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

the lower portion of the chamber, the top and larger surface resting against the rubber diaphragm. Enough water at atmospheric pressure or alcohol is poured into the upper chamber through the opening marked 1 to seal the orifice in

the inverted cup and confine the air it contains. The regulator acts as follows: The warm water from the heater moving through the lower chamber communicates heat to the water or alcohol in the upper chamber, which in turn warms the air in the inverted cup, causing it to expand. This moves the rubber diaphragm and connected levers leading to the dampers substantially as in the damper-regulator for steamheaters, already described.

The Powers regulator for hot-water heaters (see Fig. 221) is somewhat similar in construction to the one described, but acts on a different principle. A liquid which will vaporize at a lower temperature than that of the water in the heater is placed in the vessel communicating with the diaphragm, in which case considerable pressure is generated before the water in the heater reaches the boiling-point. As the water in the heater is usually under a pressure of 5 to 10 pounds per square inch, its boiling temperature is from 225 to 240 degrees, water of atmospheric pressure which boils at 212° can be used in the closed vessel, and will generate considerable pressure before that in the heater boils.