometers are used, which yield a much higher E.M.F., the use of the pointer as contact-pen becomes more feasible, and if uniform friction at all parts of the paper can be ensured, records may be taken in this manner; and a recorder so constructed is simpler and cheaper than those of the intermittent type. Contact-pen recorders are used in America to some extent, being made by Bristol, Brown, and others; but so far British makers have not developed the manufacture of these instruments. At present, contact-pen recorders must be considered less accurate and reliable than those in which the contact is intermittent.

=Installations of Thermo-electric Pyrometers.=—When a number of furnaces in the same establishment are to be controlled, considerable economy may be effected by making one indicator serve for all the couples, which in this case must necessarily be made up of wires identical in thermo-electric value. Such an arrangement is shown in fig. 27, in which H^1 and H^2 represent two couples, one wire from each being connected to one of the terminals of the galvanometer G. The other terminal of the galvanometer is connected to the arm D of a switch, and the remaining thermocouple leads are connected to the points 1 and 2 respectively on the circumference. As shown, H^1 is connected to the galvanometer, and by turning the arm D to the point 2 the other couple would then be connected. Any number of junctions may thus be arranged with a single indicator. When this arrangement is adopted in a workshop, it is advisable to construct a small wooden building at a spot convenient for most of the furnaces, in which the indicator and switchboard are kept, and which could also contain a recorder if necessary; a spot as free as possible from vibration being preferable. Separate indicators are only necessary when a furnace is used for special work.

In some instances a second indicator is kept in the shop office, to which all the pyrometers are wired, and which serves as a standard. The scale of the office indicator is checked daily at one point; and by connecting a given couple first with the shop indicator, and immediately afterwards with the office standard, any errors can be detected. It is also possible to ascertain the temperature of any given furnace in the office at any time, and so to control the whole. In fixing up such an arrangement it is necessary that each couple and its leads, up to the indicator, should possess the same resistance, or should not differ by an amount sufficient to affect the readings. The general experience of a properly managed installation is that the cost is saved in a few months in fuel alone; and, in addition, the work is carried out to much better advantage owing to complete control from the office.

=Management of Thermo-electric Pyrometers.=—Generally speaking, thermo-electric pyrometers give little trouble in practice, but the management should always be placed in skilled hands. It is advisable to test each instrument periodically at a fixed point near the working temperature, by the method explained on page 57; and if two or three pounds of material be used, the protecting shield need not be removed. A useful material for checking pyrometers near the critical range of steel is an alloy of 60 per cent. of copper and 40 per cent. of tin, which gives a well-defined freezing point at 738° C., and which may be used indefinitely in a reducing atmosphere Any serious error is easily detected by observing that the indications differ widely from those generally obtained under the same working conditions. If an error of 20° C. or more is noted, it is advisable to form a new junction, as the discrepancy will probably become greater, being due to a change at the hot junction. A small error, of the nature of 5 or 10° C., may be due to “creep” in the indicator, which may be adjusted accordingly, or a numerical correction may be made when taking a reading. An iron protecting sheath may be saved from rapid oxidation by black-leading once per week, which greatly prolongs its useful life, but should be replaced immediately it becomes dangerously thin in any part. Coating with aluminium powder also greatly prolongs the life of an iron sheath. When used in lead baths, the immersed part, if of iron or steel, should be bored from the solid, and left thick at the portion opposite the surface of the lead, where most corrosion occurs. A graphite tube, or one made of a composition containing graphite, is often useful in cases where iron is readily corroded, and can be used to much higher temperatures.

When a number of instruments are in use, it is advisable to keep a standard pyrometer for checking purposes, preferably one which has been certified by the National Physical Laboratory. In conducting a test, the couples, with protecting-tubes removed, may be placed in the tube of an electric furnace of the type shown in fig. 29, in close proximity with the standard junction. On raising the temperature gradually, the readings of each working instrument may be compared with the standard, and the necessary corrections discovered. Care must be taken to prevent contact with the furnace tube, and this may be accomplished by passing the wires through an asbestos stopper fitted into the end of the tube.

When recorders are used the attendant should make himself thoroughly conversant with the details of the mechanism, so as to be able to remedy any minor ailments, which are, as a rule, easily cured. On no account should an unskilled workman be trusted with recorders; it is better and safer to keep these in the office, where they will not be likely to be damaged or tampered with. All records should be kept for future reference, properly dated, and labelled according to the operations represented.

=Laboratory Uses of Thermo-electric Pyrometers.=—Numerous operations carried out in muffle furnaces at prescribed temperatures require no special precautions beyond those previously given. In determining the melting points of metals or alloys, however, a porcelain or silica sheath is inadvisable, as they are easily corroded. An iron sheath is proof against some metals, but not against others, and it is always safer to fix a thin fireclay sleeve, closed at the end, over the part immersed. A sheath of graphite or graphite composition may be used for temperatures above 1100° C.; and occasionally a sheath bored from a thick arc-lamp carbon, coupled to an iron tube beyond the heated part, will be found useful at high temperatures. Alundum is useful up to 1600° C, and for temperatures of this order the higher refractories such as silfrax and zirkite may also be used to advantage.

The determination of the “critical” points of steel call for special mention. In cooling down a mass of steel the fall of temperature is arrested at one or more points, observations of which are frequently of service in deciding the subsequent treatment of the steel. A method commonly employed is known as the “differential method,” and is indicated in fig. 28. The sample of steel, A, is placed side by side with a piece of nickel, B, of equal dimensions, in the tube of an electric furnace. A naked junction, C, is placed in a hole drilled in A, and is connected to the galvanometer G, which is calibrated to read temperatures. A two-junction circuit, formed of a junction D placed in the hole in A, and another junction E located in the hole in B, are connected to a delicate galvanometer H. The furnace is heated until the galvanometer G indicates 900° C., when the arrangement is allowed to cool. As A and B, under normal circumstances, cool at an equal rate, the junctions D and E will be at the same temperature, and no deflection will be observed on H. When, owing to recalescence, the cooling of A is arrested, B, not being thus affected, will continue to cool, thus producing a difference between the temperatures of D and E, and consequently a deflection on H. The temperature of A at the time this occurs is read off on G.

The furnace illustrated in fig. 29 is suited to this determination. It consists of a silica tube 1 foot long, wound with a special resistance wire and efficiently lagged, and may be heated in safety to 1000° C. for long periods, and to 1200° for a short time. It may be placed across the electric mains directly, and reaches 900° C. in less than half an hour. It consumes 600 to 700 watts at the highest temperatures, and the cost of re-winding is small. This furnace is useful as a general laboratory appliance, and may be kept at a given steady temperature by the use of an external resistance.

The wires in this experiment should be platinum and iridioplatinum or rhodioplatinum, or a good pair of base metals, and the junctions in A should be separated from each other and from the specimen by asbestos; the same precaution being taken to prevent the junction E from touching B. A thin layer of mica should be used beneath A and B, to avoid contact with the furnace tube, which, when hot, allows of leakage of current from the heating coil. Both A and B may be 1½ in. long, ¾ in. diameter, with a hole ¼ in. diameter drilled to a depth of ¾ inch.

An alternative method is to insert a junction in a hole in the specimen, and to take direct readings as the temperature slowly rises or falls, when an arrest in the movement of the pointer of the indicator shows that the change-point has been reached. Special sets are made for this purpose.

=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.