48 THE ELECTRIC ARC 



Probably the most satisfactory method of showing the 

 effect which cooling the electrode has, is to pass an arc be- 

 tween copper or carbon tubes and to observe the voltage 

 first when the tubes are hot and second when they are 

 somewhat cooled by passing water through the interior of 

 the tubes. This method has been used by Tommasi/ 

 Schultze, 2 Malcolm and Simon, 3 and myself 4 and all have 

 found that with cooled electrodes higher voltages are re- 

 quired. 5 



The changes in the anode and cathode drops were also 

 examined by myself, and it was found that the greater part 

 of the change in voltage was due to a change in the anode 

 drop. The change in the different parts of the arc is 

 roughly proportional to the potential difference existing in 

 those parts before the cathode is cooled. 



Size of the Anode Crater. It is well known that the 

 size of the anode crater increases as the current is in- 

 creased, but there seems to be no agreement as to exactly 

 how large it is, nor what the exact relation is between its 

 area and the current. Andrews 6 found the area of the 

 crater to be directly proportional to the current. Mrs. 

 Ayrton, using cored carbons, found it to be a linear func- 

 tion of the current, but not quite proportional to it. 7 She 

 states that with a current of 4 amperes the diameter was 



1 C. R., 93, 716; 1881. 



2 Ann. d. Phys., 12, 837; 1903. 



3 Phys. ZS., 8, 478; 1907. 



4 Phys. Rev., 30, 310; 1910. 



6 Other experiments were performed by the following: 



Cross and Shepard, Proc. Amer. Acad. Sc., 22, 227; 1885. 



Mitkiewicz, Beib., 29, 739; 1904. 



Stark and Cassuto, Phys. ZS., 5 265; 1904. 



6 Journ. Tel. Eng., 9, 201; 1880. 



7 Mrs. Ayrton's "Electric Arc," p. 151. 



