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.