Although it was not mentioned in the text, Newton’s law of cooling requires that the thermal conductivity k of the object be “high enough,” otherwise the surface of the object will cool below its bulk temperature T. Then the convective heat transfer from the surface would be suppressed and the temperature of the object would not be uniform. Suppose L characterizes the distance from the center of the object, where the temperature is T, to its surface. What dimensionless constant must be small to avoid this problem?