Penzig, F.
naca-tm-905
1939
This report is an attempt at a new method of coating the surface of the cylinder with materials that undergo chemical change at definite temperatures as indicated by a change in color. In this way it was hoped that the substance itself would indicate directly the position of its isotherms, which in measurements with thermocouples requires a tedious amount of labor.
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http://naca.central.cranfield.ac.uk/reports/1939/naca-tm-905.pdf