MISCELLANEOUS APPLIANCES
=Expansion and Contraction Pyrometers.=—Most substances, on
heating, exhibit an increase in size, and on cooling return to the
original dimensions. If, however, a chemical alteration occur during
the heating, the resultant material may be permanently altered in size,
so that on cooling the substance may be of less or greater dimensions
than before. Both these phenomena have been applied to the measurement
of high temperatures; the permanent shrinkage undergone by clay being
utilised by Wedgwood in the instrument which was the first practical
pyrometer; the expansion of a solid by Daniell, and of liquid by
Northrup. Both forms are still in use to a limited extent, and will now
be described.
[Illustration: FIG. 68.—WEDGWOOD’S PYROMETER.]
=Wedgwood’s Pyrometer.=—In 1782 Wedgwood introduced a method of
determining the condition of a furnace by observing the contraction
shown by cylinders prepared from a special clay. The measuring device
took the form of a tapered groove (fig. 68) made in two parts, each
6 inches long, and one a continuation of the other. Each inch of the
groove was divided into 20 equal parts, making 240 divisions in all, and
each division was called 1 degree. The width of the groove opposite the
zero mark was 0·5 inch, and opposite 240, 0·3 inch. Before firing, the
cylinders entered the groove until the lower end was opposite or near
the zero mark; and after being inserted in the furnace and allowed to
cool on removal, the cylinders were pushed as far as possible down the
groove, when the mark opposite the lower end indicated the condition of
the furnace in terms of Wedgwood’s scale. The degrees were, of course,
arbitrary; but with cylinders of uniform make a given position in the
groove after heating always represented the same furnace temperature,
and thus furnished an indication more reliable than the judgment of a
workman’s eye. Wedgwood attempted to express the divisions on his scale
in terms of Fahrenheit degrees, and by extrapolation of results obtained
at the highest limits of the mercury thermometer, where 1 degree of
contraction was caused by a rise of 130° F., arrived at figures which
now appear ludicrous, but which were accepted for forty years. As
examples, the melting point of silver was given as 4717° F.; of cast
iron, 17977° F.; and of wrought iron, 21637° F.—the last figure being
nearly 19000° higher than the present accepted value of 2770° F. The
error arose from the assumption of uniform contraction with increase
of temperature, and furnishes a striking example of the danger of
indefinite extrapolation from meagre data. But although the expression
of the result in Fahrenheit degrees was so erroneous, the observed
contraction always corresponded to a given condition of the furnace,
and the firing was continued until that known to be the best for the
work in hand was attained.
The permanent shrinkage referred to is caused by dehydration of the
clay, and it therefore follows that this method can only give uniform
results when exactly the same kind of clay is used for the test-pieces.
A given manufacturer might secure consistent indications by making
a quantity of clay, to be kept specially for this purpose; but the
same contraction at a given temperature would not be obtained by a
second observer who also had prepared a quantity of clay, as slight
differences in composition cause large variations in the observed
contraction. In practice, therefore, pyrometers of this type are not
interchangeable, and each user must standardize for his own special
conditions. Wedgwood’s pyrometer is still used to a small extent; its
replacement, however, by the more convenient and accurate instruments
now available is only a question of time.
=Daniell’s Pyrometer.=—In 1822 Daniell published an account
of a pyrometer based on the expansion of a platinum rod enclosed in
a plumbago tube. One end of the rod pressed against the end of the
tube, whilst the other end was free to move, and was connected to
a multiplying device which magnified the expansion, the increased
movement being indicated by a pointer, moving over a dial. The scale on
the dial was divided evenly into a suitable number of parts, it being
assumed that the difference between the expansions of graphite and
platinum was uniform at all temperatures. The scale was calibrated as
far as possible by comparison with a mercury thermometer, the remainder
being extrapolated. With this pyrometer Daniell obtained a value of
2233° F. for the melting point of silver, and 3479° F. for that of cast
iron—results considerably higher than those now accepted, but much
nearer than those obtained by Wedgwood. Daniell’s pyrometer was widely
used, and its modern representatives are fairly common. Platinum, owing
to its cost, is no longer used in these instruments, which are now
generally made with a graphite rod encased in an iron tube, on the end
of which the graduated dial is placed, as shown in fig. 69. Another
form, commonly used in baker’s ovens, is constructed with an iron rod
surrounded by a porcelain or fireclay tube.
[Illustration: FIG. 69.—EXPANSION PYROMETER.]
The defect of pyrometers of this type is that the coefficient of
expansion of the materials alters with prolonged heating, causing the
readings to become erroneous. Re-adjustment in boiling water or other
substance does not compensate properly for this alteration, as both
materials are not equally affected. Again, the readings will be too
low unless the whole of the expanding parts are in the interior
of the furnace, in which respect this pyrometer is inferior to a
thermo-electric instrument, which may be inserted at any convenient
depth, and may therefore be used for a greater variety of purposes.
