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Electrical Applications 2 by David W. Tyler

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Illumination 109
therefore exploits the phenomenon that when materials change
state,
from liquid to solid or liquid to gas, they do so at constant
temperature. A common example is that of water as it freezes into
ice or evaporates into steam.
Platinum melts or freezes at 1773°C and at this temperature emits
light since it is white hot. The platinum, contained in a small tube
within an insulated box, is melted electrically. During the time it is
solidifying the temperature remains constant so that the light inten-
sity at a small viewing hole at the top of the box is constant. This
light is used as the standar