The method of construction is shown at the right, in Fig. 221, as applied to a hot-water heater. The diaphragm employed consists of two layers of elastic material with compartments between and beneath; the lower one is connected to the chamber

A, which is filled with water at atmospheric pressure and is surrounded by the hot water flowing from the heater. The water in chamber A, being under less pressure, will boil before that in the heater, and will produce sufficient pressure to move the diaphragm and levers so as to close the dampers, before the water in the heater reaches the boiling-point. The compartment between the two diaphragms f, f is in communication with a vessel D, which in turn is connected by a closed pipe E with a thermostat, which may be placed at any point in the house and so arranged that if the temperature becomes too high in that room, the dampers of the heater will be closed. With this apparatus the dampers are closed either by excessive temperature of water at the heater or too great a heat in any room. The intermediate compartment is only required when the dampers are to be operated by change of temperature in the rooms.

The thermostat employed in this apparatus consists of a vessel 2, Fig. 221, separated into two chambers by a diaphragm ; one of these chambers, B, is filled with a liquid which will boil at a temperature below that at which the room is to be maintained; the other chamber, A, is filled with a liquid which

does not boil, and is connected by a tube to a diaphragm damper-regulator which moves the dampers through the medium of a series of levers.

Fig. 221, 2, shows a transverse section and I an elevation with parts broken away of a thermostat, and Fig. 222 an ele

vation with attached ther mometer. The vapor of the liquid in the chamber B produces considerable pressure at the normal temperature of the room, and a slight increase of heat crowds the diaphragm over and forces the liquid in the chamber A outward through a connecting tube which leads to the damper - regulator, one form of which has been described.

The damper-regulator as applied to a steam-heater is provided with a single rubber diaphragm with the parts arranged

FIG. 223.-DIAPHRAGM DAMPER-REGULATOR.

Many

The liquid used in the thermostat may be any which has a boiling temperature somewhat below that at which the room is to be kept. liquids are known which fulfil this condition, of which we may mention etheline, bromine, various petroleum distillates, anhydrous ammonia, and liquid carbonic acid. The liquids employed in the Powers thermostat are said to give pressures as follows at the given temperatures:

166. Regulators Operated by Direct Expansion.— Metals of various kinds expand when heated and contract when cooled, and this fact has often been utilized in the construction of temperature regulators.

A single bar of metal expands so small an amount that it is of little value for this purpose unless very long, or unless its expansion is multiplied by a series of levers. Several forms have been used, of which may be mentioned: a bent rod with its ends confined so that expansion tends to change its curvature; a series of bent rods of oval form resting on each other with the ends confined between two fixed bars; two metallic bars having different rates of expansion arranged parallel and the variation in length multiplied by a series of connecting levers an amount sufficient to be available in moving dampers; two strips of metal of different kinds bent into the form of an arc and fastened together so as to form a curved bar, with the metal which expands at the greater rate on the inside, so that expansion tends to straighten it when heated; the difference in expansion between an iron rod which is not heated and the flow-pipe of a hot-water heater multiplied by means of a series of levers. The constructions described above have all been tried for the purpose of moving the dampers of heaters or for opening and closing valves. In general, however, they have not proved satisfactory, because of the slight motion caused by expansion, and the uncertainty of operation obtained with multiplying devices.

Certain organic materials have the property of bending or curling when heated, and this has been utilized in the construction of the Howard regulator. This regulator consists of a thermostat in the form of a plaque of triangular form 11 inches. long and 9 inches wide (Fig. 224), which is located in any

living-room. As the temperature of the room increases the plaque bends. It is connected by means of cords running over pulleys to a very light and easily moved cylinder damper arranged so as to regulate both fire and check drafts. The

damper used in connection with this thermostat consists of a slotted cylinder rotating on the inside of a tube which leads in one direction to the ash-pit and in the other to the smoke-pipe. A partition separates the two parts of the tube, and the slots in the cylindric damper are so arranged that when the connection for air to the furnace is open the other is closed, and vice versa, a very slight motion serving to completely open or close the damper. The cylinder damper is connected to the plaque by a cord, and is so arranged that the drafts are opened by the motion of the thermostat and closed by gravity.

The direct expansion of a liquid or of a gas in a confined vessel has also been utilized to move a diaphragm or piston which is connected by levers to the dampers of heaters, in a manner similar to that described in the preceding article. The writer at one time constructed a regulator for a hot-water system in which the expansion of water in a closed vessel surrounding the return-pipe was employed to operate a damperregulator similar to those used in steam-heating, page 156. Péclet describes, regulators in which the expansion of air was employed to move a piston connected by cords and pulleys to the dampers.

167. Regulators Operated with Motor-General Types. -The regulators which have been described in the preceding articles operate the regulating valves with a feeble force acting through a considerable range, or with a considerable force acting through a short distance. They are consequently liable to be rendered inoperative by any accident to the levers or connecting tubes, or by any cause which renders the valves difficult to operate. To overcome such difficulties several

systems have been devised in which the power for operating the dampers should be obtained from an independent source, and in which the work required of the thermostat would be simply that of starting and stopping an auxiliary motor. In the first systems of this kind the motor employed was a system of clockwork which had to be wound at stated intervals in order to supply the force required for moving the dampers. In recent systems electricity, water, or compressed air is employed to generate the power required, and in some instances regulators are arranged to operate not only the valves which supply heat to the rooms, but also the various dampers for supplying hot or cold air in the ventilating system.

In all of the early forms of this kind of regulator the thermostat consisted of a tube of mercury or a curved strip, made of two metals of different kinds soldered together and arranged so that a given change of temperature would produce sufficient motion to make or break electric contact. A current was obtained from a battery, and connecting wires led to the motor and to the various terminals. When electric contact was made at a position corresponding to the highest temperature, the current would flow in a certain direction and cause a magnet to release a pawl which would start a motor revolving in the proper direction for closing the valves. When the temperature fell below a certain point, the thermostat would make electric connections so that the current would flow in the opposite direction and cause the motor to reverse its motion, thus opening the valve. If the motor was operated by water, the electric current would open and close a valve in the supply-pipe; if the motor was operated by electricity, the current from the battery would move a switch on the wires leading to the motor.

The valves for regulating the heat-supply are made in a great variety of ways. Dampers for regulating the flow in chimneys or flues are generally plain disks, balanced and mounted on a pivot, so that they may be turned very easily; globe- or gate-valves are usually employed in steam-pipes and must, to give satisfactory service, either be closed tight or opened wide. A system in which steam-valves are operated requires much more power than one in which dampers only are moved.