MISCELLANEOUS PRACTICAL APPLICATIONS. 



339 



Fig. 4 1 . 



intensely heated and give off a very brilliant light. The arc 

 lamp is a mechanism for automatically moving two carbon rods 

 so that a steady electric arc may be maintained between the ends 

 of the rod. There is a great variety of arc lamp mechanisms but 

 the following description will serve to give an idea of their action : 

 The current comes into the lamp and divides as shown in Fig. 41. 

 A very small portion of the 

 current flows through a 

 shunt coil B without pass- 

 ing through the arc, and the UNE wn 

 remainder flows through 

 the coil A and thence 

 through the arc. An iron 

 rod AB, passing loosely 

 into the two coils A and 

 B, is carried upon one end 

 of a lever which is pivoted 

 at the point p. The other 

 end of this lever is provided with a clutch c through which a 

 smooth brass rod bb passes. This clutch c is so constructed 

 that it releases the rod bb when the iron rod AB is raised, thus 

 allowing the carbons to come together. Each of the coils A 

 and B acts to pull the rod AB into itself, and a spring which 

 is attached to the lever is so adjusted that when the arc is burning 

 properly the combined action of this spring and the two coils A 

 and B holds the lever in such a position that the clutch grips 

 the brass rod bb. As the arc continues to burn, the carbons 

 are slowly consumed, causing the gap between the carbon tips to 

 widen. This increases the resistance of the arc and causes a 

 greater portion of the current to flow through the shunt coil B 

 which pulls up on the iron rod AB, moves the lever, releases 

 the clutch, and allows the rod bb to fall slightly, thus bringing 

 the carbons again to the proper position. 



A variety of arc lamps have been developed in which the light 

 is emitted by the arc itself. Thus we have the so-called flaming- 



