By J. C. HOADLEY.
The following description of the apparatus used for the determination
of high temperatures, up nearly to the melting point of platinum, is
offered in answer to several inquiries on the subject:
The object to be attained is a convenient and reasonably accurate
application of the method of mixtures to the determination of
temperatures above the range of mercurial thermometers, say 500° F., up
to any point not above the melting point of the most refractory metal
available for the purpose, platinum.
A first requisite is a cup or vessel of convenient form, capable of
holding a suitable quantity of water, say about two pounds avoirdupois.
Berthelot decidedly prefers a simple can of platinum, very thin, with a
light cover of the same metal, to be fastened on by a bayonet hitch. For
strictly laboratory work this may be the best form; but for the hasty
manipulation and rough usage of practical boiler testing something more
robust, but, if possible, equally sensitive, is required. The vessel I
have used is represented in section in the accompanying cut, Fig. 1.
The inner cell, or true containing vessel, is 4.25 inches in diameter;
and of the same height on the side, with a bottom in the form of a
spherical segment, of 4.25 inches radius. It is formed of sheet brass
0.01 inch thick, nickel-plated and polished outside and inside. The
outer case is 8 inches diameter and 8.5 inches deep, of 16-ounce copper,
nickel-plated and polished inside, but plain outside. There are two
handles on opposite sides, for convenience of rapid manipulation. The
top, of the same copper as the sides and bottom, is depressed conically.
like a hopper, and wired at its outer edge, forming a lip all around for
pouring out of. The central cell is connected with the outer case only
by three rings of hard rubber (vulcanite), each 0.25 inch thick, the
middle ring completely insulating the cell from its continuation upward,
and from the outer case. A narrow flange is turned outward at the upper
edge of the cell, and a similar flange is also turned outward at the
lower edge of the cylindrical continuation of the walls of the cell
upward. Between these two flanges, the middle ring of hard rubber is
interposed, and the two parts, the cell and its upward continuation,
are clamped together by the upper and lower rings of hard rubber, which
embrace the flanges and are held together by screws. The joints between
the flanges and the middle ring of hard rubber, which might otherwise
leak a little, are made tight with asphaltum varnish.
[Illustration: Fig. 1.]
Fig 1 shows two partitions, dividing the space between the cell and the
case into three compartments, and a concave false bottom. The cover is
also seen to be divided into three compartments, by two partitions, and
each compartment of the vessel and of its cover is provided with a small
tube for inserting a thermometer. This construction was adopted in the
first instruments made, for the purpose of observing the rate of heat
transmission through the successive compartments, but these parts are
without importance with respect to the practical use of the instrument,
and may as well be omitted, as they considerably increase the cost,
being nickel-plated and polished on both sides. The top and bottom
plates of the cover are of 0.01 inch brass, nickel-plated and polished
on both sides, both convex outward, the bottom plate but slightly, the
top plate to 4.25 inches radius. A ring of hard rubber connects, yet
separates and insulates these plates, and they are bound together with
the ring into a firm structure by a tube of hard rubber, having a
shoulder and knob at the top, and at the lower end a screw-thread
engaging with a thin nut soldered to the upper side of the bottom plate.
When the cover is in place, its lower plate is even with the top of
the cell; and the contained water, which nearly fills the cell, is
surrounded by polished, nickel-plated, brass plates 0.01 inch thick,
insulated trom other metal by interposed hard rubber. The spaces between
the cell and case (a single space if the partitions are omitted), the
space above the hard rubber rings, and the space or spaces in the cover
are all filled with eider-down, which costs $1.00 per ounce avoirdupois,
but a few ounces are sufficient. Soft, fine shavings, or turnings of
hard rubber, are said to be excellent as a substitute for eider-down.
