536 THE ELECTRIC ARC [Cn. XIII 



The exact nature of the resulting phenomenon will depend upon 

 the material of which the electrodes are made, upon the voltage of 

 the current supply and the resistance of the rheostat, and the kind 

 of gas surrounding the electrodes. 



741. Arc lamp. Any arrangement for holding the electrodes 

 and feeding them together as they wear away may be called an arc 

 lamp. 



It consists of three essential elements: (i) A clamp for holding 

 the positive electrode; (2) A clamp for holding the negative elec- 

 trode; (3) A mechanism for moving the holders and therefore the 

 electrodes nearer together or separating them farther apart. 



The electrode holders must be insulated so that the current must 

 flow through the electrodes and not follow any short circuits (fig. 

 270). 



For the hand-feed and the automatic types of arc lamps see 

 Chapter I, 9-11. 



742. With direct current, the arc is made up of three parts. 



1. The arc stream; a highly heated, incandescent gas which 

 conducts the current between the electrodes. 



2. The positive crater; where the current leaves the positive 

 electrode to enter the arc stream. 



3 . The negative crater ; where the current leaves the arc stream 

 to enter the negative electrode (fig. 292). 



743. Electrical behavior of the direct current arc. Measure- 

 ment of the voltage drop in various parts of the carbon arc reveals 

 the fact that the potential difference between the two electrodes 

 ( 743a) is made up of three parts. Starting from the positive 

 side, the potential difference between the positive electrode and the 

 arc stream is about 32 volts. The potential difference between the 

 arc stream and the negative electrode is about 9 volts, thus the 

 potential difference between the electrodes with the shortest possi- 

 ble arc is about 41 volts ( 743b). 



As the arc is lengthened there is an additional drop in potential 

 in the arc stream which depends mainly on the length, but partly on 

 the cross section of the arc stream. As the arc length is changed, 



