ARC IN AIR BETWEEN CARBON ELECTRODES 21 



change considered above which continues to occur for 

 several seconds. However, since the sudden change is 

 commonly less than the slower one the arc will be in equi- 

 librium for the sudden change, if it is for the slower one. 



Hissing Arc. With the currents which are ordinarily 

 used the arc makes little or no noise. Such an arc is 

 known as a silent arc." With larger currents the arc 

 gives a hissing sound which becomes very loud and dis- 

 agreeable under certain conditions. Such an arc is known 

 as a " hissing arc." The voltage required is here much 

 smaller, as is shown in the lower parts of the curves in 

 Figs. 5 and 6. 



The correct explanation of this appears to have been 

 given by Mrs. Ayrton. 1 She found that when the crater 

 fills somewhat symmetrically the end of the anode, the arc 

 is silent; when it extends up the side, the arc hisses. More- 

 over when the arc is enclosed so that no oxygen can get to 

 it, or is placed in nitrogen, there is no hissing, while there 

 is hissing when it is in oxygen. She believed that the hiss- 

 ing is due to oxygen coming in contact with the crater, and 

 combining with the hot carbon. This chemical action is 

 supposed to cause a sudden decrease in the voltage and a 

 corresponding increase in current. The increase in current 

 drives the air away from the crater, and this again causes 

 a higher voltage with decrease in current. Thus, the arc 

 fluctuates from one condition to the other with the accom- 

 panying production of sound waves. 2 That oscillations in 



1 Journ. Inst. Elec. Eng., 28, 400; 1899. Mrs. Ayrton's "Electric 

 Arc," p. 299. 



2 Further experiments were performed by the following: 



Niaudet, C. R., 92, 711; 1881. 



Gime, Lum. Elec., 18, 556; 1885. 



Craveth, Elec. World, 19, 195, and 20, 227; 1892. 



