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.
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