The chief recommendation is cheapness; but an expansion pyrometer
should never be used for work of precision. A graphite rod in an iron
enclosure gives more consistent results than other materials.
=Northrup’s Molten Tin Pyrometer.=—Tin melts at 232° C., and
boils at 2270° C. It does not give off vapour sensibly up to 1700°
C., and expands with great uniformity. It is therefore suitable for
measuring high temperatures on the same principle as an ordinary
thermometer, and Dufour, in 1900, attempted to make a high-reading
thermometer by enclosing tin in a silica bulb. Northrup has constructed
an instrument in which the bulb and stem are of graphite, and the
height of the molten tin is determined by lowering a nickel wire
through a gland until it touches the tin, thereby completing an
electric circuit and causing a bell to ring or producing a deflection
on a galvanometer. The upper end of the nickel wire moves over a
scale, which may be marked at two suitable fixed points, and the scale
divided up as in the case of an ordinary thermometer. The durability of
the graphite cover will determine the utility of this pyrometer, and
protection by some good refractory will be essential to prevent
oxidation. Such a pyrometer will not respond quickly to changes in
temperature, but may prove useful in reading temperatures at ranges
beyond the scope of present thermo-electric pyrometers. Northrup
anticipates that this instrument may be used up to 1800° C.
=Vapour-Pressure Pyrometers.=—In these instruments mercury is
placed in a stout steel tube, to which a pressure-gauge is attached,
which registers the vapour-pressure of the mercury. Readings of
pressure may be translated into temperatures by calibration with a
standard pyrometer. The range of these instruments is limited—600° or
700° C.—and they are seldom used at present, having been superseded by
more modern types.
=Water-Jet Pyrometers.=—In these instruments water is passed
through a pipe placed in the furnace or hot space at a definite rate,
and from the rise in temperature produced in the water that of the
furnace may be obtained. An outfit of this kind entails the provision
of a steady source of water-pressure, and the indications can only
remain accurate so long as the bore of the pipe remains uniform. The
calibration is made by comparison with a standard pyrometer. The
drawbacks to the method are its inconvenience, and the necessity
for continuous skilled supervision; and in consequence of these the
arrangement is seldom used.
=Pneumatic Pyrometers.=—Attempts have been made to deduce furnace
temperatures by blowing air at uniform pressure through a pipe located
in the hot space, and noticing the increase in the temperature of the
air. In the Uehling pyrometer, air from the hot space is drawn through
an opening of fixed size by means of a steam-jet, which acts as an
aspirator. The opening is placed at one end of a chamber, and the
steam-jet aspirator at the other end; and a diaphragm with a central
hole divides the chamber into two parts. The pressures existing in
the two portions of the chamber vary according to the temperature of
the air drawn in, and are measured by water-gauges, the readings of
which may be translated into temperatures by calibration against a
thermo-electric or other pyrometer. The method is ingenious, but is
elaborate and costly; and is therefore little used.
=Conduction Pyrometers.=—If one end of a rod of metal be
inserted in a furnace, heat will be conducted along it to the portion
external to the furnace, and a steady condition will be obtained when
the heat escaping from the external part of the rod, by convection
and radiation, is equal to the quantity conducted along the rod. The
hotter the portion in the furnace, the higher will be the temperature
of all parts of the external length. A series of thermometers placed
at intervals in the exterior portion would show a progressive fall in
temperature along the rod; and the hotter the furnace the higher would
be the reading on each thermometer. In applying this principle to the
measurement of high temperatures, a bar of copper or iron is passed
through the wall of the furnace, so that a length of 2 feet or more
protrudes on the outside. Near the end of the external portion a hole
is drilled to a sufficient depth to cover the bulb of a thermometer,
which is inserted in the hole, into which a quantity of mercury is
poured to make a metallic contact between the bulb and the bar. The
reading of the thermometer furnishes an approximate clue to the
temperature of the furnace, rising or falling with corresponding
changes in the hot space. A calibration might be effected by comparison
with a standard; but the method is only applied to the production
of a prescribed condition, known by experience to be attained when
the thermometer reading has a certain value—say 120° C. Changes in
atmospheric temperature, or currents of air, seriously affect the
readings, and the method at best is only approximate.
=Gas Pyrometers.=—Wiborgh, Bristol, and others have constructed
pyrometers in which the pressure of an enclosed gas is recorded by a
Bourdon pressure-gauge, the scale of which is calibrated so as to read
temperatures. A porcelain bulb, terminating in a capillary tube which
is connected to the gauge, is used to contain the air or other gas; but
at temperatures above a red heat the readings become uncertain, owing
either to leakages or the distortion of the bulb. The most suitable
material for the bulb (alloy of platinum, 80 per cent., and rhodium,
20 per cent.) is too costly to use industrially, and would deteriorate
under the influence of furnace gases. In the Bristol recording
instrument the moving index of the pressure-gauge terminates in a pen,
which touches a chart-paper revolving by clockwork. Good results are
obtained up to 400° C., but beyond this the indications are uncertain,
and the instrument is more correctly described as a recording
thermometer.
