these heaters the heating material has been surrounded with fire-clay, enamel, or some relatively poor conductor, and in other cases the material itself has been such as to give considerable resistance to the current. It is generally conceded that

the most satisfactory results are obtained with electrical as with other heaters by regulating the resistance, by change of length and cross-section of the conductor, to such an extent as to keep the heating coils at a moderately low temperature. Some of

the various forms which have been used are shown in the cuts. Fig. 216 represents a portable form of electrical heater used in the Vaudeville Theatre, London. Fig. 217 shows the interior of an office or house heater made by the Consolidated Car Heating Co., of Albany. The electrical heating surface is made in the latter by a coil of wire wound spirally about an incombustible clay core. The casing is like that for an ordinary

stove, and is built so that air will draw in at the bottom and pass out at the top.

The electrical heaters at the present time are used almost exclusively in heating electrical cars, where current is available and room is of considerable value. These heaters are generally located in an inconspicuous place beneath the seats, their general form being shown in Figs. 218 and 219.

163. Connections for Electrical Heaters.-The method of wiring for electrical heaters must be essentially the same as for lights which require the same amount of current. The details of this work pertain rather to the province of the electrician than to that of the steam-fitter or mechanic usually employed for installing heating apparatus. These wires must be run in accordance with the underwriters' specifications, so as not, under any conditions, to endanger the safety of the building from fire.

CHAPTER XV.

TEMPERATURE REGULATORS.

164. General Remarks.-A temperature regulator is an automatic device which will open or close, as required to produce a uniform temperature, the valves which control the supply of heat to the various rooms. Although these regulators are often constructed so as to operate the dampers of the heater, they differ from damper-regulators for steam-boilers, as described in Article 91, by the fact that the latter are unaffected by the temperature of the surrounding air, although acting to maintain a uniform pressure and temperature within the boiler, while the former are put in operation by changes of temperature in the rooms heated.

The temperature regulator, in general, consists of three parts, as follows: First, a thermostat which is so constructed that some of its parts will move because of change of temperature in the surrounding air, the motion so produced being used either directly or indirectly to open dampers or valves, and thus to control the supply of heat. Second, means of transmitting and often of multiplying the slight motion of the parts of the thermostat produced by change of temperature in the room, to the valves or dampers controlling the supply of heat. Third, a motor or mechanism for opening the valves or dampers, which may or may not be independent from the thermostat.

In some systems the thermostat is directly connected to the valves or dampers, and no independent motor or mechanism is employed; in this case the power which is used to open or close the valves regulating the heat-supply is generated within the thermostat, and is obtained either from the expansion or contraction of metallic bodies, or by the change in pressure

caused by the vaporizing of some liquid which boils at a low temperature. The force generated by slight changes in temperature is comparatively feeble, and the motion produced is generally very slight, so that when no auxiliary motor is employed it is necessary to have the regulating valves constructed so as to move very easily and not be liable to stick or get out of order. In most systems, however, a motor operated by clockwork, water, or compressed air is employed, and the thermostat is required simply to furnish power sufficient to start or stop this motor. The limits of this work do not permit an extended historical sketch of many of the forms which have been tried. The reader is referred to Knight's Mechanical Dictionary, article" Thermostats," and to Péclet's "Traité du la Chaleur," Vol. II, for a description of many of the early forms used. Those which are in use may be classified either according to the general character of the thermostat or the construction of the motor employed to operate the heat-regulating valves as follows:

165. Regulators Acting by Change of Pressure.-A change of temperature acting on any liquid or gaseous body causes a change in volume, which in some instances has been utilized to move the heat-regulating valves so as to maintain a constant temperature. Fig. 220 represents a regulator in which the expansion or contraction of a body of confined air is utilized to control the motion of the dampers to a hot-water heater.

It consists of a vessel containing in its lower portion a jacketed chamber connected to the hot-water heater at points. of different elevation so as to secure a circulation from the heater through the lower portion or jacket of the vessel from 2 to 3. Above this is a second chamber which is covered on top with a rubber diaphragm, and which contains a funnelshaped corrugated brass cup. The opening to the cup is in