Determine the Biot number for a thermocouple designed to measure the temperature of a gas stream, where the thermocouple's junction is modeled as a sphere with a diameter of $2\mathrm{.}0$ mm$.$ The thermal conductivity of the junction material is k $=50$ W m$-1$K$-1,$ and the heat transfer coefficient between the junction and the gas is h $=170$ W m$-2$K$-1.$