Cork insulation; a complete illustrated textbook on cork insulation ...

118 CORK INSULATION
the test box will be maintained at a lower temperature than
the outside room, and since the amount of heat required to
melt one pound of ice is definitely known, the amount of
heat lost through the walls of the test box may be determined
by weighing the water resulting from the melting of ice and
carried outside of the box through a small rubber tube.
The results are reasonably reliable since the suspended
container of cracked ice sets up a natural circulation of air
within the test box and keeps it at a very nearly uniform
temperature.
FIG. 42.—THE COLD-AIR-BOX METHOD OF TESTING HEAT
TRANSMISSION.
78.—The Hot-plate Method.—The hot-plate method has!
probably been most widely used by investigators, including!
the United States Bureau of Standards, to determine the|
relative conductivity of insulating materials. The inaccura-
cies in this method, for absolute conductivity determination,
lie in the determination of the heat loss from the edges, which
is ordinarily considerable, and the uncertainty of the contact
between the material and the plates.
The method consists of an electrically heated plate placed
between two sheets of the material to be tested, and outside
of these sheets are placed two hollow plates cooled by circulating
water. By measuring the electrical input, the amount
of heat lost through the insulating materials can be determined
by calculation. The temperature difference between
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