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.

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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.