The thermal conductivity of air

The thermal conductivity,k , of a gas is connected with its specific heat, Cv , and its viscosity,η ; by the relationk =fη Cv . The numerical constant,f , in this relation can be deduced theoretically, and various values, which are given later, from 1·25 to 2·5 have been obtained according to the law of force assumed to act in a molecular collision. Which of these laws of force holds is determined whenk ,η , and Cv are accurately known for a number of gases. A large number of determinations of both the viscosity and specific heat of each of the more simply constituted gases have been made that are in reasonable agreement with one another, but, compared with these, the independent values of the thermal conductivities are fewer, in poorer agreement, and the methods of determination are more open to criticism both in conception and performance. Eucken has determined relative to air the conductivity of a number of gases; the accuracy of his experiments made a new absolute determination ofk for air desirable. The investigation to be described was an attempt to supply an absolute value of the thermal conductivity of air by a method which would not be open to the objections which can be brought against all the methods previously used, excepting that of one observer. Heat in general is carried through a gas by conduction, radiation, and convection. In all past determinations of the thermal conductivity of gases, except those of Todd, the flow of heat by convection was present to an uncertain amount; in the experiments to be described it was absent.