50 THE ELECTRIC ARC 



between the movable carbon and the insulator about it, 

 and this is sufficient to allow the necessary amount of air 

 to enter. 



The ends of the carbons with the enclosed arc remain 

 more nearly flat, the crater of the anode being less de- 

 pressed, and the cathode point being less prominent. The 

 resulting distribution of light is somewhat different from 

 that given by the open-carbon arc. 



The great advantage of this form of arc is that the car- 

 bons do not need to be renewed nearly as often as with 

 the open arc. This is more of an advantage in this country 

 where the cost of labor is higher than in Europe. As a 

 result it has been used very largely here, and very little in 

 places where labor is cheap. 



Characteristic curves of these arcs do not seem to have 

 been published, but it may be said that in general the volt- 

 age of an enclosed arc is higher than that for an open arc 

 of the same length and current between the same carbons. 

 As an example of this it was found that the voltage of 

 the arc in a commercial lamp using 6.5 amperes and 

 having a length of 8 mm. was 80 volts. 



Miscellaneous. There are a few more or less isolated 

 facts concerning the carbon arc which should be men- 

 tioned before passing to other topics. Such, for example, 

 is the fact that an arc is deflected when in a magnetic 

 field. The first mention of this seems to have been made 

 by Davy in 1821 * and Walker 2 states that the arc will 

 rotate if one of the electrodes is a steel magnet. Blondel 3 

 took photographs of the alternating-current arc in a mag- 



1 Phil. Trans., p. 428, 1821. 



2 Pogg. Ann., 54, 514; 1841. 



3 Lum. Elec, 43, 54; 1892. 



