FROM THE PRACTICAL VIEWPOINT 



matic temperature control than to depend upon 

 man power, provided, of course, that the machine 

 works as it is intended to work. 



The machines designed to control temperatures 

 are operated in two ways. In one machine the 

 bulb which is inserted into the milk is filled with 

 some volatile liquid like ether and is connected 

 by means of a capillary tube with a pressure coil, 

 which in turn regulates the flow of compressed air 

 to a valve. The compressed air is employed as a 

 motive power to operate the valve which regulates 

 the admission of steam to the milk heater. Such a 

 system involves the installation of an air pump 

 and a tank for the storage of compressed air. To 

 one unfamiliar with the apparatus the description 

 may appear involved and the operation compli- 

 cated. Fig. 28 shows this system. 



In another form of controller, electricity is em- 

 ployed as a motive power in operating the steam 

 valve instead of compressed air. The bulb which 

 enters the milk contains two metals, which are so 

 fastened together that the difference in expan- 

 sion makes and breaks the electrical connection 

 and the steam valve is operated by this means. 

 This is shown in Fig. 29- This method can, of 

 135 



