Scientific American Supplement, No. 358, November 11, 1882
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Various >> Scientific American Supplement, No. 358, November 11, 1882
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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 deg. 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 deg. to 2,800 deg. 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 deg. F.--32 deg. to 52 deg.--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 deg.
F., it will be convenient to have thermometers of greater range, say 32 deg.
to 82 deg. F., 50 deg. 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.
HEAT CARRIERS.
For these platinum is greatly to be preferred to any other known
substance. Its rather high cost is the only objection to its use. Its
heat capacity is low, by weight, but its specific gravity is great, and
sufficient capacity can be obtained in moderate bulk, while its high
conductivity tends to shorten the duration of each experiment or
observation. A convenient outfit for each instrument consists of three
balls, hammered to a spherical form, one 1.1385 inches diameter,
weighing 4,200 grains=0.6 pound avoirdupois; one 0.9945 inch diameter,
weighing 2,800 grains=0.4 pound; and one 0.7894 inch diameter, weighing
1,400 grains=0.2 pound.
These can be obtained at 1-2/3 cents per grain, and will cost,
respectively, $70.00, $46.67, and $23.33, and collectively, $140.00.
At the assumed specific heat of Pt=0.0333+, the heat capacity of the
respective balls will be 1/100, 1/150, and 1/300 of 2 pounds of cold
water, and the two smaller balls used together will be equal to the
larger one. Corrections for varying specific heat of platinum may
be conveniently made by the tables given in a previous article.[1]
Corrections for varying specific heat of water are less important, but
may be made by the following table:
_Temperatures, Fahrenheit, and Corresponding Number of British Thermal
Units Contained in Water from Zero Fahrenheit_.
_______________________________________________________________
Deg | B.t.u. || Deg | B.t.u. || Deg | B.t.u. || Deg | B.t.u. |
----+--------++-----+--------++-----+---------++-----+---------+
32 | 32.000 || 57 | 57.007 || 82 | 82.039 || 107 | 107.101 |
33 | 33.000 || 58 | 58.007 || 83 | 83.041 || 108 | 108.104 |
34 | 34.000 || 59 | 59.008 || 84 | 84.043 || 109 | 109.107 |
35 | 35.000 || 60 | 60.009 || 85 | 85.045 || 110 | 110.110 |
36 | 36.000 || 61 | 61.010 || 86 | 86.047 || 111 | 111.113 |
37 | 37.000 || 62 | 62.011 || 87 | 87.049 || 112 | 112.117 |
38 | 38.000 || 63 | 63.012 || 88 | 88.051 || 113 | 113.121 |
39 | 39.001 || 64 | 64.013 || 89 | 89.053 || 114 | 114.125 |
40 | 40.001 || 65 | 65.014 || 90 | 90.055 || 115 | 115.129 |
41 | 41.001 || 66 | 66.015 || 91 | 91.057 || 116 | 116.133 |
42 | 42.001 || 67 | 67.016 || 92 | 92.059 || 117 | 117.137 |
43 | 43.001 || 68 | 68.018 || 93 | 93.061 || 118 | 118.141 |
44 | 44.002 || 69 | 69.019 || 94 | 94.063 || 119 | 119.145 |
45 | 45.002 || 70 | 70.020 || 95 | 95.065 || 120 | 120.149 |
46 | 46.002 || 71 | 71.021 || 96 | 96.068 || 121 | 121.153 |
47 | 47.002 || 72 | 72.023 || 97 | 97.071 || 122 | 122.157 |
48 | 48.003 || 73 | 73.024 || 98 | 98.074 || 123 | 123.161 |
49 | 49.003 || 74 | 74.036 || 99 | 99.077 || 124 | 124.165 |
50 | 50.003 || 75 | 75.027 || 100 | 100.080 || 125 | 125.169 |
51 | 51.004 || 76 | 76.029 || 101 | 101.083 || 126 | 126.173 |
52 | 52.004 || 77 | 77.030 || 102 | 102.086 || 127 | 127.177 |
53 | 53.005 || 78 | 78.032 || 103 | 103.089 || 128 | 128.182 |
54 | 54.005 || 79 | 79.034 || 104 | 104.092 || 129 | 129.187 |
55 | 55.006 || 80 | 80.036 || 105 | 105.095 || 130 | 130.192 |
56 | 56.006 || 81 | 81.037 || 106 | 106.098 || 131 | 131.197 |
----+--------++-----+--------++-----+---------++-----+---------+
[Footnote 1: _Journal_ for August, pp. 97, 98, and errata in _Journal_
for September, p. 172.]
