The thermometers and pyrometers usually employed are almost all based on
the expansion of some fluid or other, or upon that of different metals.
The first can only be constructed with glass tubes, thus rendering them
fragile. The second are often wanting in exactness, because of the
change that the molecules of a solid body undergo through heat, thus
preventing them from returning to exactly their first position on
cooling.
[Illustration: Fig. 1.--Pyrometer with Electric Indicator.]
The principle of the Seyfferth pyrometer is based on the fact that
the pressure of saturated vapors, that is, vapors which remain in
communication with the liquid which has produced them, preserves a
constant ratio with the temperature of such liquid, while, on the other
hand, the temperature of the latter when shut up in a vessel will
correspond exactly with that of the medium into which it is introduced.
[Illustration: Fig. 2.--Method of Mounting by means of a cone on vacuum
apparatus.]
[Illustration: Fig. 3.--Mounting by means of a sleeve on vacuum
apparatus.]
This instrument is composed of a metallic vessel or tube which contains
the liquid to be exposed to heat, and of a spring manometric apparatus
communicating with the tube, and by means of which the existing
temperature is shown. The dial may be provided with index needles to
show minimum and maximum temperatures, as well as be connected with
electric bells (Fig. 1) giving one or more signals at maximum and
minimum temperatures. The vessel to contain the liquid may be of any
form whatever, but it is usually made in the shape of a straight or
a bent tube. The nature of the metal of which the latter is made is
subordinate, not only to the maximum temperature to which the apparatus
are to be exposed, but also to the nature of the liquid employed. It is
of either yellow metal or iron. To prevent oxidation of the tube, when
iron is employed, it is inclosed within another iron tube and the space
between the two is filled in with lead. When the apparatus is exposed to
a high temperature the lead melts and prevents the air from reaching the
inner tube, so that no oxidation can take place.
_Pyrometers filled with Ether._-These are tubular, and constructed of
yellow metal, and are graduated from 35° C. to 120°. They are used for
obtaining temperatures in vacuum apparatus, cooking apparatus, diffusion
apparatus, saturators, etc. Figs. 2, 3, 4, and 5, show the different
modes of mounting the apparatus according to the purpose for which it is
designed.
_Pyrometers filled with distilled water_ are used for ascertaining
temperatures ranging from 100° to 265° C., 80° to 210° R., or 212° to
510° F.
_Pyrometers filled with mercury_ are constructed for ascertaining
temperatures from 360° to 750° C.
[Illustration: Fig. 4.--Mounting on horizontal pipes by thread on the
tube.]
[Illustration: Fig. 5.--Mounting by means of a clasp in reservoirs.]