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American Society of Heating and Ventilating Engineers Guide, 1934 directly the rate of heat loss from an object maintained at the temperature of the body, irrespective of air temperature. The apparatus for this purpose consists essentially of a hollow sphere, or cylinder, containing a fluid which can be maintained accurately at 83 F (the accepted mean surface temperature of the human body), with an accurate means of measuring the rate of heat supply required to maintain the temperature at that exact point. The latter measurement can be made with sufficient accuracy by electrical methods. Although a BET of 72 deg Fahr is desirable, the mean radiant and air temperatures may both vary, provided the heat loss by radiation and convection from a surface at 83 F is maintained at the rate of 15.4 Btu per square foot per hour, 15.4 which corresponds to 3.415 surface. 4.5 watts pier square foot of exposed This instrument, the eupatheoscope, can readily be adapted as a thermo stat by electrical control to shut off or turn on heat when the critical temperature of 83 F in the vessel is increased or decreased. A modifi cation of the instrument is called the eupatheostat. Another instrument for maintaining comfort conditions is at present available only in a model adapted to British practice as it is designed for a temperature of 75 F. It consists of a blackened copper sphere of approxi mately 6 in. diameter in which is housed a cylindrical sump containing a volatile liquid. In operation, a small electric heating coil drawing about 5 watts creates in the sphere a vapor pressure which is constant as long as the heat losses from the sphere are standard. If the temperature of the air or the MRT becomes too high for comfort, a greater pressure is created, owing to a smaller loss of heat from the sphere. This increase of pressure acts on a diaphragm and shuts off the supply of heat to the room. For testing work, the globe thermometer is a very useful instrument. It consists of an ordinary mercury thermometer, with its bulb placed in the center of a sphere about 6 in. to 9 in. in diameter, usually made of thin copper and painted black. The temperature thus recorded is termed the radiation-convection temperature. REFERENCES A.S.H.V.E. paper' entitled Room Warming by Radiation, by A. H. Barker (A.S.H.V.E. Transactions Vol. 38, 1932). Panel Warming, by L. J. Fowler (A.S.H.V.E. Transactions, Vol. 36, 1930). Calculations for Radiant Heating, by T. Napier Adlam (Heating and Ventilating, October, 1931). Principles of Calculation of Low Temperature Radiant Heating, by A. H. Barker (paper presented before The Institution of Heating and Ventilating Engineers, London, December, 1931). Application of the Eupatheoscope for Measuring the Performance of Direct Radiators and Convectors in Terms of Equivalent Temperatures, by A. C. Willard, A. P. Kratz and M. K. Fahnestock (A.S.H.V.E. Journal Section, Heating, Piping and Air Con ditioning, July, 1933). 540 Chapter 38 ELECTRICAL HEATING Resistors, Heating Elements, Electric Heaters, Unit Heaters, Central Fan Heating, Electric Steam Heating, Electric Hot Water Heating, Heat Pump, Control, Calculating Capacities, Potoer Problems, Electric Heating Data WHILE it is improbable that electricity will ever replace fuels as the main source of heat, this type of heating has a logical and a rapidly growing place in the heating industry due to its advantages, such as flexibility, cleanliness, safety, convenience, and ease of control. Electric heating practice has many basic principles in common with fuel heating, blit there are also important differences. The advantages of good building insulation are even more important in electric heating than for fuel heating, because the initial cost per Btu is usually higher. All heat is a form of energy. Fuels hold stored chemical energy which is released into heat by combustion. Electrical power is a form of energy which can be released into heat by passing it through a resisting material. Both fuel and electric heating have two divisions: first, the conversion of energy into heat; second, the distribution and practical use of the heat after it is produced. In converting the chemical energy of fuels into heat by combustion, there is necessarily a considerable variation in thermal efficiency. This is not true, however, when converting electric power into heat, because 100 per cent of the energy applied in the resistor is always transformed into heat. In electric heating practice the engineer need not be concerned about efficiencies of heat production, but rather about efficiencies of heat utilization. DEFINITIONS Definitions of terms used in fuel heating are given in Chapter 42. The following terms apply particularly to electric heating: Electric Resistor: A material used to produce heat by passing an electric current through it. Electric Heating Element: A unit assembly consisting of a resistor, insulated supports, and terminals for connecting the resistor to electric power. Electric Heater: A complete assembly of heating elements with their, enclosure, ready for installation in service. RESISTORS Solids, liquids, and gases may be used as resistors, but most'com mercial electric heating elements have solid resistors, such as metal alloys, and non-metallic compounds containing carbon. In some types of electric boilers, water forms the resistor and is heated by an alternating current of electricity passing through it. 541