In Fig. 166 is shown the arrangement of the thermostat _T_ as it appears on the wall. Air from the supply tank is conveyed by the pipe _A_ through the thermostat _T_ to the motor valve _V_ attached to the radiator. The function of the thermostat is that of so controlling the radiator valve by means of the motor _V_ that the radiator will give out just sufficient heat to keep the room at the desired temperature. A closer view of the thermostat is given in Fig. 167. [Illustration: FIG. 167.--Outside view of thermostat as it appears in use.] The thermostat illustrated in Fig. 167 is that employed by the National Regulator Co. The drawing shows the exterior and interior construction of the parts enclosed in the previous illustration. The pipe _C_ at the right and opening _P_ at the left are the same as _A_ in Fig. 169; likewise, the pipe _D_ connects at the opening _M_ of the motor valve in Fig. 169. Referring again to Fig. 168, the sensitive part consists of a tube _A_ of vulcanized rubber. It is the dark-shaded part in the left-hand drawing. Any change in the air temperature influences the length of this tube. The changing length of the tube effects the air supply to close the radiator valve when the temperature rises above the desired amount and to reopen it when more heat is required. A finely threaded screw passes through the plug _H_ at the top and to this is secured the indicating disc _X_. The bottom of this screw is cupped to receive the point of the rod _K_, which connects with the piece _L_. Any change in length of the sensitive tube moves the valve lever _O_, and thus opens or closes the air port _G_. Air under pressure is supplied by the pipe _C_, connected to the air supply, flowing into the thermostat through the filter _P_, the restriction _S_, the passage _T_, and the port _G_. The adjustment of the thermostat for different temperatures is provided for by the screw _J_ through the top plug _H_, and the indicating disc _X_. The screw _R_ in the connector _Q_ at the base of the thermostat is a needle valve which opens or closes the connection with the air supply, and is used as an air shut-off valve when it is desired to remove the thermostat. The screw _S_ is a restriction valve which controls the supply of air to the thermostat, and this screw is set so as to allow the air to pass in a restricted quantity. [Illustration: FIG. 168.--Internal construction of the National Regulator Co.’s thermostatic regulator.]