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1046 CHAPTER 40 1958 Guide 11 Sound Absorption in Rectangular Ducts, by L. L. Beranek, (Journal Acoustical Society of America, Vol. 12, October 1940, pp. 237-228). ' 11 The Transmission of Sound Inside Pipes, by Philip M. Morse (Journal Acoustical Society of America, Vol. 11, October 1939, pp. 205-210). " Acoustics, by L. L. Beranek, Chapter 10 (McGraw-Hill Book Co., 1954). CHAPTER 41 ELECTRIC HEATING Resistors, Heating Elements, Electric Heating Units, Applications of Electric Heat, Types of Electric Heating Systems, Equipment and Installation Methods, Calculating Capacities, Heating Water by Electricity, Induction and Dielectric Heating, Power Problems ELECTRICITY as a source of heat represents thermal energy in a re fined form, easily applied tospace heating by a variety of methods, and readily distributed and controlled. However, it usually is more expensive on a direct-heat-equivalent basis than heat from conventional fuels, and for economical service requires careful application in the design of system; adaptation of building structure, choice of control devices, and in method of operation by the user. For special applications and particularly when used to supplement heating systems of other types, the compactness, simplicity responsiveness, accuracy of control, safety, and other charac teristics of electric heating may carry greater weight in choice of method, than operating expense or initial investment. The basis of all electric heating methods and devices, except so-called heat pumps, is in the power-to-heat conversion constant: 1 kilowatt equals 3413 Btu per hr. Because ratings of electric heating equipment are ex pressed in terms of watts input and volts at the terminals, heating capacity calculations are translated directly to electric power requirements without the necessity of considering ampere currents or ohm resistances. Definitions of Resistor, Heating Element, Electric Heating Unit and other terms applying to heating practices will be found in Chapter 1. It is strongly recommended that compliance with the National Electrical Code and approval by Underwriters' Laboratories Inc., or other recog nized certifying agency, be required in the specifications for all equipment, materials, and construction used in electric heating systems. RESISTORS AND HEATING ELEMENTS Electric resistors usually are composed of metal alloys such as nickelchromium wire or ribbon, or non-metallic compounds containing carbon formed into rods or other shapes. Heating elements may have resistors either in the form of exposed coils mounted on insulators or of metallic conductors embedded in a refractory insulating material, encased in a protective sheath of metal. Fins or extended surfaces may be used to add neat-dissipating area. Elements are made in many forms, such as wires, strips, rings, tubes, plates and panels. Strip elements are used for clamping to surfaces requiring heat transfer by conduction, in some types of con vection air heaters, and in low-temperature radiant heaters. Ring and P ate elements are common in electric ranges and many small air heaters. * or oxide conductive films on glass and ceramics have been applied, ually in the form of panels. Tubular elements may be immersed in liqs, and when cast into metal or formed into coils, are used in water eTers> electric ranges and air heaters. th fabrics, woven from flexible resistor wires and asbestos or glass 1047