OCT. 19, 1921 ROBERTS: FURNACE REGULATOR 403 



primary P connected through a "thermal lamp flasher" F to the 

 battery corners of the bridge. The operation of the main relay C 

 increases or decreases the current through P and induces a momen- 

 tary surge of current through the coil of the galvanometer. The 

 current is caused to pass through the primary P in such a direction 

 that this surge causes a rather violent deflection of the boom away 

 from the contact stop. 



The "lamp flasher" F also causes sudden changes in the current 

 through P and corresponding deflections of the galvanometer, which 

 occur independently of the movement of the relay. The violence 

 of the deflection serves to make contact when the contacts have been 

 brought together but have been prevented from touching by a chance 

 accumulation of dirt. 



CYCLE OF OPERATION OF THE RELAYS 



In describing the various operations of the relays by which the 

 regulator controls the temperature of the furnace we shall arbitrarily 

 assume that a rise in the temperature of the furnace causes a movement 

 of the boom of the galvanometer A toward the left; and that clos- 

 ing the galvanometer contacts to the left sends a current through the 

 polarized relay B in such a direction as to cause its boom to move 

 over to the left-hand contact. 



At a given instant, let us say, the contacts of the polarized relay 

 B are closed on the left; the main relay C is then short-circuited 

 and open ; the current to the furnace is passing through the resistance 

 Rz, being cut down thereby to its lower value, and the furnace is 

 therefore cooling. 



As it cools, the boom of the galvanometer .4 moves toward the 

 right, and when it touches the right-hand contact, completes a cir- 

 cuit through the coils of the polarized relay B in parallel with the re- 

 sistance Ri. This causes the armature of the polarized relay to start 

 to move over towards its right-hand contact. The main rela}^ C 

 is now no longer short-circuited, and closes, cutting out the resistance 

 Rz. 



When the armature of the polarized relay reaches the right-hand 

 contact it short-circuits Ri and thus stops the flow of current through its 

 own coils and through the right-hand contacts of the galvanometer A . 

 Since it is a polarized relay, its armature does not swing away from 

 the contact, but remains pressed against it because of the action of 

 the permanent magnetic field of the relay. 



