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 of temperature of the radiator and surrounding objects, and is independent of currents of air. This is by no means the case with the convected heat, which is increased greatly by a slight draft from any extraneous source. In this connection it is very remarkable what a great effect an almost imperceptible draft will have on the heat given out by a radiator. This is partly due to the lowering of the temperature of the air between the loops, but also to the fact that with the same temperatures any increase in velocity increases the amount of heat the air absorbs.

Radiator tests.—Numerous tests of radiators have been made since those of Mills, Richards and others in the early '70's, but there is a wide variation in the results obtained, due partly to the different kinds of radiators tested and partly to the different methods of testing. As yet, no standard means of testing radiators has been adopted. The steam radiator as a heat-using device is theoretically perfect; that is, all of the heat that is put into the radiator by the latent heat of the steam condensed is given out to the air and objects surrounding. Its efficiency is therefore 100 per cent. The question of practical efficiency is, therefore, more strictly speaking, only one of effectiveness of surface. That is, of two radiators under exactly the same conditions of temperature and surroundings, that one which has such an arrangement of its surfaces as to give out the most heat per square foot is the most effective, usually called the most efficient. In all tests of radiators, the heat given off is measured by connecting them so that the steam which condenses can be accurately weighed, its pressure, quality and temperature being determined at the same time. The results are generally reduced to British thermal units given off per square foot per hour per degree difference of temperature between the air of the room and the steam of the radiator.

Tests of radiators have been made in various ways by Mr. George H. Barrus, by Profs. Denton and Jacobus of the Stevens Institute, by Prof. R. C. Carpenter, of Sibley College, Cornell University, and by the author. The results of Prof. Carpenter's tests are published in detail in his valuable work on "Heating and Ventilation of Buildings." In these tests the radiators were located in separate compartments, 7x10 feet, built together in a large room, and as shown in Figures 21 and 22. In order to allow some circulation of air so that the temperature of air of the compartments might