The controller appears in Fig. 54 as commonly seen in use. The upper part carries a thermometer and the pointer A indicates the temperature to be maintained in the room. The middle division indicates 70°F. Each division to the right of the middle point raises the temperature 5°. Each division to the left lowers the temperature a like amount.
In addition to the ordinary type this controller is furnished with a time attachment by means of which the controller may permit the temperature of the room to fall to any desired degree at night and raise it again in the morning at the time for which it is set.
This is accomplished by a little alarm clock shown at the bottom of the controller in Fig. 54. The indicator B is arranged to correspond with the indicator A; the middle point representing 70°F. To set the time attachment, the alarm is wound and set as in any alarm clock, ½ hour earlier than the desired time for rising. The indicator B is set for the day temperature and A is set for the temperature desired during the night. At the appointed time the alarm moves the indicator A to the desired point for the day and the controller raises the temperature accordingly.
Fig. 55 shows the mechanism that is exposed to view when the cover of the controller is removed. The bent strip C is the part that is influenced by the change of temperature. It is made of two thin strips of metal, one of brass and the other of steel. The two strips are soldered firmly together. Any change in temperature will affect the strip and cause it to bend and touch the contact point—K or J. The bending of the strip is due to the unequal expansion of the brass and steel due to the change of temperature. Brass expands 2.4 times as much as steel with the same change of temperature. The amount of bending is sufficient to make an appreciable movement in a small fraction of a degree change. The brass part of C is on the left and since it expands the greater amount, a rising temperature causes C to come into contact with the point J. When this happens the motor is started and makes one-half cycle. In so doing it shuts off the air supply of the furnace, opens the check draft and at the same time the motor changes the electric contact from J to K. When the temperature begins to fall, the brass contracts in the same ratio to the steel as it expands during the rising temperature and as a consequence the bar bends to the left. When the strip touches the point K the motor again makes one-half circle, admitting air once more to the furnace, closes the check draft and shifts the electric contact back to K. When properly started the thermostat will regulate the temperature within a degree of temperature.
The Thermostat Motor.
—The thermostat motor automatically opens and closes the furnace dampers or the valve that admits steam to the radiators as heat is demanded by the controller.
The motor, as shown in Fig. 53, consists of a system of gears and a brake S, which regulates the speed, a cam M, and armature I, for starting and stopping the motor, and the electromagnet H-H which operates the bar I. Two lever arms L, one in front and the other at the back of the motor furnish means for attachment to the valve or furnace dampers. An emergency switch at D is shown in detail in Fig. 56. The battery B furnishes the current which energizes the magnets and an iron weight supplies the motive power for the motor.
The description of the operation of the motor applies to the steam valve shown in Fig. 53. The same motor might be used for opening and closing of the dampers of the furnace in any kind of heat supply. The method of communicating the motion of the motor arms to the dampers of the furnace will be described later. The connections with the furnace drafts are shown in Figs. 3, 6, 8, 34, etc.
Suppose that the valve for admitting steam to the radiators, as that in Fig. 53, is closed and that the temperature of the house is falling. The strip C of the thermostat controller is moving toward J. When contact is made, the current from the battery B energizes the magnets H-H and the bar I is lifted. As the bar I is raised the catch J is released and permits the motor to start. The bar I is held suspended by the cam M until the arm L has made one-half revolution, when the lug K drops into the depression in the cam made to receive it and the catch J engages with the brake and stops the motor.