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° 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.