If R1 and R2 are the inner and outer radii of a cylinder, the heat conduction through a cylinder is proportional to
1
(R1 – R2)
2
(R1 × R2)
3
\(\frac{1}{{\left( {{R_1} - {R_2}} \right)}}\)
4
\(\frac{1}{{{{\log }_e}\left\{ {\frac{{{R_1}}}{{{R_2}}}} \right\}}}\)