Heat cannot be confined by any known method. Its transmission can be in
some degree retarded, and in a greater degree, perhaps, regulated. Some
heat will be promptly absorbed by the sides, bottom, and cover of the
cell, and by the agitator; but this does no harm, as its quantity can
be accurately ascertained and allowed for. Some will be gradually
transmitted to the eider-down, filling the spaces, and through this to
the outer casing; but this can be reduced to a minimum by rapid and
skillful manipulation, and its quantity, under normal conditions, can
be ascertained approximately, so as not to introduce large errors. But
varying external influences, such as currents of air, caused by opening
doors, or by persons passing along near the apparatus during
the progress of an experiment, which would introduce disturbing
irregularities, can best be guarded against by such spaces as I have
described, filled with the poorest heat-conductor and the lightest
_solid_ substance attainable. Air, although a poor heat-conductor, and
extremely light, is diathermous, and offers no obstruction to the escape
of radiant heat.
The agitator is an important part of the apparatus. Its object, in
this instrument, is twofold. _First_, it serves to produce a uniform
temperature throughout the body of water in the instrument; and
_secondly_, it answers as a support to the heat-carrier of platinum or
other metal, often intensely hot, which would injure or destroy the
delicate metal of the bottom if allowed to fall on it. For this second
purpose, no spiral revolving agitator, such as that commended by
Berthelot, would suffice. The best form is such as I have shown in Fig.
1. A concave disk of sheet-brass, made to conform to the shape of the
bottom of the cell, with a narrow rim turned up all around, of about
0.02 inch thickness, is liberally perforated with holes to lighten it,
and to give free passage to water. The concave form causes the streams
of water, produced by slightly raising and lowering the agitator, to
take a radial direction downward or upward, so as to cross each other
and promote rapid mixing. By a slight modification small vanes might be
turned outward from the surface of the metal, which would produce mixing
currents if the agitator were given a slight reciprocatory revolving
motion, thus avoiding the alternate withdrawal and re-immersion of any
part of the stem so strongly deprecated by Berthelot; but for several
reasons I think an up and down motion of the agitator desirable in this
instrument. The platinum heat carrier, sometimes at a temperature of
2,500° to 2,800° F., is thereby brought into more rapid and forcible
contact with the water, steam or water in the spherical condition is
washed away from its surface, and by cooling it more rapidly, the
duration of the observation is lessened, and errors due to transmission
of heat through the walls of the instrument are diminished. The upper
part of the agitator stem is of hard rubber, and the brass portion,
which terminates at the under side of the cover when the agitator is in
its lowest position, suspended by the shoulder at the upper end, need
never be lifted for the purpose of mixing out of the hard rubber tube at
the cover, so that loss of heat from this cause must be very slight.
The brass tube is very freely perforated with holes to admit water,
streaming radially through the holes in the agitator, to contact with
the thermometer. The hole in the stem at the top is flared, to receive
a cork, through which the thermometer is to be passed. The bulb of the
thermometer should be elongated, and very slightly smaller in diameter
than the stem. After passing it through the cork, a very slight band--a
mere thread--of elastic rubber should be put around the bulb, near its
lower end, or a thin, narrow shaving of cork may be wound around and
tied on, to keep it from contact with the brass tube, for safety; and a
little tuft of wool, curled hair, or hard rubber shavings should be
put in the bottom of the brass tube to avoid accidents. For the same
purpose, a light, but sufficient fender of brass wire, say 0.03 inch
diameter, might be judiciously placed around the brass tube at a little
distance, to protect it and the thermometer inside of it from shocks
from the platinum ball when hastily thrown in, as it must always be.
I have had delicate and costly thermometers broken for want of such a
fender. Thermometers cannot be too nice for this work. For accurate work
at moderate temperatures, they should be about 14 inches long, having a
"safe" bulb at the upper end, with a range of 20° F.--32° to 52°--in a
length of 10 inches, giving half an inch to a degree F., and carefully
graduated to tenths of a degree, so that they can be read to hundredths,
corresponding to single degrees of the heat-carrier in the normal use of
the instrument.
For the determination of the highest temperatures, up closely to 2,900°
F., it will be convenient to have thermometers of greater range, say 32°
to 82° F., 50° in a length of 12.5 inches, or a quarter of an inch to
a degree F., also graduated to tenths, or at the least, to fifths of a
degree. Such thermometers will be about 17 inches long.
It is very satisfactory to have _two_ instruments and a good outfit of
thermometers and heat-carriers, in order to take duplicate observations
for mutual verification and detection of errors.