_______________________________________________________________
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.
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.
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.