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.
[Illustration: FIG. 30.—DIFFERENTIAL GALVANOMETER METHOD OF MEASURING
RESISTANCE.]
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.
[Illustration: FIG. 31.—PRINCIPLE OF WHEATSTONE BRIDGE.]
=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.
[Illustration: FIG. 32.—CONNECTION BETWEEN RESISTANCE OF PLATINUM AND
TEMPERATURE: A, ON GAS SCALE; B, ON PLATINUM SCALE.]
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.
[Illustration: FIG. 33.—PLATINUM RESISTANCE PYROMETER.]
=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.
[Illustration: FIG. 34.—WHEATSTONE BRIDGE AS USED WITH A RESISTANCE
PYROMETER.]
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.
[Illustration: FIG. 35.—SIEMENS’ DIAL INDICATOR.]
=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.
[Illustration: FIG. 36—WHIPPLE’S INDICATOR.]
=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.
[Illustration: FIG. 37.—THE HARRIS 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.
[Illustration: FIG. 38.—CALLENDAR’S RECORDER.]
[Illustration: FIG. 39.—RECORD OBTAINED WITH CALLENDAR’S RECORDER.]
=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.
[Illustration: FIG. 40.—PRINCIPLE OF LEEDS-NORTHRUP RECORDER.]
[Illustration: FIG. 41.—LEEDS-NORTHRUP RECORDER.]
=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.