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

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HEAT CONDUCTIVITY 119<br />

the hot and the water-cooled plates is measured by thermal<br />

juncti<strong>on</strong>s. Knowing these factors, also the area and the<br />

thickness, the relative c<strong>on</strong>ductivity of the materials under test<br />

may be computed with precisi<strong>on</strong>.<br />

An instrument of this general character, which shows re-<br />

finements over previous apparatus, has lately been designed<br />

and c<strong>on</strong>structed. The hot plate c<strong>on</strong>sists of two 5^-inch cop-<br />

per plates 12 inches square, between which are the heating<br />

FIG. 43.—THE HOT-PLATE METHOD OF TESTING HEAT TRANSMISSION-<br />

GENERAL VIEW OF THERMAL CONDUCTIVITY APPARATUS, 8-IN.<br />

SQUARE.<br />

coils c<strong>on</strong>sisting of nichrome resistance ribb<strong>on</strong> wound with<br />

even spacing <strong>on</strong> a slate core and insulated from the copper<br />

plates by two sheets of mica b<strong>on</strong>d, /n order to minimize the<br />

loss of heat from the edges of the hot plate, each copper plate<br />

is divided into an inner test area 8x8 inches and an outer<br />

guard ring. A compensating winding for furnishing auxiliary<br />

current is wound around the outer edges of the plate, to<br />

prevent the lateral flow of heat from the inner test area to<br />

the outer guard ring, a 1/16-inch air space being left between

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