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A small diameter NiCr high-temperature wire is
stretched across an inclosure, and is heated by an electric current from a Variac until it glows red hot. In
the center of this wire is threaded a 2 mm, OD, hollow glass tube. Because the tube diameter is less than the
critical radius it allows more heat transfer from the wire thus allowing that portion to be cooler so
the wire doesn't glow where it is covered by the glass tube. Thus, when smaller in diameter than the critical
radius, an insulator promotes heat transfer.
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