It appears that four factors determine the rate of heat impartation from the gas to any small area of the metal[17]:
(1).—The temperature difference between the body of the gas and the metal;
(2).—The weight of the gas per cubic foot, which is proportional to the number of molecules in any unit of volume;
(3).—The bodily velocity of the motion of the gas parallel to any small area under consideration; and (probably),
(4).—The specific heat of the gas at constant pressure.
The apparatus consists of an electric resistance furnace containing coils of nickel wire, a small (interchangeable) multi-tubular boiler, and a steam-jet apparatus for reducing the air pressure at the exit end, so as to cause a flow of air through the boiler. A surface condenser was attached to the boiler’s steam outlet, the condensed steam being weighed as a check on the feed-water measurements. A number of thermometers and thermo-couples were used to obtain atmospheric-air temperature, temperatures of the air entering and leaving the boilers, and feed-water temperature.
The apparatus is now being reconstructed with appliances for measuring the quantity of air entering the furnace, and an automatic electric-furnace temperature regulator.
Three sizes of boiler have been tested thus far, the dimensions being as given in Table 4.
Each of the three boilers was tested at several temperatures of entering air, up to 1,500° Fahr., about ten tests being made at each temperature. It is also the intention to run, on these three boilers,
about eight tests at temperatures of 1,800°, 2,100° and 2,400° Fahr., respectively. A bulletin on the work already done, together with much incidental matter, is in course of preparation.[18]