The ratio of the axial heat loss by convection to the radial heat loss by conduction for a wire mounted on the axis of a vertical tube is calculated for two cases: (a) when the tube is open at both ends to allow unrestricted circulation, and (b) when the tube is closed at both ends.The calculations are based on the assumption that the axial flow is not so large as to cause the form of the radial temperature drop to depart appreciably from the logarithmic and that the system is sufficiently long for end effects to be negligible. The elementary calculations made possible by these simplifying assumptions show that for a radius of wire of 0.05 cm and of the cylinder 0.25 cm, the axial heat loss for the closed case is about 7.5 times smaller than for the open case. For a 10 cm long tube with temperature drop between the wire and the surrounding tube of 1°C, the calculation for water shows that the axial heat loss is 2.5% of the radial heat loss for the open case and 0.3% for the closed case. For air under the same conditions the axial heat loss is 0.02% of the radial for the open case and 0.003% for the closed case.