The radiation from the hot plate was suppressed, a thin
aluminum foil was glued to the plate carefully, in order to
achieve good contact of the two surfaces. The outside sur-
face of the foil is specular, and its emissivity is as low as 0.04
[21]. Experiments without the foil were also conducted.
In those experiments, the hot plate emissivity was 0.85
and for that reason the thermocouples were shielded by
small screens. The screens could not alter the ¯ow inside the box, but could eliminate radiation errors in the measure-
ments. The screens were covered on both sides by the same
aluminum foil.