CH. XIII] 



POLARITY TESTS 



507 



the current and strike the arc. After the lamp has burned a 

 minute or two open the switch or pull the separable plug apart and 

 watch the ends of the carbons. The one that remains red-hot the 

 longer is the positive one, and the wire leading to it is the positive 



B 



FIG. 271. 



SIDE AND FRONT VIEWS OF THE RIGHT-ANGLE CARBON ARC WITH 

 CORRECT AND INCORRECT POLARITY. 



A The upper figures show the correct polarity, that is, with the positive 

 crater on the upper carbon. 



B The lower figures show reversed polarity, that is, with the lower carbon 

 positive and hence the large crater on it. 



The photographs were made with a color screen in order to bring out the posi- 

 tive and the negative craters with the greatest clearness. The exposure for 

 the craters was instantaneous, then there was an additional exposure of 90 

 seconds without a color screen, and with an illumination from a mazda lamp 

 to bring out the carbons and give the appearance seen by the human eye (see 

 also fig. 292-293). 