=Wiborgh’s Thermophones.=—These consist of infusible clay
cylinders, 2·5 cms. long and 2 cms. in diameter, which contain an
explosive. When placed in a hot space, the explosion occurs after
a definite time, the interval being less at high temperatures than
at lower, as the rate at which heat is conducted through a solid
varies directly as the difference between the external and internal
temperatures. The interval elapsing between placing in the furnace and
the subsequent explosion is noted on a stop-watch to the nearest 1/5
second, and from the observed time the temperature is obtained from a
table, drawn up from the results of experiments under known conditions.
If the cylinders be kept dry, an observer experienced in the use of
thermophones may secure a reading to within 40° C.
=Joly’s Meldometer.=—This device, due to Dr Joly, is intended
for laboratory determinations of melting points. It consists of a
strip of platinum, heated by electricity, upon which a tiny fragment
of the material is placed, which is viewed through a microscope. The
temperature of the platinum is regulated by means of a rheostat in the
circuit, and in making a test the temperature is gradually raised until
the material is observed to become globular, or to flow over the
platinum strip. The temperature at which this occurs is deduced from
the linear expansion of the platinum strip, which is measured by a
micrometer attached to the instrument. When carefully used, very
accurate determinations may be made by the meldometer, the results,
moreover, being obtained rapidly, and with the use of the minimum of
material.
=Brearley’s Curve Tracer.=—This apparatus made by the Cambridge
and Paul Instrument Company, is designed to take a large-scale
record of an operation which only occupies a short period of time.
It consists of a drum, round which the record paper is wound, and
capable of rotating on its axis once in ten or thirty minutes by the
aid of clockwork. Attached to the arm of the pen is a pointer, which
moves along the scale of a sensitive mirror galvanometer to which a
thermocouple is connected. The operator, by turning a handle, moves
the drum longitudinally so as to keep the pointer opposite the centre
of the spot of light, and this movement is traced on the chart,
combined with the rotary movement, by the pen. In this manner the
large change in deflection, due to a few degrees increase or decrease
in temperature, can be recorded in ink. This instrument is of special
service in recording the critical points of steel, or any operation
which involves delicate readings over a limited range of temperature.
Index
Air pyrometers, 219.
Anti-vibration stand for galvanometers, 47.
Atmospheric temperatures, measurement of, 96, 132.
Automatic compensators, 66.
_Barrett, Sir W._, discovery of recalescence, 4.
_Becquerel, Ed._, optical pyrometer, 5.
Black-body radiations, 136.
_Brearley_, sentinel pyrometers, 208.
— curve-tracer, 221.
_Bristol_, air-recording pyrometer, 219.
— compensator, 66.
_Byström_, calorimetric pyrometer, 201.
Calorimetric or “water” pyrometers, 195.
— — — special uses of, 203.
Centigrade scale of temperatures, 6.
Clay-contraction pyrometers, 211.
Colour-extinction pyrometers, 189.
Colour, in relation to temperature, 167.
Comparison of gas and platinum scales, 111.
Compensators for cold junctions, 66.
Conduction pyrometers, 218.
Constant temperature cold junction, 71.
Contact-pen recorders, 88.
“Critical” points of steel, 94.
_Daniell_, expansion pyrometers, 2, 214.
_Darling_, automatic compensator, 68.
— and _Grace_, liquid element thermocouples, 43.
_Day_, extension of gas scale, 15.
Electromotive force (E.M.F.) developed by junctions, 31.
— — measurement of, 62.
Expansion pyrometers, 214.
_Fahrenheit_, temperature scale, 7.
_Féry_, lens pyrometer, 142.
— mirror pyrometer, 143.
— optical pyrometer, 174.
— spiral pyrometer, 150.
Fixed points for calibration of pyrometers, 16, 17.
_Foster_, base-metal pyrometer, 39.
— fixed-focus pyrometer, 152.
— recorder, 82.
Furnace, electric tube, 95.
— temperatures, control of, by pyrometers, 87.
Fusible metals, 209.
— pastes, 210.
Fusion pyrometers, 204.
Gas pyrometers, 219.
— scale of temperatures, 11.
— thermometer, constant volume, 11.
_Hadfield_, effect of temperature on hardness of steel, 4.
_Harris_, indicator, 121.
_Holborn-Kurlbaum_, optical pyrometer, 181.
_Holden-d’Arsonval_, galvanometer, 45.
_Holman_, formula for thermal junctions, 62.
_Howe_, colour-temperature table, 167.
Indicators, for radiation pyrometers, 156.
