Small graphite crucibles with covers, as shown in section, in Fig. 2,
serve to guard against losing the ball, to handle it by when hot, and to
protect it against loss of heat during transmission from the fire to the
pyrometer. To guard against overturning the crucibles, moulded firebrick
should be provided to receive them, two crucibles being put into one
brick, in the same exposure, whenever great accuracy is desired, each
serving as a check on the other, and their mean being likely to be more
nearly correct than either one if they differ. The firebrick cover
is occasionally useful to retard cooling, if, by reason of local
obstructions, some little delay is unavoidable in transferring the
balls from the fire to the water of the pyrometer. With convenient
arrangements, this may be done in three seconds. After observing the
temperature of the water, make ready for the immersion of the heat
carrier by raising the agitator until a space of only about 1.5 of an
inch is left between its rim and the cover. An instant before putting
in the heat carrier--"pouring" it from the crucible--lift the cover and
agitator both together, so that the rim of the latter is level with the
sloping top of the instrument. The agitator then receives the hot ball
without shock, and no harm is done. If the ball goes below the agitator,
it is likely to injure the bottom of the cup. If, on taking the
temperature of the water before the immersion of the heat carrier, any
change is observed, either rising or falling, the direction and rate of
such change, and the exact interval of time between the last recorded
observation and the immersion, should be noted, in order to determine
the exact temperature of the water at the instant of immersion. The
temperature of the water will continue to rise as long as the heat
carrier gives out heat faster than the cell loses it. The rise will grow
gradually slower until it ceases, and the maximum can be very accurately
determined. Examples of the mode of using the tables, and of determining
the true temperature of the heat carrier at the instant of immersion
from the observations with the instrument, are given in the table on
pages 170 and 171 of this Journal for September. A method of using the
tables, by which a closer approximation to the true temperature may be
reached, will be pointed out in a subsequent article.
[Illustration: Fig. 2.]
DETERMINATION OF THE CALORIFIC CAPACITY OF THE METALS OF THE PYROMETER,
in terms of water, i.e., in British thermal units.
First. Weigh the cup, or cell, the lower plate of the cover and the
metallic portion of the agitator, and compute their heat-capacity by the
specific heat of the respective metals. Compute also the heat capacity
of the thermometer; or, if it be long, of so much of it as is found to
share nearly the temperature of the immersed portion. The result will
be a minimum--indeed, in so small a vessel the inevitable loss by
conduction and radiation will amount to more than one-third as much as
the simple heat capacity of the metals.[1] The total must be ascertained
by an application of the method of mixture. Ascertain the temperature of
the interior of the instrument simply; pour in quickly but carefully a
known quantity of water, say about two pounds, of known temperature, say
about 100° F., and ascertain the temperature as soon after pouring as
mixing can be properly performed. But a correction is necessary for
loss of heat in the act of pouring. To ascertain the amount of this
correction prepare a bath of tepid water, and bring all parts of the
instrument--outside, inside, and interior portions, together with the
vessel to pour from--exactly to one common, carefully ascertained
temperature. Now take two pounds of the water and pour it into the
cell in the same manner as before. Exposure of so thin a stream on
two surfaces to the air of the room will produce a certain degree of
refrigeration in the water, which is supposed to be warmer than the air,
say at about 160° F. This effect will be due to conduction, by contact
with the air, to radiation, and to evaporation; and by so much the
refrigeration observed in mixing is to be diminished.
[Footnote 1: In our case the heat-capacity, thermometer included, was
0.0757; total, 0.1053; radiation, etc., 0.0296. Respectively, 71.9 per
cent, and 28.1 per cent. of the total.]
Four experiments, carefully conducted, gave the following results:
Loss of temperature by pouring at 170° F., 0.81°, 0.86°, 1.00°, and
1.07° F.; mean, 0.935° F.
The following are values of the calorific capacity of my pyrometers,
that is, of those parts of each which share directly the temperature
of the inclosed water, including the thermometer to be used with the
instrument, and the heat communicated to the eider-down and otherwise
lost during an observation, expressed in decimals of a British thermal
unit, or in decimals of a pound of cold water:
0.1048, 0.1052, 0.1077, 0.1008, 0.1028, and 0.1104.
Mean 0.1053 = 0 lb. 1 oz. 11 drms.
Add water 1.8947 = 1 " 14 " 4 "
------ - -- --
2.0000 = 2 " 0 " 0 "
This was the value used. The instrument, being put on delicate coin
scales and counterbalanced, weights equal to 1.8947 lb. avoirdupois = 1
lb. 14 oz. 5 drms., were added to the counterbalancing weights, and cold
water was poured in until the scales again balanced.
The pyrometer with its contained water was then just equal in heating
capacity, while the temperature was not above 38° F. to two pounds of
cold water. The two instruments were sensibly alike, but were numbered
No. 1 and No. 2, and at each observation the one used was noted.
The process of preparation and testing appears long and tedious, and
is indeed somewhat so; but the instruments once well made are durable,
convenient in use, and with care reasonably accurate.
Compared with mercurial thermometers between 212° and 600° F., I believe
them to be much more accurate, although less convenient.
For a range of temperatures from 212° to 900° F. they are certainly
more trustworthy than anything save an air thermometer of suitable
construction; and for all temperatures from 800° to 900° F. up nearly
to the melting point of platinum they are without a rival, so far as I
know.
For some situations the ball can best be inserted in the fire or other
situation where an observation is desired, and withdrawn for immersion
by means of long, slender tongs, with jaws resembling bullet moulds.
A word about the melting point of platinum. My balls certainly began to
melt below 2,950° F., but I am by no means sure that they do not contain
any silver, although their specific gravity gives assurance that they
are at least nearly pure.--_Franklin Journal_.
* * * * *