A composite heat-carrier, of iron covered with platinum, answers well
for temperatures up to about 1,500 deg. F. A ball of wrought iron 0.88 inch
diameter will weigh 700 grains, and a capsule of platinum spun over it
0.048 inch thick, making the outside diameter 0.976+ inch, will also
weigh 700 grains. Upon the assumption of 0.0333+ for the specific heat
of Pt and 0.1666+ for that of Fe, the composite ball will have a heat
capacity equal to that of 4,200 grains of Pt, and equal to 0.01 of that
of 2 pounds of cold water. A patch, about 0.35 inch diameter, has to be
put in to close the orifice where the Pt capsule is spun together, and
a slight stain will show itself at the joint around this patch, from
oxidation of the iron, but the latter will be pretty effectually
protected. Difference of expansion, which will not exceed 0.007 inch
in diameter, will not endanger the capsule of Pt. The interruption of
conductivity at the surface contact of the two metals makes the process
of heating and cooling a little slower, but not noticeably so.
Such composite balls can be obtained for $20 each, $50 less than the
cost of an equivalent ball of solid platinum, which is preferable in all
but cost. Iron balls could be used for a few crude determinations. Cast
iron varies too much in composition, and wrought iron oxidizes rapidly.
While the oxide adheres it gains in weight, and when scales fall off it
loses; and the specific heat of the oxide differs from that of
metallic iron. Whatever metal is used, care must be taken to apply the
appropriate tabular correction for PtFe, or Pt and Fe.
MANIPULATION.
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 deg. 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 deg. 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 deg. F., 0.81 deg., 0.86 deg., 1.00 deg., and
1.07 deg. F.; mean, 0.935 deg. 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 deg. 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 deg. and 600 deg. F., I believe
them to be much more accurate, although less convenient.
For a range of temperatures from 212 deg. to 900 deg. F. they are certainly
more trustworthy than anything save an air thermometer of suitable
construction; and for all temperatures from 800 deg. to 900 deg. 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 deg. 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_.
* * * * *
LOCOMOTIVE PAINTING.
[Footnote: A paper read before the Master Car Painters' Association,
Chicago, September, 1883.]
By JOHN S. ATWATER.
The subject of locomotive painting has been pretty well discussed at the
former meetings of the association, and we have heard many excellent
suggestions regarding the use of oils, mineral paints, and leads from
gentlemen of long experience. But as the secretary has invited a display
of my ignorance I will endeavor to explain as clearly as possible
the methods I pursue, which, though not new or original, have been
productive of good results.
If time enough can be had we can prime with oil alone, or in connection
with the leads or minerals, and be sure of durability; but in these
days of "lightning speed," "lightning illuminations," and "lightning
painting," we must look about for something with "chain lightning" in
it, which, unlike the lightning, will remain bright and stick after it
strikes. We all have to paint according to the time and the facilities
we have for doing the work.
The scale on iron or steel is the only serious trouble which the painter
has to contend with. Rust can be removed or utilized with the oil,
making a good paint, but unless time can be given it is better to remove
the rust.
If possible let tanks get thoroughly rusted, then scrape off scale and
rust with files sharpened to a chisel edge, rub down large surfaces with
sandstone, and use No. 3 emery cloth between rivet heads, etc., then
wash off with turpentine. This will give you a good solid surface to
work upon.
For priming I use 100 pounds white lead (in oil), 10 pounds dry red
lead, 13 pounds Prince's metallic, 8 quarts boiled oil, 2 quarts
varnish, 6 quarts turpentine, and grind in the mill, as it mixes it
thoroughly with less waste. I mix about 250 pounds at a time (put into
kegs and draw off as wanted through faucets).
This _o-le-ag-in-ous_ compound can be worked both ways, quickly by
adding japan, slower by adding oil, and reduce to working consistency
with turpentine.
Without the oil or japan it will dry hard on wrought iron in about seven
days, on castings in about four days. When dry putty with white-lead
putty, thinned with varnish and turpentine, and knifed in with a
"broad-gauge" putty knife. Next day sandpaper and apply first coat
rough-stuff, which is, equal parts, in bulk, white lead and "Reno's
umber," mixed "stiff" with equal parts japan and rubbing varnish, and
thin with turpentine. Next morning, second coat rough-stuff, made with
Reno's umber, fine pumice stone, japan, and turpentine. At 1 o'clock
P.M. put on guide coat for the benefit of the small boys, which is
rough-stuff No. 2, darkened with lamp-black and very thin. The addition
of fine pumice to rough-stuff No. 2 encourages the boys in rubbing, and
prevents the blockstone from clogging.
By the time the last end of the tank is painted the first end is ready
for rubbing, though it is better to stand until next day.
After rubbing sandpaper and put on very thin coat of varnish and
turpentine (about equal parts). This soaks into the filling, hardening
it and making a close, smooth, elastic surface, leaving no brush marks
and being more durable than a _quick_-drying lead. This can be rubbed
with fine sandpaper or hair to take off gloss, and colored the next
morning, but it is better to remain 24 hours before coloring.
Upon this surface an "all japan color" would, before night, resemble a
map of the war in Egypt, but by adding varnish and a very little raw oil
to the "japan color," making it of the same nature as the under surface,
will prevent cracking.
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