— for resistance pyrometers, 118-124.
— special range, 71.
— standardizing of, 54, 108, 157.
— for thermo-electric pyrometers, 45-53.
Installations of resistance pyrometers, 130.
— of thermo-electric pyrometers, 89.
_Joly_, meldometer, 220.
_Kelvin_, thermodynamic scale of temperature, 9.
_Kowalke_, base-metal couples, 30.
_Lambert_, anti-vibration stand for galvanometers, 47.
_Le Chatelier_, optical pyrometer, 177.
— thermo-electric pyrometer, 5, 22.
_Leeds-Northrup_, indicator, 122.
— recorders, 85, 127.
— resistance pyrometers, 115.
Liquid element thermocouples, 43.
_Lovibond_, optical pyrometer, 186.
Low temperatures, measurement of, 97, 132.
Meldometer, 220.
Mercury thermometer, limits of, 1.
_Mesuré and Nouel_, optical pyrometer, 187.
_Morse_, optical pyrometer, 182.
National Physical Laboratory, scale of temperatures, 17.
_Newton_, researches on high temperatures, 2.
_Northrup_, molten tin pyrometer, 216.
— pyrovolter, 74.
Optical pyrometers, 167.
— — management of, 192.
— — special uses of, 193.
_Paul_, base-metal pyrometer, 39.
— compensator, 70.
— radiation pyrometer, 155.
— recorder, 83.
— uni-pivot indicator, 49.
_Peake_, compensated leads, 67.
Pivoted galvanometers, 49.
_Planck_, modifications of Wien’s formula, 171.
Platinum scale of temperatures, 106.
Pneumatic pyrometers, 217.
Potentiometer indicators, 73.
— method for measurement of E.M.F., 63.
_Prinsep_, gas pyrometer, 3.
Protecting sheaths for pyrometers, 34.
Pyrometer, definition of, 1.
Pyromike, 189.
Radiation pyrometers, 134.
— — calibration of, 157.
— — indicators for, 156.
— — management of, 161.
— — recorders for, 161.
— — special uses of, 164.
_Rasch_, luminosity formula, 168.
Recalescence of steel, 4, 94.
Recorders for radiation pyrometers, 161.
— — resistance pyrometers, 124.
— — thermo-electric pyrometers, 75.
Resistance, measurement of, 102.
— of platinum, 105.
Resistance pyrometers, 101.
— — indicators for, 118.
— — management of, 130.
— — recorders for, 124.
— — special uses of, 132.
— pyrometry, terms used in, 111.
_Roberts-Austen_, recorder, 76.
Salts, melting points of, 204.
_Seebeck_, discovery of thermo-electricity, 3, 20.
_Seger_, pyramids or “cones,” 205.
_Siemens_, calorimetric or “water” pyrometer, 201.
— indicator for resistance pyrometer, 118.
— — — thermo-electric pyrometer, 48.
— optical pyrometer, 182.
— recorder, 81.
— resistance pyrometer, 114.
Specific heat of nickel, 197.
Standardizing of indicators, 54, 108, 157.
Standards of temperature, 9.
Steam, measurement of temperature of, 98.
_Stefan-Boltzmann_, fourth-power law, 139.
_Stone_, pyrometer, 209.
Surface temperatures, measurement of, 97.
Suspended-coil galvanometers, 45-48.
Temperature differences, measurement of, 99.
— scales, 7-9.
— fixed points of, 16-17.
Thermal junctions, changes in, 29.
— — choice of metals for, 21.
— — E.M.F., developed by, 31.
— — methods of joining, 26.
— — used in pyrometers, 27.
Thermo-electric circuits, 22.
— pyrometers, 20.
— — calibration curves for, 59.
— — for surface temperatures, 97.
Thermo-electric circuits, indicators for, 45-53.
— — management of, 91.
— — practical forms of, 32.
— — recorders for, 75-87.
— — standardization of, 54.
Thermodynamic scale of temperatures, 9.
Thermometer, constant volume gas, 11.
— mercury, 1.
Thermophones, 220.
Thread recorder, 78.
_Uehling_, pyrometer, 218.
Uni-pivot galvanometer, 49.
Vapour-pressure pyrometers, 217.
_Wanner_, optical pyrometer, 178.
Water-cooled cold junction, 33.
Water equivalent of calorimeter, 200.
“Water” pyrometer, 201.
Water-jet pyrometer, 217.
_Watkin_, heat recorder, 207.
Wedge pyrometer, 190.
_Wedgwood_, pyrometer, 211.
— test-pieces, 205.
_Wheatstone_ bridge for measuring resistance, 104-114.
_Whipple_, indicator, 120.
_Whipple-Féry_, pyrometer, 154.
_Wiborgh_, gas pyrometer, 219.
— thermophones, 220.
_Wien_, luminosity law, 171.
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