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.
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