The accurate measurement of very high temperatures is a matter of great
importance, especially with regard to metallurgical operations; but it is
also one of great difficulty. Until recent years the only methods
suggested were to measure the expansion of a given fluid or gas, as in the
air pyrometer; or to measure the contraction of a cone of hard, burnt
clay, as in the Wedgwood pyrometer. Neither of these systems was at all
reliable or satisfactory. Lately, however, other principles have been
introduced with considerable success, and the matter is of so much
interest, not only to the practical manufacturer but also to the
physicist, that a sketch of the chief systems now in use will probably be
acceptable. He will thus be enabled to select the instrument best suited
for the particular purpose he may have in view.
The first real improvement in this direction, as in so many others, is due
to the genius of Sir William Siemens. His first attempt was a calorimetric
pyrometer, in which a mass of copper at the temperature required to be
known is thrown into the water of a calorimeter, and the heat it has
absorbed thus determined. This method, however, is not very reliable, and
was superseded by his well-known electric pyrometer. This rests on the
principle that the electric resistance of metal conductors increases with
the temperature. In the case of platinum, the metal chosen for the
purpose, this increase up to 1,500°C. is very nearly in the exact
proportion of the rise of temperature. The principle is applied in the
following manner: A cylinder of fireclay slides in a metal tube, and has
two platinum wires one one-hundredth of an inch in diameter wound round it
in separate grooves. Their ends are connected at the top to two
conductors, which pass down inside the tube and end in a fireclay plug at
the bottom. The other ends of the wires are connected with a small
platinum coil, which is kept at a constant resistance. A third conductor
starting from the top of the tube passes down through it, and comes out at
the face of the metal plug. The tube is inserted in the medium whose
temperature is to be found, and the electric resistance of the coil is
measured by a differential voltameter. From this it is easy to deduce the
temperature to which the platinum has been raised. This pyrometer is
probably the most widely used at the present time.
Tremeschini's pyrometer is based on a different principle, viz., on the
expansion of a thin plate of platinum, which is heated by a mass of metal
previously raised to the temperature of the medium. The exact arrangements
are difficult to describe without the aid of drawings, but the result is
to measure the difference of temperature between the medium to be tested
and the atmosphere at the position of the instrument. The whole apparatus
is simple, compact, and easy to manage, and its indications appear to be
correct at least up to 800°C.
The Trampler pyrometer is based upon the difference in the coefficients of
dilatation for iron and graphite, that of the latter being about
two-thirds that of the former. There is an iron tube containing a stick of
hard graphite. This is placed in the medium to be examined, and both
lengthen under the heat, but the iron the most of the two. At the top of
the stick of graphite is a metal cap carrying a knife-edge, on which rests
a bent lever pressed down upon it by a light spring. A fine chain attached
to the long arm of this lever is wound upon a small pulley; a larger
pulley on the same axis has wound upon it a second chain, which actuates a
third pulley on the axis of the indicating needle. In this way the
relative dilatation of the graphite is sufficiently magnified to be easily
visible.
A somewhat similar instrument is the Gauntlett pyrometer, which is largely
used in the north of England. Here the instrument is partly of iron,
partly of fireclay, and the difference in the expansion of the two
materials is caused to act by a system of springs upon a needle revolving
upon a dial.
The Ducomet pyrometer is on a very different principle, and only
applicable to rough determinations. It consists of a series of rings made
of alloys which have slightly different melting-points. These are strung
upon a rod, which is pushed into the medium to be measured, and are
pressed together by a spiral spring. As soon as any one of the rings
begins to soften under the heat, it is squeezed together by the pressure,
and, as it melts, it is completely squeezed out and disappears. The rod is
then made to rise by the thickness of the melted ring, and a simple
apparatus shows at any moment the number of rings which have melted, and
therefore the temperature which has been attained. This instrument cannot
be used to follow variations of temperature, but indicates clearly the
moment when a particular temperature is attained. It is of course entirely
dependent on the accuracy with which the melting-points of the various
alloys have been fixed.
Yet another principle is involved in the instrument called the
thalpotasimeter, which may be used either with ether, water, or mercury.
It is based on the principle that the pressure of any saturated vapor
corresponds to its temperature. The instrument consists of a tube of metal
partly filled with liquid, which is exposed to the medium which is to be
measured. A metallic pressure gauge is connected with the tube, and
indicates the pressure existing within it at any moment. By graduating the
face of the gauge when the instrument is at known temperatures, the
temperature can be read off directly from the position of the needle. From
100° to 220°F. ether is the liquid used, from thence to 680° it is water,
and above the latter temperature mercury is employed.
Another class of pyrometers having great promise in the future is based on
what may be called the "water-current" principle. Here the temperature is
determined by noting the amount of heat communicated to a known current of
water circulating in the medium to be observed. The idea, which was due to
M. De Saintignon, has been carried out in its most improved form by M.
Boulier. Here the pyrometer itself consists of a set of tubes one inside
the other, and all inclosed for safety in a large tube of fireclay. The
central tube or pipe brings in the water from a tank above, where it is
maintained at a constant level. The water descends to the bottom of the
instrument, and opens into the end of another small tube called the
explorer (_explorateur_). This tube projects from the fireclay casing into
the medium to be examined, and can be pushed in or out as required. After
circulating through this tube the water rises again in the annular space
between the central pipe and the second pipe. The similar space between
the second pipe and the third pipe is always filled by another and much
larger current of water, which keeps the interior cool. The result is that
no loss of heat is possible in the instrument, and the water in the
central tube merely takes up just so much heat as is conducted into it
through the metal of the explorer. This heat it brings back through a
short India-rubber pipe to a casing containing a thermometer. This
thermometer is immersed in the returning current of water, and records its
temperature. It is graduated by immersing the instrument in known and
constant temperatures, and thus the graduations on the thermometer give at
once the temperature, not of the current of water, but of the medium from
which it has received its heat. In order to render the instrument
perfectly reliable, all that is necessary is that the current of water
should be always perfectly uniform, and this is easily attained by fixing
the size of the outlet once for all, and also the level of water in the
tank. So arranged, the pyrometer works with great regularity, indicating
the least variations of temperature, requiring no sort of attention, and
never suffering injury under the most intense heat; in fact the tube, when
withdrawn from the furnace, is found to be merely warm. If there is any
risk of the instrument getting broken from fall of materials or other
causes, it may be fitted with an ingenious self-acting apparatus shutting
off the supply. For this purpose the water which has passed the
thermometer is made to fall into a funnel hung on the longer arm of a
balanced lever. With an ordinary flow the water stands at a certain height
in the funnel, and, while this is so, the lever remains balanced; but if
from any accident the flow is diminished, the level of the water in the
funnel descends, the other arm of the lever falls, and in doing so
releases two springs, one of which in flying up rings a bell, and the
other by detaching a counterweight closes a cock and stops the supply of
water altogether.
It will be seen that these instruments are not adapted for shifting about
from place to place in order to observe different temperatures, but rather
for following the variations of temperature at one and the same place. For
many purposes this is of great importance. They have been used with great
success in porcelain furnaces, both at the famous manufactories at Sevres
and at another porcelain works in Limoges. From both these establishments
very favorable reports as to their working have been received.--_W.R.
Browne, in Nature_.
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[NATURE.]