The drawing shows a solid cylindrical rod made from a center cylinder of lead and an outer concentric jacket of copper. Except for its ends, the rod is insulated (not shown), so that the loss of heat from the curved surface is negligible. When a temperature difference is maintained between its ends, this rod conducts 1/7 the amount of heat that it would conduct if it were solid copper. Determine the ratio of the radii r₁/r2.

The drawing shows a solid cylindrical rod made from a center cylinder of lead and an outer concentric jacket of copper. Except for its ends, the rod is insulated (not shown), so that the loss of heat from the curved surface is negligible. When a temperature difference is maintained between its ends, this rod conducts 1/7 the amount of heat that it would conduct if it were solid copper. Determine the ratio of the radii r₁/r2.